Coal Ball Paleobotany: Introduction and Overview

Smithsonian Scholarly Press

smithsonian contributions to paleobiology • number 111

Coal Ball Paleobotany Introduction and Overview

Tom L. Phillips, W. John Nelson, Scott D. Elrick, and William A. DiMichele

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smithsonian contributions to paleobiology • number 111

Coal Ball Paleobotany Introduction and Overview

Tom L. Phillips, W. John Nelson, Scott D. Elrick, and William A. DiMichele

Smithsonian Scholarly Press

WASHINGTON, D.C.

2026

ABSTRACT Phillips, Tom L., W. John Nelson, Scott D. Elrick, and William A. DiMichele. Coal Ball Paleobotany: Introduction and Overview. Smithsonian Contributions to Paleobiology, number 111, viii + 80 pages, 49 figures, 2026. —­Coal balls are concretions, specifically permineralized peat stages of coal, formed during the process of conversion of peat to coal. Thus, they preserve the original peat fabric of the coal at various stages of that conversion, providing insights into the taphonomic processes involved in the formation of peat during the late Paleozoic. Coal balls have been prized by paleobotanists because they entomb the various organs of the original peat-­swamp plants preserved anatomically, thus carrying more information about the plants from peat-­swamp environments of this time period than any other form of fossil preservation. They also permit insights into the paleoecology of the individual plants and the communities they composed. We discuss in detail the methods involved in paleoecological analyses. Typically, coal balls are permineralized mainly by calcium carbonate (calcite) with various admixtures of magnesium carbonate (dolomite) and iron sulfide (pyrite and marcasite), and minor amounts of siliciclastic sedimentary particles. The conditions leading to the formation of these carbonate concretions, in what were arguably low-­to medium-­pH environments (pH 3–5), remain a topic of research, and several models have been proposed, some of which are mutually incompatible; none thus far explains all of the observed data. This type of fossil is known exclusively from coal beds of late Paleozoic age, primarily in Late Mississippian and Pennsylvanian coal basins of Euramerica, and from Permian-­age coals of China. They require collecting procedures similar to those employed for other kinds of plant-­fossil remains (digging tools), but require specialized procedures for processing, storing, and studying, all of which are covered here in some detail. We also illustrate and discuss some of the adventures and vicissitudes encountered in coal ball collecting by examining specific times and places when and where specimens were located, collected, and brought back to the laboratory for study.

KEYWORDS: Late Paleozoic, peat, taphonomy, stratigraphy, coal mine

Cover image: Cross section of a coal ball aggregate from the Calhoun Coal (Upper Pennsylvanian), East locality, Richland county, Illinois. Black coal regions separate coal ball concretions. (Natural size.)

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Contents

LIST OF FIGURES

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INTRODUCTION The Coal Age Time and Place The Late Paleozoic Ice Age Peat-­Forming Swamps

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COAL BALLS: THE ORIGINAL PEAT FABRIC Caveat: Most Coal Ball Peat Is Not Well Preserved Coal Ball Formation in Brief GEOLOGY OF AMERICAN COALFIELDS American Chronostratigraphy Appalachian Coalfield Illinois Basin Coalfield Midcontinent Coalfield Texas Coalfield Other Areas COAL BALL COLLECTION, PREPARATION, AND QUANTITATIVE ANALYSIS METHODS Coal Ball Occurrences in the Field Coal Ball Collecting: Background Collecting Coal Ball Vertical Sections Laboratory Preparation of Coal Balls Cutting the Coal Ball Surface Preparation Peel Preparation Siliceous Coal Ball Preparation Caveats Thin-­Section Preparation Quantitative Analysis of Coal Balls Coal Ball Storage Summary: Coal Ball Collection, Preparation, and Storage

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COAL BALLS IN TIME, SPACE, AND HISTORY A Brief History of Coal Ball Studies Selected American Coal Ball Occurrences Herrin Coal, Illinois Basin Springfield Coal, Illinois Basin Houchin Creek Coal, Northeastern Illinois Murphysboro-­Equivalent Coal, Maple Grove Mine, Illinois Basin Mineral and Fleming Coals, Kansas, U.S. Midcontinent Iron Post Coal, Peabody Rogers County Mine No. 2, Vinita, Oklahoma, U.S. Midcontinent Cliffland/Blackoak Coal, Oskaloosa, Iowa, U.S. Midcontinent Friendsville Coal, Allendale, Illinois, Illinois Basin Calhoun Coal, Berryville, and New Calhoun Locations, Illinois, Illinois Basin

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CONCLUDING REMARKS

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ACKNOWLEDGMENTS

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REFERENCES

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INDEX

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Figures

1. Stratigraphic subdivisions of the late Paleozoic 2. Pangea: Pennsylvanian paleogeography 3. Pennsylvanian peat-­forming swamp reconstructions 4. Forms of preservation 5. Coal ball tissue preservation 6. Coal balls 7. Coal ball occurrence map, Herrin Coal 8. “Normal” coal ball preserved within organic matter 9. Coal ball aggregate cross section 10. United States coalfields map 11. Appalachian coalfield map 12. Pocahontas Basin cross section 13. Cross section from northwestern Pennsylvania to the Anthracite region 14. Illinois Basin coalfield map 15. Midcontinent coalfield map 16. Exposure of coal and coal balls 17. Various forms of coal ball exposures 18. Coal ball collecting in action 19. Mine apparel and equipment 20. Views of mines 21. Coal balls in situ, Galatia Mine 22. Coal ball storage 23. Coal ball processing laboratory 24. Older methods of coal ball preparation 25. Peel removal and treatment 26. Observing and quantifying peels 27. Coal ball cross section with overlay grid 28. Overlay grid area enlargement 29. Coal ball collection storage facilities 30. Herrin Coal coal ball localities map 31. Springfield Coal coal ball localities map 32. Herrin Coal mining in the early twentieth century 33. Herrin Coal cyclothem

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34. Sahara Coal Company Mine No. 6 35. Coal ball profile, Old Ben Mine No. 24 36. Old Ben Mine No. 24 and coal ball collectors 37. Houchin Creek coal ball cross sections 38. Houchin Creek coal ball aggregate 39. Murphysboro-­equivalent coal ball aggregate 40. Houchin Creek coal ball aggregate 41. Murphysboro-­equivalent coal ball aggregate 42. Murphysboro-­equivalent coal ball aggregate from top 43. Murphysboro-­equivalent coal ball cross section 44. Pittsburg and Midway Coal Co. Mine No. 19 45. Western Interior coal mines 46. Coal ball with a marine-­sediment-­filled burrow 47. Allendale Mine coal ball locality 48. Berryville, Illinois, coal ball locality, 1930s 49. Berryville locality, 1990

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Coal Ball Paleobotany Introduction and Overview Tom L. Phillips,1† W. John Nelson,2 Scott D. Elrick,2 and William A. DiMichele3*

INTRODUCTION THE COAL AGE

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†Deceased. Formerly with Department of Plant Biology, University of Illinois, Urbana, Illinois, 61801, USA. 2 Illinois State Geological Survey, University of Illinois, Champaign, Illinois, 61820, USA. 3 Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560, USA. * Correspondence: dimichel​@si​.edu Manuscript received 3 March 2025; accepted 29 January 2026.

“The Coal Age” is a keynote term that marries two concepts central to the study of the fossils known as coal balls—­concretions found in coal beds that preserve the plant remains of the peat from which coal formed. “Coal Age” first refers to the Carboniferous Period of geologic history, when vast peat swamps flourished across central and eastern North America and from the British Isles to Russia. Second, the term refers to the early stages of the Industrial Revolution, fueled by the coal that developed from those Carboniferous peat swamps. As those coal deposits were first exploited to fuel furnaces, smelters, and engines, a lively scientific debate raged around the origin of this rock that burns. “Condensed petroleum,” weathering of Precambrian crystalline rocks, and a surplus of atmospheric carbon dioxide were among the theories offered. Although authors more than four centuries ago suggested that coal originated from compressed plant matter, the debate was not settled until the early twentieth century, and coal balls played a leading role.

As revealed in mines throughout the American industrial heartland, these concretions of mineralized peat proved beyond doubt that coal was composed of the compressed remains of land plants. Stutzer (1940), in his book The Origin of Coal, discusses the history of this understanding and the role played by coal balls in its development.

Simultaneously in Great Britain, the science of stratigraphy had its beginning, and the Carboniferous Period was named for its bountiful coal deposits. Economic demand for coal, metallic ores, and other mineral deposits drove the study of geology. Coal balls were first observed and described in mines, and recognition of their contained fossil record and significance to the origin of coal came from specimens collected in mines.

Without coal mining, coal balls would remain geologic curiosities, known from rare chance encounters in stream banks and highway cuts. The mass collecting of coal balls in active underground and surface coal mines enabled the research reported here. Yet today, as the Coal Age of industrial history winds down, opportunities for future collection of coal balls become tightly constrained.

Because of the discovery of coal balls and their intensive study for over 170 years, the tropical world of the Pennsylvanian Coal Age is one of the best understood times in Earth’s history. As a result of these studies, we now know that more than 300 million

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years ago the Earth was in the midst of a long and complex ice age. The dominant plants in the wetlands of this time were almost completely different from those in similar habitats today, and included tree lycopsids, marattialean tree ferns, extinct groups of seed plants, and giant horsetaillike sphenopsids, all in important ecological roles. The diversity of plants in these tropical lowland habitats was less than in the modern tropics; species pools were composed of a few hundred species at most rather than the thousands typical of modern tropical rain forests. Yet this smaller cohort of species created ecosystems that persisted in composition and structure for millions of years, and in the face of periodic fluctuations in climate and sea level brought about by the waxing and waning of high-­latitude glaciers. Giant insects, amphibians, and food webs based on detritivory, as much as or more than herbivory, combined with the plants to create an unfamiliar biosphere, the strangeness of which museum dioramas and artistic reconstructions have endeavored to capture. All in all, therefore, this was a world much like our own in some ways, yet profoundly foreign in others, and a compelling example of how long-­extinct organisms lacking modern analogues could be organized into ecosystems of nearly modern complexity.

TIME AND PLACE

The Pennsylvanian Subperiod of the Carboniferous (Figure 1) encompasses a time interval, by current estimations, of about 33 million years, approximately 323.4 to 289.9 million years before the present day (Lucas et al., 2022). Physical conditions similar to those of the Pennsylvanian began to appear in the latter part of the Mississippian, extending the Coal Age back another seven million years or so (e.g., Gastaldo et al., 2009). During this time, the Earth’s continental configuration was very different from that of today, with the continents in the final phases of aggregating into the Pangean supercontinent (Lucas et al., 2022), a continent that straddled the equator and across which there was a central mountainous region (Hatcher, 2002, 2010; Kroner et al., 2008) known as the Variscan-­Appalachian belt (Figure 2). The classic wetland regions of the Carboniferous were concentrated in the eastern and central United States, Canada’s Maritime Provinces, the British Isles, France, Belgium, Germany, Poland, the Czech Republic, Ukraine, and Russia. Farther to the west in Pangea, there were few coals, and wetland-­plant fossils are often or generally intermixed with drought-­tolerant taxa, indicative of conditions that were drier and more seasonal than in more central regions of the Pangean tropical belt (Bashforth et al., 2021). On the other side of central Pangea, to the east, tropical peat-­forming wetland vegetation persisted into and throughout the Permian (Li et al., 2018). Wetland vegetation in eastern Pangea, also known as Cathaysia, differed in some ways from that typical of central Pangea, but nonetheless retained many similarities, including taxa that had largely disappeared from Euramerica at the end of the Middle Pennsylvanian (e.g., Wang, 2004; Hilton and Cleal, 2007; Zhou et al., 2008; Seyfullah et al., 2009; Wang et al., 2009, 2012).

FIGURE 1. Stratigraphic subdivisions of the late Paleozoic as used in this report. Terms used in the United States are compared with those of western Europe and the global standard. We have not shown the Cantabrian Stage in the western European terminology; it has been proposed to cross the Middle–Upper Pennsylvanian boundary (*).

Despite disputes, we take the position that the Cantabrian (not shown) cannot be correlated outside northern Spain, where it was originally established (Nelson et al., 2023).

The land surface of the Pennsylvanian differed greatly from our modern world in numerous ways. Perhaps most obvious were the arrangement of its continents and the height of those continents above sea level (e.g., Blakey, 2003). The tropical land surface of Pangea was complex, including actively rising mountain

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FIGURE 2. Pangea: Pennsylvanian paleogeography showing the major continental regions, the central Pangean mountainous area, and the major oceanic areas. Recent research suggests that the southern polar region had multiple noncontinuous ice centers during episodes of glacial maxima. Map created by Ron Blakey, used with permission.

ranges such as the Ancestral Rocky Mountains in the western and central United States (McBride and Nelson, 1999). However, much of the continental surface comprised broad, flat coastal plains that bordered shallow epicontinental seaways, from which the sea rapidly and repeatedly advanced and retreated, a pattern unfamiliar to us today (e.g., Archer and Kvale, 1993; Archer et al., 2016). Thus, large areas of the Pennsylvanian lowlands alternated between exposed, fully terrestrial conditions and flooded, fully marine environments. Driven by the waxing and waning of glaciers, these sea-­level oscillations took place in concert with climate changes in the equatorial regions: changes in both rainfall patterns (e.g., Cecil et al., 2014) and temperature (Matthaeus et al., 2021). It was during such glacial intervals, when the land surface was widely exposed, that the continental surface in the tropics was covered by peat-­forming wetlands (Figure 3) (Cecil et al., 2003, 2014; Greb et al., 2003). These peat beds were the precursors of coal beds, which gave the Pennsylvanian the name of Coal Age (e.g., Binder, 1974).

THE LATE PALEOZOIC ICE AGE

The time interval from the Late Devonian through the early Permian encompasses the Late Paleozoic Ice Age (LPIA) (Isaacson et al., 2008; Isbell et al., 2008; Brezinski et al., 2010; Montañez

and Poulsen, 2013; Montañez, 2022). For more than 60 million years the late Paleozoic world was in the on-­and-­off grip of a so-­ called icehouse climate, with large glaciers in the southern polar regions and mountain glaciers in many areas (Isbell et al., 2008; Fielding et al., 2008; Henry et al., 2008). The LPIA had many parallels with the Pleistocene “ice ages,” including multiple, periodic glacial–interglacial fluctuations and accompanying fluctuations in sea level and climate through several million years (Cecil et al., 2003, 2014; Heckel et al., 2007; Greb et al., 2008; DiMichele et al., 2009; Eros et al., 2012; Montañez and Poulsen, 2013; van den Belt et al., 2015). Because it lasted much longer than the Pleistocene–Holocene, the LPIA encompassed numerous glacial–interglacial cycles, covering a broad spectrum of intensities. This dynamic alternation of glacial and interglacial conditions, on time scales of Milankovitch frequency (Heckel et al., 2007; Eros et al., 2012; van den Belt et al., 2015; Montañez, 2022), was punctuated by periods of warming, during which glaciers almost entirely disappeared, and by periods of intense cooling, during which large volumes of ice formed and the sea was withdrawn from much of the continental surface (Fielding et al. 2008; Heckel, 2008; Rygel et al., 2008). This was the last time prior to the Quaternary icehouse that the Earth experienced such climate dynamics over an extended period of time (Gastaldo et al., 1996).

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FIGURE 3. Pennsylvanian peat-­forming swamp reconstructions. Top: Late Pennsylvanian, tree-­fern-­dominated swamp with subdominant medullosan pteridosperms, calamitalean sphenopsids, and sigillarian lycopsids. Bottom: Middle Pennsylvanian, lycopsid-­dominated swamp with subdominant marattialean tree ferns, medullosan pteridosperms, and calamitalean sphenopsids. Artist: Mary Parrish, Smithsonian Institution.

PEAT-­FORMING SWAMPS

Tropical peat-­forming swamps are the focus of this and subsequent parts of this treatise. They characterize the Pennsylvanian time period in Euramerica and, as coal, are one of the dominant features preserved in the geological record of the terrestrial land surface (Averitt and Berryhill, 1950; Phillips and Peppers, 1984).

They were part of a larger lowland landscape characterized by a diversity of wetland habitats, best known from the extensive, and long-­studied, record of compression and impression plant fossils (together called adpressions by Shute and Cleal, 1987) preserved primarily in siliciclastic rocks. We will mention, illustrate, and refer to adpressions occasionally, to the extent that they illuminate studies of the peat-­forming vegetation, which is known, if imperfectly due to taphonomic factors, from the fossils found in coal itself (coal balls). The wetlands described here were concentrated in the central and west-­central portions of Pangea, today’s Europe and eastern North America (Figure 2). The plants that dominated the swamps (Phillips et al., 1985; DiMichele and

Phillips, 1994, 1996a; Montañez, 2016) and the broader tropical wetlands landscape (e.g., Blake et al., 2002; Wagner and Álvarez-­ Vázquez, 2010; Cleal et al., 2012; Opluštil et al., 2016, 2022) changed over time; many of the principal lineages persisted into the Permian in eastern Pangea/Cathaysia (Hilton and Cleal, 2007; Seyfullah et al., 2009; Wang et al., 2012; Opluštil et al., 2022). It is the peat accumulations, however, that are the signature of high-­rainfall climates with short or even no significant seasonality. These are climatic conditions described as humid and perhumid (sensu Cecil, 2003), where rainfall exceeds evapotranspiration for 10 months or more of the year.

The flora of Pennsylvanian tropical wetlands may be the best known of the pre-­Neogene terrestrial geological record.

This knowledge has been made possible primarily by the preservation, in abundance and high fidelity, of multiple kinds of plant remains in the siliciclastic rocks associated with coal beds, as well as in the coal beds themselves (Figure 4). The most widely and intensively studied of these have been the adpressed remains of compressions and impressions found in shales

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FIGURE 4. Forms of preservation. (a) Coal balls. A mass including several discrete layers (zones) from the Murphysboro-­ equivalent coal bed of Indiana. (b) Adpression fossil from the nonmarine roof shale of the Corinth Coal, Illinois.

Macro­neuropteris scheuchzeri, showing both compression preservation (C + arrow) of organic matter and impression preservation (I + arrow) where organic matter has flaked off the surface of the rock. USNM specimen 698097; also illustrated in DiMichele et al. (2023, fig. 10A). Scale bar = 1 cm. (c) Adpression fossil preserved in a siderite nodule from the marine roof shale of the Henryetta Coal, Oklahoma. Macroneuropteris scheuchzeri impression preservation. USNM specimen 546962; also illustrated in Moore et al. (2013: pl. VI.4). Scale bar = 1 cm. (d) Calamitalean spore Calamospora from the Corinth Coal, Illinois, specimen also illustrated in DiMichele et al. (2023a: fig. 20K). (e) Cordaitalean pollen Florinites from the Corinth Coal, Illinois; specimen also illustrated in DiMichele et al. (2023a: fig. 20N). Scale bar in (e) = 50 µm, also applied to (d).

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overlying, and sometimes immediately below, coal beds. The literature on these floras is vast. “Effective publication” begins in the early 1800s with Sternberg’s (1820–38) Flora der Vorwelt.

Since that time, the Pennsylvanian flora has been globally characterized, and detailed analyses of spatiotemporal patterns have become possible due to the density of the published data and extent of publicly accessible collections (e.g., Pfefferkorn and Thomson, 1982; Thomas, 2007; Cleal, 2008; Cleal et al., 2012; Opluštil et al., 2016, 2022; DiMichele et al., 2017a; Schneider et al., 2020).

The compression–impression record of Pennsylvanian wetland plants has been greatly supplemented by the presence of the unique concretions known as “coal balls” (Figure 4a) present in many Pennsylvanian-­age coal beds. Coal balls are deposits of mineral matter, mainly calcium carbonate, sometimes silica or other minerals, that formed within the peat body at various stages prior to its conversion to coal. The timing of their formation is uncertain. The quality of peat preservation varies from early in the process of litter conversion to peat to much later, when decay, shrinkage, compaction, and chemical changes had greatly altered the original plant litter. Coal balls preserve the original peat fabric in a permineralized, three-­dimensional state, and thereby allow study of anatomical detail of the plant remains. Once recognized, over 170 years ago (Hooker and Binney, 1855; Binney, 1862), this form of preservation became a mainstay of paleobotanical systematic and morphological research because of the often exquisite preservation of the plant tissues (Figure 5), including fine cellular detail (e.g., Williamson, 1871). Also informing the record of Pennsylvanian plants are numerous investigations of the spore and pollen composition of coal beds (Figure 4d,e), including major summaries aimed at stratigraphic correlation (e.g., Butter­ worth and Smith, 1976; Peppers, 1996; Eble, 1996) and those more focused on paleoecology (e.g., Smith, 1962; Corsin et al., 1967; Eble and Grady, 1990; Willard, 1993; Jasper et al., 2010), the latter often integrating geochemical and petrographic data.

The combined information from coal balls, coal palynology, and adpression fossils has created a record of Pennsylvanian wetlands unrivaled in the geological history of plants.

COAL BALLS: THE ORIGINAL PEAT FABRIC The main foci of this study are the plants and the processes contributing to the formation of Pennsylvanian-­ age tropical peat, the precursor of coal. Coal balls are, therefore, the form of fossil preservation on which this understanding will be based (Figure 6). The occurrence of coal balls within coal beds means that they record the plants that lived in peat-­forming swamps, minimally transported from their site or environment of growth.

In contrast, the plant remains that are preserved as adpressions commonly have been transported away from their sites of growth, although plant remains preserved in volcanic-­ash beds within coals are a notable exception (e.g., Opluštil et al., 2014). Coal balls, therefore, provide insights into the botanical composition

of coal, the various organ and tissue types that contributed to Pennsylvanian peats, and indications of the dynamics of the peat-­forming process, such as decay, compaction, and rooting.

This potential was recognized long ago (Schopf, 1939), but years passed before coal balls were examined specifically as indicators of the parent material of coal (Phillips et al., 1977). Since that time, there have been additional coal ball studies focused on peat diagenesis and the processes attendant on the transition of organic matter from plant litter to coal (e.g., Anderson et al., 1980; Hatcher et al., 1982; Lyons et al., 1984, 1985; Winston, 1986; Raymond, 1987; Covington and Raymond, 1989; Phillips and DiMichele, 1990; DiMichele and Phillips, 1994; Zodrow et al., 1996; Boyce et al., 2010; Raymond et al., 2012; Siewers and Phillips, 2015), but these are relatively few in number compared to the hundreds of papers focused on morphology and taxonomy of coal ball plants.

Although coal balls reveal aspects of the original peat composition of Pennsylvanian coal, they are less reliable as indicators of the standing vegetation from which that peat was derived. It is important to recognize that numerous factors affected the extent to which coal ball peat reflects the standing vegetation, including patterns and causes of litter production as well as physical and chemical processes acting on that litter.

Thus, numerous interpretive filters must be applied when attempting to use coal balls (e.g., Phillips and DiMichele, 1981), palynology (e.g., Eble and Grady, 1990), or coal petrography (Diessel, 1982) to infer parent vegetation. For understanding the transition from parent vegetation to peat, however, coal balls are unrivaled because they capture a broad spectrum of the states between litter and coal, and these states provide insights into the process of peat diagenesis.

Coal balls preserve a range of conditions in the peat-­ formation process, various stages of the conversion of aerial litter and roots to peat. They also vary in their position and density within a coal bed and have shapes and sizes that vary and often are characteristic of a particular coal bed. Their spatial distributions within a coal bed (Figure 7) provide insights into the microenvironments of the original peat swamp, as do aspects of their geochemistry and petrography (e.g., Mamay and Yochelson, 1962; Anderson et al., 1980; DeMaris et al., 1983; Zodrow et al., 1996; Raymond et al., 2012; Siewers and Phillips, 2015).

Based on studies of modern peat-­accumulating environments, diagenesis of organic matter in a peat body during and after accumulation can progress at different rates and is affected by a variety of factors. Acting simultaneously, these factors include decay, shrinkage, autocompaction, rooting by the vegetation growing actively on the peat surface, and a changing chemical environment as diagenesis proceeds. Furthermore, not all plant tissues and organs are affected by or respond to these factors in a similar manner or at the same rate. Coal balls indicate that the Pennsylvanian peat-­formation process was highly dynamic, as is the formation of peat today, and encompasses various effects that are difficult to tease apart or generalize.

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FIGURE 5. Coal ball tissue preservation. (a) Lyginopteris olhamia, thin section of stem showing pith, xylem cylinder, and surrounding cortical tissues. LY = Lepidophylloides, leaves of arboreous lycopsids transverse sections. USNM 775806. (b) Magnification of part of image (a), illustrating cellular detail of wood, pith, and some of the extraxylary tissues. (c) Marattialean tree fern roots from the outer root mantle. These roots are free and develop outside the dense inner root mantle. (d) Magnification of part of image (c) showing stelar xylem, trabecular cortical tissue (the root is mostly air spaces), and origin of a lateral root (arrow). Scale bars = 1 cm.

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FIGURE 6. Coal balls. (a) Coal balls in situ. Galatia Mine, Herrin Coal, Illinois. Coal bed is approximately 2 m thick. (b) A small, extracted mass of coal balls in coal, cut and etched (see text for details). Maple Grove Mine, Murphysboro-­equivalent coal, Indiana.

FIGURE 7. Map of coal ball occurrences within the Herrin Coal in a portion of Old Ben Coal Company Mine No. 24.

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CAVEAT: MOST COAL BALL PEAT IS NOT WELL PRESERVED

There is a general assumption among those who read or make use of the coal ball literature that Pennsylvanian coal balls offer abundant, exquisitely preserved specimens for study. To the contrary, as known to those who work firsthand with them from collection through processing to study and publication of findings, most coal balls preserve peat in various highly decayed and compacted states, often so intensely rooted and decayed that ­little remains but roots and the most decay-­resistant aerial tissues. The misunderstanding of the quality of coal ball preservation has been mitigated to some extent by paleoecological studies that often publish images of entire coal balls (e.g., Phillips and DiMichele, 1998; Willard et al., 2007) rather than only specific plants of interest. Such images demonstrate the great range of preservational states. We focus here on the full range of preservational states and hope to demonstrate the processes involved in formation of Pennsylvanian peats and the interactions of those processes.

COAL BALL FORMATION IN BRIEF

Coal balls are a type of concretion that developed in peat before the process of coalification began (Scott et al., 1996).

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They entomb organic matter in a mineral matrix and that matter then remains in the same structural state as it was at the time of coal ball formation, terminating the processes of decay and compaction (Figure 8). The process of chemical alteration of the entombed organic matter appears to proceed, however, and the remains of plant organic material in coal balls, where examined, have proven to be coalified to a rank similar to the surrounding coal (Hatcher et al., 1982; Lyons et al., 1984, 1985).

Most coal balls are composed of carbonate minerals, chiefly calcite (CaCO3) and dolomite (CaMg(CO3)2), although silica permineralized coal balls are found rarely. Carbonate precipitation requires alkaline conditions (pH > 8.0) in those areas of the swamps where coal balls formed; carbonate precipitation may have been bacterially mediated (Hammes and Verstraete, 2002).

The requirement for low to moderate acidity in order for coal balls to form also suggests formation in planar (topogenous, eutrophic) peat bodies because ombrotrophic or “domed” peat bodies generally have low pH/high acidity (Andriesse, 1988), which would not have been conducive to carbonate precipitation.

Various lines of evidence indicate that carbonate coal balls generally were initiated as multiple nucleation points at one or more layers in the peat body, and that these areas of nucleation coalesced to form larger concretions (Figure 6b). Furthermore,

FIGURE 8. “Normal” coal ball preserved within organic matter that has continued to decay to a lignitic state. Charcoal, although subject to crushing during compaction, is decay resistant. A layer visible in the coal ball is likely traceable into the lignitic material.

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the quality of peat preservation within some coal balls indicates that carbonate precipitation occurred in multiple discrete intervals, with decay and shrinkage (devolatilization) of the organic matter continuing between these intervals in the peat surrounding the coal balls (Figure 9).

Coal balls occur at various levels in a coal bed and may be found as isolated individuals, as clusters, and as intergrown masses (Figure 6a) that may extend for tens of meters laterally.

These masses also may extend through the entire thickness of the coal or be localized to a portion of the bed. Within the masses, individual coal ball layers remain recognizable, often separated by layers of coal millimeters to centimeters in thickness. Within the individual layers the coal ball shape, preservational state of the entombed peat, and plant composition of the peat tend to be distinctive. These features reflect variation in the preservation of peat from one layer to another. At a still larger spatial scale, coal ball masses, where they have been mapped, may occur in a series of subparallel, semilinear patterns. The origin of such linear patterns is not fully understood.

As recognized by the earliest authors (e.g., Hooker and Binney, 1855; Stopes and Watson, 1909), coal balls occur almost exclusively in coal directly beneath rocks that contain marine fossils, indicating that marine water had come into direct contact with the peat body in those places where coal balls are present. Such strata are, for the most part, black fissile shales or limestones. Where the coal bed is covered by nonmarine rocks, coal balls are rarely, if ever, found. These nonmarine strata often are found in areas close to ancient river channels where overbank muds from rivers buried the swamp as sea level rose and the rivers were converted to estuaries flanked by tidal flats (e.g., Elrick et al., 2017a; Nelson et al., 2020). In areas where such overbank muds were eroded, often in narrow areas carved out by tidal channels, marine rocks may lie directly on the coal bed (Greb et al., 1999; DeMaris, 2000); it is in these areas that coal balls may occur, emphasizing the relationship between coal balls and marine strata in direct contact with the coal bed.

GEOLOGY OF AMERICAN COALFIELDS In order to provide context for the discussions that follow, a brief review of the geology of Pennsylvanian-­age coalfields of the United States is in order. Although coal balls are known from Pennsylvanian-­age strata in several areas of Great Britain, continental western Europe, and the Donets Basin (Mackowsky, 1968), and from the Permian strata of the Kuznetsk Basin and several areas in China, most of the coal balls discussed in this book were collected in American coalfields.

There are three major Carboniferous coalfields in the eastern and central United States (Figure 10). From east to west, these are the (1) Appalachian Basin, (2) Illinois Basin, and (3) Midcontinent. Although the coal deposits were originally continuous, uplift and erosion have reduced them to these three distinct geographic and geologic regions. To begin, the chronostratigraphic subdivisions of Pennsylvanian time used by American geologists are explained.

AMERICAN CHRONOSTRATIGRAPHY

Three Carboniferous time scales are referred to here (Figure 1). Although the global and western European time scales for the Carboniferous series and stages are widely used, including in the United States, those developed in the United States also are widely employed. There are two U.S. schemes. One, the Midcontinent provincial time scale, is shown in Figure 1 along with the global and western European scales. Less commonly used is the U.S. Geological Survey (USGS) time scale, which​.is mostly restricted to the Appalachian coalfield.

The Pennsylvanian portion of the Midcontinent time scale currently has five divisions, ranked as stages (Figure 1). The respective type areas, from oldest to most recent, are: Morrowan Stage, northwestern Arkansas; Atokan Stage, southeastern Oklahoma; Desmoinesian Stage, central to southeastern Iowa; Missourian Stage, near Kansas City, Missouri; and Virgilian Stage,

FIGURE 9. (Opposite) Cross section of a coal ball aggregate demonstrating multiple episodes of permineralization between which peat decay and compaction continued and increased. Compaction, or autocompression, of the peat occurs in concert with, and is likely driven by, decay of organic matter. Decay is particularly intense in the litter layer and in the upper layers of the peat, the acrotelm, through which the water table fluctuates and is aerobic. Deeper in the peat, in the permanently water-­saturated catotelm, decay may continue at a reduced pace but compaction becomes more severe. This specimen illustrates various degrees of decay and compaction, parts of the organic matter in a nearly lignitic state prior to permineralization. Secondarily, this specimen illustrates the type of poor preservation state of the peat found in many coal balls. University of Illinois Coal Ball Archival Sheet 0007, Peabody Northern Illinois Mine, Pit 14, South Wilmington, Illinois. Houchin Creek Coal (Summum Coal in older literature).

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FIGURE 10. Map of the United States showing coalfields of Pennsylvanian age and their relationship to major tectonic features.

northeastern Kansas. Boundaries of these stages are defined principally on fusulinid biostratigraphy, increasingly supplemented in recent years by conodont biostratigraphy. As for series, the Lower Pennsylvanian coincides with the Morrowan Stage, the Middle with Atokan and Desmoinesian Stages, and the Upper with the Missourian and Virgilian Stages.

The USGS time scale has Lower, Middle, and Upper Pennsylvanian Series; no stages are recognized. Boundaries of the USGS series do not coincide with the Midcontinent series.

According to Bradley (1956) and Peppers (1996), the USGS boundary between Lower and Middle Series is the top of the New River Formation in West Virginia, and the Middle–Upper boundary is the contact between the Allegheny and Conemaugh Formations. Using these definitions, the USGS series are not chronostratigraphic units, they are lithostratigraphic. Therefore, we avoid using the USGS series as time units and mention them in historical context only.

APPALACHIAN COALFIELD

The Appalachian coalfield occupies foreland basins on the northwest side of the Appalachian Mountains in the eastern United States (Figure 11). The coal deposits of the Canadian Maritime Provinces and in New England are a northeastern continuation of the Appalachians; however, on the eastern flank of the mountains, geological and paleobiological affinities are more closely aligned with Europe than with the United States.

To the south, the Appalachian Mountains and their coal-­bearing rocks disappear beneath the Gulf Coastal Plain, reemerging as the Ouachita Mountains in the southern Midcontinent region.

Mountains and basins are products of several successive Paleozoic orogenies culminating in the Alleghenian orogeny of Pennsylvanian and Permian time. The Alleghenian orogeny was continuous with the Variscan or Hercynian orogeny of Europe prior to the breakup of Pangea and formation of the Atlantic

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FIGURE 11. Map of the Appalachian coalfield and sub-­basins. Marked cross sections in Kentucky–­ Virginia and Pennsylvania are shown in Figures 12 and 13, respectively.

Ocean beginning during the Triassic Period. Multiple similarities between the coal measures of the Appalachian Basin and those of northern Europe, therefore, are evident.

The present Appalachian Mountains began to rise near the end of Mississippian time. As this happened, the flanking foreland basin subsided most strongly along its southeastern margin, adjacent to the mountains. The three areas of greatest subsidence are (1) Black Warrior Basin in Alabama, (2) Pocahontas Basin in southern West Virginia and westernmost Virginia, and (3) Anthracite coalfields of eastern Pennsylvania (Figure 11). Sedimentation was essentially continuous across the mid-­Carboniferous boundary in these three deep basins, where major coal deposits accumulated during the Morrowan and Atokan Ages (Englund, 1974; Englund et al., 1986; Greb et al., 2009). By contrast, in distal portions of the Appalachian foreland basin sedimentation was much slower.

The mid-­Carboniferous boundary is a major unconformity, and Morrowan–Atokan rocks wedge out against it (Figures 12, 13).

The triangular Black Warrior Basin developed near the right-­angle juncture of the Alleghenian and Ouachita orogenic

belts (Figure 10). Rapid subsidence commenced during the Late Mississippian and continued at least into Atokan time; younger Pennsylvanian rocks are eroded. This basin contains the thickest Morrowan succession in North America, more than 2,500 m thick. Coal-­bearing Morrowan and Atokan rocks were deposited in fluvial, alluvial, deltaic, and marginal marine settings (Pashin and Gastaldo, 2009). Coal balls have been collected at a single locality: the Cedrum Mine in Walker County, Alabama (Winston and Phillips, 1991).

The Pocahontas Basin occupies a narrow strip parallel to the Appalachian Mountain front. Sedimentation apparently was continuous in the deep, proximal part of the basin. Toward the northwest, the mid-­Carboniferous boundary becomes an angular unconformity, and older Pennsylvanian units successively wedge out against this surface (Figure 12). Maximum Pennsylvanian thickness is more than 1,500 m with the top eroded. Sedimentary environments were similar to those in the Black Warrior Basin (Greb et al., 2009). No coal balls have been reported from this region.

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FIGURE 12. Cross section of the Pocahontas Basin with the mid-­Carboniferous boundary horizontal. Line of section is shown in Kentucky and Virginia in Figure 11. Modified from Greb and Chesnut (2009). Abbreviations: Ck. = Creek, Fm. = Formation, L. = Lower, Ls. = Limestone, Miss. = Mississippian, Penn. = Pennsylvanian, Pt. = Point, Sh. = Shale, Ss. = Sandstone, U = Upper.

The Pennsylvania Anthracite fields occupy a series of syn­ clines (Figures 11, 13). Although coal is present in rocks ranging from Morrowan to Virgilian age, most of the thickest and most extensive coal beds are of Desmoinesian age. Peat in this region accumulated on alluvial and deltaic plains that experienced occasional marine incursions and were coalified during the Alleghenian orogeny mainly during the Permian Period (Wood et al., 1986; Greb et al., 2009). No coal balls from the Anthracite region have come to our attention.

Morrowan and Atokan coal beds in the Appalachian field are locally thick and generally high quality, but continuity is limited and correlations uncertain. Many units are named as coal zones rather than discrete coal beds or members. Desmoinesian coal beds are mined extensively in Ohio, Pennsylvania, and northern West Virginia. Several beds, notably the Lower, Middle, and Upper Kittanning and Lower and Upper Freeport, have wide lateral con­ tinuity and have been correlated into the Illinois and Midcontinent

coalfields with confidence (Peppers, 1996; Heckel, 2013). Although uncommon, coal balls have been collected from several Atokan and Desmoinesian coals in the Central Appalachian Basin of Atokan and Desmoinesian age in Tennessee, eastern Kentucky, Ohio, and Pennsylvania (Cross, 1967; Taylor and Smoot, 1985; Feng, 1989; Phillips and Cross, 1995; Greb et al., 1999).

Missourian and lower Virgilian rocks of the Appalachian Basin contain only a few thin coal beds of limited extent, whereas thick sandstone units and variegated claystone (paleosols) are prevalent in these rocks. This “barren” interval is attributed to an overall drier climate, during both glacial and interglacial phases of such cycles, contrasting with the wetter regimes during Early and Middle Pennsylvanian time (Phillips and Peppers, 1984; DiMichele et al., 1985; Cecil et al., 1985). Coal balls have been reported and some collected from coals of this age, primarily in Ohio (Rothwell, 1976, 1988; Pryor, 1996). Periodically humid conditions returned later in the Virgilian, culminating in the most

FIGURE 13. Cross section from northwestern Pennsylvania to the Anthracite region. Line of section is shown in Figure 11. Modified from Greb and Chesnut (2009). Cgl./cgl. = Conglomerate, Fm. = Formation, Gp. = Group, Miss. = Mississippian, Mbr. = Member, Mt./Mtn. = Mount/Mountain, Penn. = Pennsylvanian, Ss./ss. = Sandstone.

extensively mined coal bed in the United States, the Pittsburgh Coal. Over 18 billion short tons (16.3 billion metric tons) have been extracted from the Pittsburgh Coal bed (Greb et al., 2009).

So far as is known, the Pittsburgh Coal is devoid of coal balls, the only documented mineralized plant matter being upright, silicified trunks of Psaronius at a locality near Athens, Ohio (Cross, 1952). Other upper Virgilian coal beds, such as the Redstone and Sewickley, also are important economically. Lower Permian rocks are present in the Dunkard Basin (Figure 11) (Cecil et al., 2013), a broad, shallow depression centered southwest of Pittsburgh.

Permian coal is thin, sporadic, and suitable for local mining only.

Sedimentary environments in the Appalachian coalfield pro­ gressed from piedmont on the southeast through coastal plain to marginal marine (e.g., deltas, estuaries, and bays) on the

northwest. Distance from the coastline diminished the occurrence of marine rocks and rapid influx of terrigenous detritus from the rising mountains further suppressed carbonate precipitation, so limestone layers are few and sandstone widely prevalent.

Although eustatic processes were operative, their cyclic signal is muted and commonly overwhelmed by tectonic and autocyclic processes. Only major marine incursions are reflected in the Appalachian sedimentary record. In combination with climatic effects, the relative rarity of marine incursions contributed to the lower sulfur content of Appalachian coals, particularly those of Early and early Middle Pennsylvanian age, in comparison to those of the Illinois and Midcontinent regions. The scarcity of marine strata is consistent with the rarity of coal balls. Thus, most Appalachian coal ball occurrences are in the later Middle

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Pennsylvanian and early Late Pennsylvanian and are associated with marine strata.

ILLINOIS BASIN COALFIELD

Encompassing parts of Illinois, Indiana, and Kentucky (Figure 14), the Illinois Basin is an interior cratonic (or intracratonic) basin situated in the interior of the North American craton remote from orogenic belts. The basin developed incrementally throughout Paleozoic time in response to stresses transmitted from distant plate collisions. A Cambrian failed rift beneath the southern part of the basin served as a locus of subsidence (Kolata and Nelson, 2020). The Alleghenian orogeny during Pennsylvanian time induced renewed subsidence and also served as one of several sources for clastic sediments. Simultaneously, the Ancestral Rocky Mountains (ARM) orogeny, induced by plate collisions far to the southwest, reactivated faults in Precambrian basement that propagated upward through sedimentary cover as anticlines, domes, and monoclines (McBride and Nelson, 1999).

The principal Pennsylvanian sediment source to the Illinois Basin lay to the northeast on the Canadian Shield and northern Alleghenian orogenic belt (Potter and Pryor, 1961; Nelson and Jacobson, 2010). Recent research on zircons suggests dispersal of sediment from the Appalachian orogen through the Appalachian Basin (Thomas et al., 2020). Low positive areas closer to the Illinois Basin, such as the Ozark Dome and Transcontinental Arch,

were minor contributors (Nelson and Jacobson, 2010; Thomas et al., 2020). An epicontinental seaway stretched at times of sea-­ level highstand across the Midcontinent through the Illinois Basin toward the northern Appalachian Basin. Its shoreline fluctuated by hundreds of kilometers in response to eustatic sea-­ level changes, producing classic cyclothems (Langen­heim and Nelson, 1992; Cecil et al., 2014) in which marine shale and limestone overlie coal formed as peat in freshwater swamps. This setting was conducive to forming high-­sulfur coal and coal balls.

Most coal mined in the Illinois Basin is 1 to 3 m thick, a comfortable walking height above 2 m being the norm in underground mines, especially in Illinois (parts of western Indiana and western Kentucky also are part of the basin). The maximum thickness is about 4.5 m; coal as thin as 30 cm is extracted in multi-­bed surface operations. Reflecting the generally stable tectonic regime, many coal beds exhibit great lateral continuity and were correlated into the Midcontinent coalfield based on physical character alone before the advent of refined biostratigraphy.

Two beds, the Springfield and the Herrin, together account for more than 70% of mapped resources and more than 90% of all the coal mined to date in Illinois. Not surprisingly, concretions from the Springfield and the Herrin make up a large portion of the coal balls collected by scientists and are an important baseline in this study. Many other Illinois Basin coal beds are generally too thin to mine yet exhibit basinwide extent and are confidently correlated into the Midcontinent and Appalachian regions (Peppers, 1996; Heckel, 2013).

Morrowan rocks are confined to the southeastern part of the Illinois Basin; coal beds in this interval are thin and highly discontinuous. Atokan coals are somewhat thicker and less lenticular, supporting large surface mines on the eastern side of the basin in Indiana and western Kentucky. Lack of coal bed continuity in older rocks reflects ARM tectonic activity combined with rolling paleotopography that developed above the deeply incised mid-­Carboniferous unconformity (Bristol and Howard, 1971).

More than 90% of coal production has taken place in rocks of the Desmoinesian Stage, when favorable climate, basin infilling, and reduced ARM tectonic activity fostered the development of thick and laterally extensive peat deposits. Although laterally extensive, Missourian and Virgilian coals of the Illinois Basin rarely exceed 1 m thick and have been extracted only in relatively small surface mines. The drier and more seasonally variable Late Pennsylvanian climate was probably the main factor retarding peat development. Not surprisingly, the preponderance of coal ball collections from the Illinois Basin coalfield came from coal of Desmoinesian age with lesser, but still significant, numbers from coals of early Late Pennsylvanian (Missourian) age.

MIDCONTINENT COALFIELD

FIGURE 14. Map of the Illinois Basin coalfield.

The Midcontinent coalfield extends across parts of the states of Iowa, Nebraska, Missouri, Kansas, Arkansas, and Oklahoma (Figure 15). The small bituminous coal deposits of north central Texas also may be included. Coal mining in the Midcontinent

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FIGURE 15. Map of the Midcontinent coalfield.

peaked during the early and middle twentieth century and has declined significantly since. As the industry declines, with it goes the opportunity to collect coal balls.

Midcontinent coal deposits encompass several structurally distinct provinces (Figure 15). In Arkansas and southeastern Oklahoma, the Arkoma Basin bears the same relationship to the Ouachita orogenic belt as the Pocahontas Basin has to the Alleghenian (Appalachian) fold and thrust belt. The Ouachita, Appalachian, and Variscan orogenic belts originally were con­ tinuous. Coal in the Arkoma Basin is of early to middle Des­ moinesian age and formed from peat having little or no marine influence. The strata are strongly folded and coal rank increases from high-­ volatile bituminous on the west to semianthracite near the eastern terminus of the field (Trumbull, 1957; Haley, 1960). The most important bed economically is the Hartshorne, which resides near the base of the Desmoinesian and locally attains a thickness of about 2.5 m. Coal balls are known from the Oklahoma portion of the Arkoma Basin in Haskell and Pittsburg Counties. Other coal ball localities in northeastern Oklahoma (e.g., Craig County) are on the Northeast Okla­­­homa shelf.

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The Forest City Basin is a shallow interior cratonic basin that developed during the late Paleozoic (Anderson and Wells, 1968). It is less pronounced than the Illinois Basin. Surface bedrock is mostly Missourian and Virgilian age and includes coal beds that have been mined in the past (Pepperberg, 1910; Landis and Van Eck, 1965; Robertson and Smith, 1981). Older coal is probably present but has never been exploited.

The rest of the Midcontinent coalfield occupies broad, tectonically stable areas here termed the Iowa-­Missouri and Cherokee Platforms (Figure 15). Nearly all the coal that has been mined on the platforms is of Desmoinesian age and is ranked as high-­volatile bituminous. More than two-­thirds of historic production and mapped resources in Iowa belong to the lower Cherokee Group, of early Desmoinesian age (Landis and Van Eck, 1965). These beds are lenticular, difficult to correlate, and commonly exhibit trough or basin structure, having the thickest coal (locally more than 3 m) in structural lows, pinching out on the margins. Younger coals are more widely continuous but mostly thinner than 1 m. Most coal beds are directly overlain by marine shale or limestone, and coal balls have been reported at many localities but collected only sporadically.

In platform areas of Missouri, Kansas, and Oklahoma the lower Cherokee (lower Desmoinesian) coal beds tend to be thinner and more extensive than their counterparts in Iowa, but upper Cherokee and Marmaton Group coals (middle and upper Desmoinesian, respectively) accounted for a larger share of production (Trumbull, 1957; Brady et al., 1976; Robertson and Smith, 1981). Most of these coal beds are 75 cm or less in thickness and have been mined chiefly by surface mining. Most coal beds in the region are directly overlain by marine shale or limestone. Coal balls have been reported at many localities but only collected sporadically.

TEXAS COALFIELD

The bituminous coalfield of Texas (Figure 10) extends from the central to the north central part of the state and includes beds of Desmoinesian through late Virgilian age (Mapel, 1967; Finley et al., 1979). Thin beds (mostly less than 75 cm) have high ash and sulfur content, and are of low rank (borderline subbituminous), which retarded commercial development in a state having prolific oil and gas production (Evans, 1974).

Coal balls have been collected from the Newcastle coal bed at two sites in Young County; their age is close to the Pennsylvanian–Permian boundary. They also recently have been reported from the Dalton coal bed in Palo Pinto County, where they are of middle Missourian, early Late Pennsylvanian, age (Chrpa et al., 2023).

OTHER AREAS

Meta-­ anthracite or graphite of Pennsylvanian age exists in the Narragansett Basin of Rhode Island and Massachusetts in the northeastern United States. These deposits are analogous

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to those of eastern Pennsylvania but have undergone more intense metamorphism. The graphitic material was mined for fuel and metal smelting, most recently during World War II (Toenges et al., 1948).

Rocks of Pennsylvanian age are widely distributed in the western United States but are devoid of coal except locally. Coal occurs in small areas near the upper Pecos River in northern New Mexico (Figure 10). Coal beds there are in the Sandia Formation of Morrowan and Atokan age and locally exceed 1 m in thickness. They were mined for local use during the early twentieth century (Hoffman, 1996). Coal of Atokan age also has been reported from the Pinkerton Trail Formation of the San Juan Mountains in southwestern Colorado (Connelly and Gianinny, 2012). As Cecil et al. (2003) demonstrated, there was a pronounced climatic gradient, from humid and perhumid climates in the central Pangean tropics to arid and semiarid in the west, intensifying progressively during Pennsylvanian time. No coal balls have been reported from the coals in these Western areas, even though conditions should have been suitable. Their absence possibly reflects limited coal-­mining activities and thus limited exposures.

COAL BALL COLLECTION, PREPARATION, AND QUANTITATIVE ANALYSIS METHODS Markedly different from most other kinds of fossils, rocks, and minerals, coal balls require specialized techniques for collection, preparation, and analysis. In this section, we focus on the approaches used in each phase of coal ball study. The techniques described here have changed only a little since the initial discovery of these concretions in the mid-­1800s, although the specifics of the technology and the form of the materials used has evolved.

Similarly, most of the objectives of coal ball analysis have remained the same for the past 150 years: understanding the systematics, anatomy, and evolution of these early land plants. In the latter part of the twentieth century and into the twenty-­first, these studies have been extended to encompass the plant composition of Pennsylvanian coal and to examine the ecology of Pennsylvanian peat swamps quantitatively. As of this writing, the study of coal balls is at a comparatively low point in terms of the number of dedicated laboratories. The 1930s and early 1940s were similar but were followed by a renaissance during the 1950s through the 1990s. We anticipate a resurgence of interest in coal balls as studies of plant developmental biology, physiology, and chemistry (and as-­yet unimagined pursuits) begin to encompass a broader range of extinct plants, and indications of such interest have appeared recently in laboratories in the United States, Western Europe, and China.

COAL BALL OCCURRENCES IN THE FIELD

To find coal balls one must, for the most part, find coal—­ this is not always the case, because in rare instances coal balls

may have been eroded out of a coal bed in the deep past and redeposited. Most commonly, however, the most likely place to find coal balls is in a coal mine (Figure 16a,b). Natural outcrops and artificial exposures, such as railroad and highway cuts, also occasionally reveal coal balls (Figure 16c) (Rothwell, 1988; Willard et al., 2007). On the Pangean continent coal balls tend to be concentrated at certain places and certain times in the stratigraphic record—­so simply finding coal will not usually lead to the discovery of coal balls. However, where coal balls are found, they often occur in great numbers, to the point where they become an obstacle to mining.

The principal problem, then, in locating coal balls is to find people who are likely to hear about their occurrence and can arrange access to exposures from which they may be collected.

Making contact with geologists who study coal and visit coal mines, and staying in direct contact with the geological or engineering staff of those mines, are the best ways to learn of coal ball occurrences. Acquiring access to coal mines will vary with the company and depends heavily on scheduling a mine visit such that it does not interfere with coal production.

The University of Illinois, which was for many years a center for coal ball studies, is ideally situated to investigate American coal balls, being centrally located among the American coalfields of Pennsylvanian age. The state of Illinois is a major coal producer and, as a result, the Illinois State Geological Survey (ISGS) on the Urbana-­Champaign campus of the university has pursued intensive research on coal throughout its history. ISGS geolo­gists routinely visited active mines and maintained close ties with mine operators, who reported any condition, such as “rocks in the coal,” that interfered with coal extraction. The ISGS communicated with University of Illinois scientists studying coal balls, and with other state geological bureaus and federal agencies, such as the U.S. Geological Survey and Bureau of Mines, that were closely involved with the coal industry, yielding opportunities to collect coal balls outside Illinois.

Nevertheless, most coal ball discoveries were, and continue to be, made by serendipity. No one ever thoroughly and systematically searched a mining district or coalfield for coal balls. Had such searches been made, the number of known localities would have multiplied manyfold. As an example, the authors made a five-­day tour of surface mines in southeastern Kansas and northeastern Oklahoma in the summer of 1979. Ten mines were visited, and coal balls collected at five, nearly doubling the number of known localities for the two states. Among those collections was the only vertical profile of coal balls from the entire Midcontinent coalfield at that time. In 1979 there were approximately 180 active strip mines in Oklahoma alone (Friedman, 1982).

Coal balls in place, in the coal bed (Figures 16a, 17a), provide the most information of value to studies of peat formation and coal ball origins. In situ occurrences, if the specimens are carefully logged with regard to position in the bed, and position relative to one another, also are crucial to studies of paleoecology, allowing compositional changes through time to be assessed and related to position in the bed (such as in the color-­coded

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FIGURE 16. Exposure of coal and coal balls. (a) Coal balls in situ. Galatia Mine, Illinois, underground, Springfield Coal. (b) Coal balls in situ.

Creek Paum Mine, Illinois, surface exposure. Mount Rorah Coal. Note that coal balls replace most of the bed and thicken it. The mattock used for scale is 66 cm in length. (c) Stream exposure of the Calhoun Coal, Illinois. This coal is generally too thin to mine but it does have coal balls at other nearby exposures. The individuals standing next to the coal are 1.8–2 m in height.

zonation of coal balls in Figure 17a, preserving their relative position in the coal bed; this procedure is explained in more detail in Figure 34 and the associated text). Lithologically distinct layers, called “partings,” within the coal also allow distinct layers of coal balls to be identified and delimited. Such partings may represent floods (siliciclastic layers), fires (layers of charcoal), intervals of advanced peat decay (dull coal bands), or the introduction of marine water (layers containing marine invertebrates with or without siliciclastic sediment). Coal balls collected ex situ from mine spoil (waste material) areas are valuable, too, for various kinds of studies beyond those of plant morphology and systematics. In many instances, these are the only specimens available because coal balls in place are inaccessible. Additionally, “loose” coal balls are more likely to preserve the full shape and size of the specimen (Figure 17b). Sometimes coal ball masses are removed from the bed in large pods (Figure 17c), which subsequently weather and fall apart or may even be run through a coal crusher, in either instance producing loose coal balls. Being

harder and more resistant to weathering than the surrounding coal, coal balls may be set free of adherent coal matrix under the right conditions. In contrast, coal balls in situ can be challenging to remove in one piece, depending on how long the coalface has been exposed to weathering or subject to industrial activity.

COAL BALL COLLECTING: BACKGROUND

Until the advent of systematic in situ coal ball collection methods, the process of acquiring specimens for study was largely serendipitous and somewhat haphazard. A few researchers had field agents who scouted local mines and natural outcrops in search of petrifactions. For example, William Darrah’s associate F. O. Thompson acquired large collections of coal balls from the Shuler Mine west of Des Moines, Iowa, but may never have gone underground himself. As related by Phillips and Cross (1995: 328), “An official of the [Shuler] mine said he would fill a car with coal balls and have it pulled out by mules to save

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FIGURE 17. Various forms of coal ball exposures. (a) Coal balls in situ, prepared for collections, distinct layers marked by different colors of spray paint. Sahara Mine No. 6, Illinois, Herrin Coal. Coal bed is approximately 2 m thick. (b) Coal balls in spoil, removed from their in-­bed context. Peabody Northern Illinois Mine, Houchin Creek Coal. Hammer is 32–33 cm long. (c) Large, thick mass of coal balls removed from the coal bed, but retaining their original orientation, marked by intact roof strata at the top of mass. Peabody Cottage Grove Mine, Illinois, Herrin Coal. People in the photo are approximately 1.8 m tall.

carrying the stuff up” and noted further that Darrah himself wrote to T. L. Phillips, “I have photographs of Thompson’s back yard with a 5-­ton coal truck dumping a full load of coal balls.”

Such efforts spared the scientists the danger and arduous labor of going underground and hacking the concretions out of virgin coal. Other investigators visited tipples of active or abandoned mines and gleaned coal balls from the reject piles. Whereas coal

balls from any source are scientifically valuable, those obtained secondhand are divorced from context and provide only limited insights into the development of ancient peat deposits.

The technique of collection of coal balls in vertical succession rests upon finding coal ball concretions either still in place, within a coal bed, or as an intact mass that was bypassed during mining. The specimens must, of course, occur in multiple layers,

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FIGURE 18. Coal ball collecting in action demonstrating various tools. (a) Coal balls in stream exposure, Calhoun Coal, Illinois. In preparation for extraction, the coal ball zones have been painted distinctive colors. Note equipment and supplies: small pick mattock, long chisel, cans of spray paint, whisk brook, small crack hammer. (b) Tom Phillips breaking up a mass of coal balls in the spoil with a sledgehammer. Lemmon Brothers Mine, Indiana, Springfield Coal. (c) Collecting coal ball layers using a sledgehammer and chisel. Note handle (arrow) welded onto the chisel to protect the hands of a helper. Star Mine, Laddsdale coal zone, Iowa. (d) Using the pick-­mattock to extract coal balls. Another pick is shown in foreground. AMAX Chinook Mine, Survant Coal, Indiana.

which can be extracted systematically, as described below. The analysis of coal ball profiles provides the opportunity to examine the history of the peat-­forming swamp vegetation, and also the processes that acted upon the organic matter during the process of coalification.

Coal ball collecting is a multifaceted procedure. The collecting method depends on many factors, beginning with the accessibility of the coal balls—­are they in situ (Figures 16a,b; 17a; 18a) or present only in mine spoils? And if the latter, are coherent masses of coal balls, preserving original zonation, still intact (Figures 17c, 18b)? If coal balls are accessible at the coalface, or as masses in the spoils, it must be determined whether they are removable and separable as zones using hand tools, such as rock hammers, chisels, and small sledgehammers (Figure 18a), or if larger sledgehammers (Figure 18b,c), chisels (Figure 18c), and

picks (Figure 18d) are required. The collecting techniques also may shift as conditions change with excavation or as coal ball distribution dictates decisions on zonation.

Since the late 1960s most of our efforts have been focused on collecting profiles of in situ coal balls (Figures 17a, 18a), where possible, and supplementing these with “random samples” collected from spoils or the tipple area of a mine (Figure 18b). We found it useful to maintain spatial information for spoils collections, where conditions permit and where evidence suggests that the coal balls have been moved minimally. Coal ball composition can differ significantly from place to place within a single mine. Profiles of coal balls are crucial to determining the sequencing of both peat accumulation patterns and the formation of the coal balls themselves. Surprisingly, though, there are few illustrations of coal balls in situ (e.g., Phillips et

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FIGURE 19. Mine apparel and equipment. (a) Geologists dressed for winter work in a surface mine, mid-­2000s. Individuals shown are wearing hard hats, high visibility vests, gloves, metatarsal boots, and protective eyewear. (b) Geologist dressed for work in an underground mine, 1970s; individual is wearing less reflective material than is currently required. Note hard hat, head lamp, mine belt, boots with steel toes, and rock hammer. (c) Inactive area of a surface mine, 1970s, with geologists demonstrating one of the many uses for a long-­handled shovel, here addressing one of the many exigencies (deep mud!) that can be encountered in a surface mine.

al., 1976, figs. 1, 7, 8; Phillips and DiMichele, 1981, fig 7.4; Di­ Michele and Phillips, 1996b, fig. 1).

If collecting coal balls in a mine, certain equipment is generally required, some of which may be lent by the mine operator: a Mine Safety and Health Administration (MSHA) approved hard hat, steel-­toe (or steel-­toe and metatarsal-­plate) boots, eye protection, and high-­visibility clothing or a vest (Figure 19a). In addition, if working underground, additional equipment is required, such as a self-­rescuer, a cap lamp, and additional high-­ visibility clothing or reflective appliques (Figure 19b). Mining companies may require additional safety equipment, including such items as gloves, side shields for prescription glasses, and so on. In both surface and underground mines, special training is required to impart familiarity with hazards, traffic circulation patterns, blasting signals, in-­mine escape routes, airflow patterns, and the use of specialized emergency equipment, both personal and of a larger nature, located in the mine (Figure 20). It is a certainty that in order to enter a mine, accompaniment by mine personnel will be necessary. This requirement often constrains the

amount of time permitted in the mine and makes it imperative that the collecting work be done efficiently and quickly.

When collecting from spoils in a surface mine or on the surface at a deep mine, certain pieces of equipment are indispensable. These include rock hammers and 5–7 lb. (2.3 to 3.2 kg) crack hammers to trim and disaggregate coal balls. Larger, 9–16 lb. (4.1 to 7.3 kg) sledgehammers also may be useful in breaking up aggregates or to reduce the size of large specimens.

In some instances, chisels are useful; both small handheld chisels, which will be used by a single person, or larger chisels, which will be held by one person for another who is swinging a crack hammer or sledgehammer. We strongly recommend having handles (pieces of steel rebar of the type used to reinforce concrete) welded onto the upper parts of large chisels should the plan be to use them in a two-­person team (Figure 18c). Sturdy bags in which to carry and store the coal balls also are highly recommended; canvas bags are most suitable. Cardboard boxes are not recommended—­their construction is not strong enough and they are not water resistant. Labels should be affixed to each bag to

FIGURE 20. Views of mines. (a) Surface mine in Illinois, mining two coal beds, the Herrin (below) and Baker. The upper coal bed can be seen to warp downward and thicken into what was, at the time of formation, a shallow abandoned channel. Note the truncation of horizontally disposed strata on either side of the channel. Surface mines often provide excellent stratigraphic context for interpretation of the environments of peat/coal formation. (b) Dual entry portals to an underground mine. This is a drift mine, meaning that the mine portals are in the coal bed itself. A box cut down to the coal bed allows direct access. One entry is for human and equipment traffic, and the other for removal of the coal. (c) Entry to a shaft mine. Headframe supports a large elevator from the surface to the coal bed below. Persons in photo are geared up for underground work. (d) People gathered at the shaft bottom, about 165 m (550 feet) below the surface. Note the low-­profile vehicle for transporting people to sites in the interior of the mine. (e) In an underground mine. The white color of walls (“rib”) and ceiling (“roof”) results from spraying with rock dust, a suspension of powdered limestone, applied to dilute and cover coal dust and mitigate its explosive hazard. The height of the exposure is the thickness of the coal bed. In many areas of an underground mine, the roof strata are well exposed, permitting close study and collection of plant fossils. Furthermore, direct access to the coal is easy and safe, permitting detailed study of its physical characteristics.

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identify the location of the collection and any additional information that might accompany the collection (e.g., coal name and mine name).

If collecting from the coalface, in surface, underground, or on a natural outcrop, the same tools and supplies as described above are needed. However, in these instances, chisels are almost obligately required to remove coal balls. In addition, pickaxes or pick mattocks (Figure 18d) and pry bars may prove useful in undercutting the coal, for removing coal from around coal balls, or for prying coal balls out after they have been partially exposed. We recommend keeping long-­handled shovels in the toolkit, given that moving soil, mud, and general rock debris is often necessary (Figure 19c).

COLLECTING COAL BALL VERTICAL SECTIONS

Coal balls most often occur in layers of varying thickness and lateral extent (Figure 21a). They may be present through the entire thickness or in only part of a coal bed, and in extreme cases nearly the entire original peat bed may have been permineralized locally (e.g., Rothwell, 1976; Willard and Phillips, 1993).

Information on such layered distribution can be preserved when collecting is done directly from the coalface. Layering also may be preserved in spoils collections if coal balls have remained aggregated (Figures 16c, 18b); however, unless some of the roof rock remains attached, orientation will be lost.

The occurrences of coal balls within a coal bed tend to be podlike, meaning that any given occurrence is spatially limited, with an extent generally measurable in meters to perhaps tens of meters. Such pods of coal balls may have a predictable distribution throughout a coal bed, often being curvilinearly disposed (Figure 21b).

We refer to the vertical disposition of coal balls at any one natural or in-­mine outcrop as a “peat profile” or “vertical section” (VS). An individual layer of coal balls is referred to as a “zone.” In peat-­profile sampling, coal balls are collected zone by zone across a face about 1 m wide. Coal ball zones within a vertical section are delineated by continuity of the coal ball layers, distinctive benches or characteristics of the coal, mineral (siliciclastic) partings, charcoal (fusain) layers, and/or other distinctive markers visible on the coalface at the site of coal ball sampling (Figure 21a, note clastic partings as marker horizons separating some coal ball zones).

Once zones have been identified and delineated, we use the following procedures to collect specimens. First, we spray-­paint the individual zones different colors to make them visually separable (Figures 17a, 18a) and respray as collecting proceeds and coal balls are removed. As they are removed from the coal bed, the top–bottom orientation of each coal ball is identified and the upper surface is spray-­painted the color of the zone from which the coal ball was removed. These individual coal balls are the basic collection units that will be subject to processing and analysis, the results of which will be summarized by zone. The collected coal balls are sorted into piles by zone in a nearby area, away from the working face (see subsequent discussion). Once collecting is completed, the coal balls are packed into bags, which

are closed with twine, to which a label is attached (Figure 22a).

The color of the zone is spray-­painted along the outer edge of the label. Label information includes the name and number of the vertical profile (e.g., Sahara VS 4), the zone number, and the zone color (Figure 22). These bagged specimens are then carried to the transport vehicles and brought back to the processing center.

From this kind of detailed collecting, directly from the coalface, we can reconstruct a sequence of events that, allowing for gaps in coal ball formation, led to the various types of peat represented by the in situ coal ball concretions. Any one coal ball and, by extension, a coal ball zone, represents an indeterminate temporal interval of peat accumulation. Distinct time horizons are often recognizable within an individual coal ball or entire zone, represented by such things as a charcoal layer, a layer of small ground-­cover plants, or by distinct changes in the peat characteristics, such as the degree of compaction, extent of decay, or amount of rooting. Such inferred surfaces permit effectively instantaneous “snapshot” views of the peat swamp at the site of coal ball formation; such time horizons, however, are difficult to trace through a coal ball zone, from one coal ball to the next.

Consequently, the full thickness of a coal ball or coal ball zone represents some amount of time averaging, if the zone is treated as a unit. Thus, in further derived analyses, particularly in studies of paleoecology or peat taphonomy, coal balls, even when zoned, mostly provide a time-­averaged look at the parent plant community and the processes acting on the plant litter derived from it, which distorts the relationship between peat composition and the parent vegetation.

LABORATORY PREPARATION OF COAL BALLS

Although identifiable plant parts sometimes are visible on the surface of coal balls, generally the composition of the entombed peat remains unknown until the coal ball is sliced, exposing its content in anatomical section. In this way, coal balls, like other three-­dimensionally preserved plant remains preserved in various types of concretions, differ significantly from typical compression or impression fossils. Thus, in order to study their composition, specialized laboratory equipment is needed (Figure 23).

Below we describe the now widely used methods of preparing coal balls for microscopic analysis. There are other methods, some still in use, some now obsolete. The earliest used, still employed today, was the thin-­section method (Figure 24a,b), a technique for coal ball study pioneered by James Lomax in Great Britain (Howell, 2005). Though their preparation is time-­ consuming and laborious, thin sections have advantages in that they do not use acid treatments that etch away the carbonate matrix. Recent studies (Taylor et al., 2011) have demonstrated that remains of microscopic organisms can be preserved within this matrix but are often lost during the kinds of preparation we describe below.

An advance over the thin-­section method, in terms of rapidity and ease of preparation, was the liquid-­peel technique (e.g., Graham, 1933). This method uses nearly all the technology described below except that the final application of the peel involves a liquid nitrocellulose solution poured over the prepared

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FIGURE 21. Coal balls, Galatia Mine (underground), Herrin Coal, Illinois. (a) Coal balls in situ. Coal ball zones (layers) can be seen, including a mineral (siliciclastic) layer (“parting”) separating zones and demonstrating, by its undulations, the incompressibility of the coal balls relative to unpermineralized peat, which became coal. (b) From left to right, map of Illinois showing location of the Galatia Mine, a general map of the mine works showing location of the coal ball pods, and the location of coal ball profiles in one of the pods.

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FIGURE 22. Coal ball storage. (a) Cloth bags with tags identifying the mine, geographic location, and identifying information. (b) Example of the type of storage label used on wooden flats and cardboard boxes, identifying geological and specimen information. (c) Example of storage boxes used to store coal balls that are part of a vertical section. Label information is self-­explanatory.

FIGURE 23. (Opposite) Coal ball processing laboratory at the Paleobotanical Research Center, University of Illinois, Champaign-­Urbana; demonstration of steps in coal ball preparation and the peeling technique. (a) Siliceous pea gravel fills the “pit” work area, permitting leveling of the upper surface of cut coal ball specimens. The etched surface will be flooded with acetone in preparation of rolling on a peel of clear cellulose acetate. (b) Lapidary saws with industrial diamonds embedded in the metal blades easily cut calcareous coal balls with a cooling and lubricating cutting oil. (c) The coal ball (top orientation color coded) is locked in a vise. (d) The vise is pulled by a hydraulic system that slowly pushes the coal ball against the cutting blade, like slicing a loaf of bread. Angles of cut may be changed depending on what each cut reveals. (e) Oil-­covered slices of coal balls are scrubbed in hot water with strong detergents prior to further treatment. Coal ball cuts with severe saw kerfs or irregular edges are usually ground level on a revolving lap or with a grinder (not shown). (f) Previously smoothed and peeled coal ball faces are usually resmoothed after each peel with silicon carbide (carborundum) of 400–600 grit on a wet glass plate (1 cm thick). (g) All carborundum powder is scrubbed off the coal ball surfaces prior to further processing. (h) The wetted smoothed coal ball surface is submerged in 5% hydrochloric acid for about 17 seconds (15–20 s range) while being held by the sides of the specimen. (i) The coal ball slice shown here has been submerged fully in 5% HCl; the surface is turned upward only to demonstrate the haze of tiny carbon dioxide bubbles emanating from the etching surface. A longitudinal section of a Lepidostrobus oldhamius cone is evident on the etched coal ball surface. (j) The etched specimen is rinsed with tap water to remove the acid. (k) The wet specimen with the etched surface upward and level is placed on the gravel pit to dry. Drying may be achieved rapidly by flooding with acetone and providing increased air flow. The specimen surface should be completely dry before proceeding. If water is present, the resulting acetate peel will be cloudy. (l) Etched surfaces usually exhibit a bright white aspect (compared to non-­etched); acetone is applied to the surface so that a pre-­cut, clear sheet of cellulose acetate can be deftly rolled across it, pushing the acetone ahead of it as the peel is rolled down. The acetone partially dissolves the cellulose acetate film, allowing it to penetrate the microrelief on the smooth, etched surface of the coal ball. (m) The applied peel is permitted to dry for several hours, preferably overnight. Caution: Except for the final stages of peel removal, all stages of coal ball processing require the use of protective gloves, aprons, and safety glasses, and should be carried out with adequate ventilation that permits removal of oil, acid, and acetone fumes. Weak acid should be flushed and spent oil sludge or other harmful chemicals disposed of in an approved manner.

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FIGURE 24. Older methods of coal ball anatomical preparations. (a) Photograph of a James Lomax thin section of a coal ball from Great Britain, shown in reflected light. USNM specimen 789512. (b) Photograph of the same thin section as in part (a) shown in transmitted light. (c) Photograph of a coal ball surface covered with liquid peel solution. (d) Photograph of the same specimen as in part (c), in oblique view.

coal ball surface (Figure 24c,d). Firsthand experience has taught the authors that this method requires excellent ventilation! Liquid peels become brittle and yellow severely with age. Consequently, this method is little used today. Also, nitrocellulose, aka guncotton, is highly flammable and can be explosive. Many disastrous fires consumed early motion-­picture films made from “celluloid” prepared from guncotton.

Cutting the Coal Ball The first step in preparation is to remove from the coal ball surface any loose material, such as dirt or coal. If coal is adherent to the coal ball surface, it may be best to leave it attached because it can be quite informative about the relationship of the coal ball to the surrounding coal bed in which it was originally embedded (showing layering, extent of compaction, and even permitting individual plant organs to be traced from the coal ball into the adjacent coal; e.g., Winston, 1986). This step should be done

with a dry brush. If water is used, the specimen should be allowed to dry prior to further processing, unless that processing is to be done with a water-­cooled saw.

Once initial preparation is complete, the coal ball must be sawn to expose its inner contents (Figure 23b–d). For nearly all specimens, several cuts will be necessary, in effect slicing the coal ball much like the slices of a loaf of bread. Each slice is generally about 2.5 cm (1 inch) thick. This step is carried out most often with a saw that has small industrial diamonds embedded in the edge of its steel blade. Such saw blades are generally lubricated with a special oil, particularly for larger saws (Figure 23c). Smaller saws may utilize water as the lubricating and cooling agent. In the oil saws we use, the coal ball is held in place firmly with a vise clamp (Figure 23d); movement during cutting should be prevented to stop the saw blade from jamming and possibly being damaged. After each slice is cut, it is removed from the saw, and the coal ball is repositioned for the next cut.

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The removed slice must be cleaned of oil. This is done with a strong detergent, under hot, flowing water, with a scrubbing brush (Figure 23e). It is important to remove as much oil as possible because it will interfere with subsequent preparation of acetate peels. When coal balls have been exposed to weathering in the field, they may have become decalcified to some extent, which can lead to the absorption of significant amounts of cutting oil. Once washed, the coal ball slice may be permitted to dry, although this is not necessary prior to the next steps.

A second stage of cutting is sometimes necessary. Coal balls often break slightly in the final phases of cutting in an oil saw, leaving a raised edge along one side. If this is extensive, it may be necessary to remove this edge or lip prior to further preparation.

Such cutting generally is done by hand. In one method, the cut is made on a table saw fitted with a carborundum blade. The coal ball slice should be completely dry. Proper safety equipment must be used when hand-­cutting dry coal balls, including the wearing of eye protection, a dust mask, and protective gloves. Carborundum blades may break or shatter suddenly and coal ball pieces can be violently dislodged; we therefore recommend a plastic shield between the operator and the blade. Handheld cuts of smaller specimens (generally < 10 cm) also can be accomplished on a water-­cooled saw with a water line alongside a diamond-­ edged blade. In both dry and wet cutting, where coal balls are handheld, slight deviations in the path of the cut can cause binding and damage to the cutting blade, breakage of the specimen, or even injury to the operator, which is why these methods are recommended only for minor trimming of small specimens.

eight across the plate surface. As with the grinding wheel, it is necessary to add grit periodically and to keep the surface of the grinding plate wet.

Once grinding is completed, it is imperative that the coal ball slice again be washed thoroughly in water (Figure 23g). All grit should be removed prior to further processing.

Carborundum abrasives come in a variety of different degrees of coarseness. The larger the number, the finer the grit; we generally use grit sizes 100, 250, 400, and 600. The choice of grit size depends on how rapidly one wishes to flatten the surface and how much of that surface one wishes to remove.

The highest grit grades are used for polishing to achieve high degrees of surface smoothness. Grit sizes 100 and 250 are generally used on the grinding wheel. Sizes 400 and 600 are used on the glass plate.

Surface Preparation

Step One

Cutting procedures can leave a coal ball slice with irregular surfaces. It is sometimes necessary, therefore, to smooth the cut surface. This step can come before or after the preparation of a cellulose acetate peel of the coal ball cut surface. We recommend preparation of “test peels” prior to undertaking the laborious procedure of smoothing coal ball cut surfaces. See the next section for a discussion of peel preparation.

Surface smoothing is done first on a grinding wheel (lap): a mechanized horizontal steel grinding surface. Carborundum (silicon carbide, SiC) abrasive, “grit,” is sprinkled on the surface of the lap, which has been wetted so that the abrasive will stick to its surface. The wheel spins when activated and the coal ball surface is held firmly against the surface of the grinding wheel.

Because the grit is harder than the calcium carbonate matrix of the coal ball, the surface of the latter is ground away slightly. The gritty surface of the grinding wheel should be kept moist by the occasional addition of water (generally from a sprinkler or spray bottle). It may be necessary to clean the surface periodically.

Further grinding of the coal ball cut surface is done by hand on a glass plate (Figure 23f). The procedure is similar to that of the grinding wheel, except that the grinding plate is stationary. After water and grit are applied to the plate, the coal ball cut surface is held against the glass plate and moved in a figure

The first step of this procedure is to etch the clean, wet, cut surface of the coal ball. That surface is immersed in the solution of 5% HCl for several seconds (Figure 23h,i). In our experience, between 10 and 15 (+/−) seconds is sufficient, but more or less may be necessary depending on the density of the carbonate matrix and amount of dolomite in that matrix; the more dolomitic in the coal ball matrix, the longer the etching time will be. The acid dissolves the carbonate matrix of the coal ball, exposing to a very slight degree (at the etching times recommended above) the organic matter in the coal ball.

Peel Preparation Once the cut surface of a coal ball has been properly cleaned of cutting oil, of rock chips, and of any grit used in surface smoothing, a cellulose acetate thin section, also known as a “peel,” can be prepared from that surface (Joy et al., 1956; Phillips et al., 1976; Galtier and Phillips, 1999; Lakeram et al., 2023). This process requires a container to hold a solution of dilute, 5% by volume, hydrochloric acid (HCl), running water, sheets of cellulose acetate of either .005 or .001 mil thickness, and acetone. Proper ventilation and personal protection (breathing, eye, and skin) should be observed.

Step Two The etched surface of the coal ball must be rinsed gently in running water to remove all traces of HCl (Figure 23e).

Step Three The etched surface must then be permitted to dry thoroughly prior to the first steps of peel preparation. It is important to have some surface on which to permit specimens to dry. We generally use wooden boxes or a specially designed bench filled with pea gravel (Figure 23a,k). This permits residual water to drain away,

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allows air to reach all surfaces of the coal ball slice, and provides a simple means to level the cut surface.

Once dry, the etched surface of the coal ball will appear white (Figure 23l). This surface should not be touched, or the exposed organic matter may be damaged.

Step Four Prepare the peel by following these steps: (a) Cut a sheet of cellulose acetate of a size slightly larger than the coal ball surface. (b) Flood the etched surface with acetone. (c) Lay the acetate sheet onto the surface of the coal ball (Figure 23m). This may take some practice to avoid getting air bubbles between the coal ball surface and the acetate sheet.

We suggest rolling the acetate across the coal ball surface from one side to the other, rather than simply laying it flat against that surface. (d) Permit the peel to dry for several hours, so that all the acetone will evaporate.

The logic of this procedure is as follows: When the acetate sheet comes into contact with the acetone on the coal ball surface, the acetate partially melts. The melted acetate then entombs the exposed organic matter. As the acetone evaporates, the acetate sheet rehardens and the organic matter is then firmly embedded in the thin acetate sheet.

Step Five Once dry (Figure 25a), the peel may be removed (slowly and carefully) from the coal ball surface (Figure 25b,c,d). This produces a thin section of the surface that can be examined with a hand lens or, more typically, a dissecting microscope (Figure 25e,f).

There are some generally encountered problems with peel removal. The most common is difficulty in removing the peel

due to its tenacious adherence to the coal ball surface. This can result in tearing during the attempt to pull it off. The most common cause of a sticky peel is over-­etching, which can be resolved to some degree by experimenting with etching times. To remove stubborn peels, we suggest sliding a single-­edged razor blade between the coal ball surface and the peel. A second common problem is overadherence of the peel to the edge of the coal ball surface, which may result in tearing of the peel at that edge. This problem is most often caused by too much acetone, overmelting the acetate sheet where that acetone accumulates at the edge of the coal ball surface. This is a problem that can be exacerbated by a thin lip at the edge of the coal ball, left by the cutting process or if the surface was not completely smoothed during grinding.

The most common other issue that may be encountered in peel preparation is the presence of air bubbles that leave a portion of the peel not in contact with the coal ball surface. These bubbles can be caused by several factors, most often small amounts of coal or other contaminants on the etched surface of the coal ball. A second cause is the insufficient application of acetone, or its evaporation, leaving areas of the etched surface too dry at the time of acetate-­sheet application. A third cause, and one that can be difficult to resolve, is air trapped in the coal ball matrix itself due to porosity of the coal ball; there are specimens that will always have air bubbles as a consequence of this latter problem.

Step Six The peel should be labeled with a grease pencil or indelible marker once it is removed from the cut/etched surface (Figure 25g). The label can be written on the excess clear, marginal part of the peel (which is why we recommend that the peel be slightly larger than the coal ball surface). We use the following procedure for labeling coal ball peels: Coal ball number (if the specimen has been curated), slice designation, and top or bottom of the slice relative to the end at which slice designation started.

We generally designate slices as A, B, C, and so on. If the coal ball is part of a vertical section, we will also mark the upper surface with the marker in order to retain a record of original orientation.

FIGURE 25. (Opposite) Peel removal and treatment. (a) Peels placed on etched coal ball surfaces should be ready for removal after several hours or can remain attached for months in storage. However, early removal (0.5 hours) works for short teaching labs, although such peels tend to curl. The plant anatomy visible in the peel is a longitudinal section of a Lepidostrobus oldhamius microsporangiate cone and numerous Lepidocarpon aquacarps. (b–e) Steps in peel removal: (b) Begin along one relatively straight edge, (c) firmly grasp the sides with each hand, freeing the margin upward. (d) As the peel comes free, adjust fingers to retain tautness close to the attached peel base, maintaining tension until the peel is removed. (e) Removal of peel here shows the rough side that was against the coal ball and in which the etched surface has become embedded. (f) Once peel is fully removed, most observations will be made with the shiny (outer) side up, including high-­resolution microscopic examination or electronic image scanning. (g) Peels usually have irregular margins that may be trimmed with scissors so that they will lie flat.

Many peels can be stored in envelopes.

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Step Seven We store peels in 8.5 × 11-­inch manila envelopes. All the peels from the cut surfaces of one coal ball, and one coal ball only, are stored in each envelope. Information about the coal ball is written on the outside of the envelope. This includes the name of the coal bed, the location where the collection was made, and any additional relevant information, such as vertical section number and zone. If there are particular plant fossils of interest in that coal ball, those too can be noted on the envelope.

Siliceous Coal Ball Preparation Caveats In the rare instances where coal balls are siliceous, making peels is more complex than for carbonate coal balls. Sawing and grinding the surface of a silica concretion is slow because this rock is so much harder than carbonates (Mohs scale 7 for silica versus 3 to 4 for calcite, dolomite, and siderite). Even more problematic is the etching. The only common reagents that etch silica are hydrofluoric acid (HF) and hexafluorosilicic acid (H2SiF6), both of which (liquids or fumes) are highly caustic and toxic, potentially lethal, requiring use of a specially designed laboratory, including an HF-­adapted fume hood, and personal protective equipment: face shield, apron, rubber gloves, and gauntlets. Potential users are referred to OSHA Materials Safety Data Sheets.

Those intent on examining siliceous concretions might consider foregoing peels in favor of polished slabs and thin sections.

Thin-­Section Preparation Since its development by Henry C. Sorby during the 1840s, thin-­sectioning has been a standard procedure for petrographic study of rocks and minerals. This technique was widely used during early studies of coal balls, well into the twentieth century.

Many major fossil-­plant collections in museums and universities have collections of coal ball thin sections, particularly those made by the well-­known preparator James Lomax, who established a commercial operation and produced many thin sections

(Figure 24a,b), particularly of English coal balls, during the late 1800s and early 1900s (Howell, 2005).

A rectangular slab a few centimeters on a side is cut and one surface polished, then cemented to a glass slide. Next, excess rock is sawn away and the remainder carefully ground until light passes through—­typically a thickness of only 30 microns.

After final polishing, a cover slip is applied. For mineralogical specimens, petrographic microscopes used for thin-­section study are equipped with various lenses and filters, used to determine optical properties of components of the specimen. Coal ball thin sections are generally observed under plain transmitted light on either a dissecting or compound microscope. Making thin sections is time consuming and requires practice (there are labs that do this work for a fee), but the most serious limitation for coal ball study is the small size of the slide. Plant parts preserved in coal balls may have dimensions measured in decimeters, whereas making uniform thin sections larger than a few centimeters on a side without “scalping” part of the slide is a challenge for the most skillful technician. Nonetheless, as noted above, thin sections have certain advantages over the peel method if thicker sections are desired and if organisms living in spaces within and between plant cells, such as fungi and bacteria, are the objects of study (Taylor et al., 2011). Thin sections also permit more detailed examination of the coal ball mineral matrix.

QUANTITATIVE ANALYSIS OF COAL BALLS

Whether applied to random samples or to specimens collected from a profile, quantitative analysis of coal balls follows the procedures of Phillips et al. (1977; Phillips and DiMichele, 1999). The objective is to obtain a consistent, statistically valid sampling of the plant parts (anatomically and taxonomically) that compose a coal ball.

To begin, a single, generally middle peel from each coal ball in a zone is chosen as representative of that specimen. Each peel is then attached beneath a transparent acetate overlay ruled in square-­centimeter grids (Figure 26a,b,c); the grids themselves are numbered (Figure 26d; Figure 27). The outline of the coal

FIGURE 26. (Opposite) Observing and quantifying peels. (a) Examination of peels is generally done at a dissecting microscope, and therefore at a relatively low magnification. This illustrates a special, purpose-­built, double-­headed microscope. The boom mount permits observation of even very large peels. (b) In quantifying peels from vertical sections or random samples, the peel of interest is placed under and attached to a numbered heavy cellulose acetate overlay laid out with a square-­centimeter grid system. Peel “readers” Tom Phillips, (left) and Bill DiMichele (right) determine identification and abundance of plant tissues and organs with preservational states such as charcoal. This is done on a grid basis with one coal ball middle peel or more—­depending on coal ball shape and size. (c) The dissecting-­scope field of view permits a sweep of the coal ball surface in order to have a general understanding of its composition before detailed analysis. (d) A coal ball peel is observed one grid cell at a time, and a determination of that cell’s content is made. (e) Identifications and other notations are recorded on a paper grid sheet in a simple code of letters and numbers that represent the botanical constituents and various preservation states of the peat. (f) Trimmed peels are taped onto white paper and (g) scanned at high resolution. (h) These scans permit microscope-­like examination on computer screens with a larger field of view, rapid manipulation, and identification of evidence regarding peat taphonomy. They also facilitate sharing of coal ball images with other investigators who may wish to make use of the coal ball collection.

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FIGURE 27. Coal ball cross section with overlay grid of square centimeters. Each grid square is 1 cm on a side. This image together with Figure 28 demonstrate the basic peel quantification method. This is a cellulose acetate peel made from a cross section of coal ball. Each cell is numbered in a grid overlay and the overlay covers the entire peel. The grid is analyzed quantitatively cell by cell for botanical constituents. One identifier is applied to each grid, as shown in Figure 28. Source: Secor VS 2, Zone 5, 26499 DTOP, Westhoff Brothers Enterprise Mine No.1, Haskell County, Oklahoma. Secor Coal.

ball peel is traced onto a paper copy of the grid sheet; the grid numbers on the acetate overlay and those on the paper copy are aligned such that the grid numbers are identical (Figure 26e).

Results of the quantitative analyses reported thus far in the literature, and in subsequent volumes of this series, were based on the following method. The coal ball peel is mounted below the acetate grid sheet and then is examined under a dissecting microscope (Figure 26a–d), thus at relatively low magnifications of 15–40×. With the advent of high-­resolution scanning, it is now possible to prepare a selected peel for scanning (Figure 26f,g) and make observations and determinations from a computer screen (Figure 26g). One of the advantages of computer analysis is that

multiple people can examine the image together. In addition, the size and field of view is much greater than under a microscope.

In this way, computer projection of coal ball scanned images facilitates study in new ways, including sharing those images in a multi-­person videoconference.

For either of these techniques, a record is made of the most abundant plant taxon in each square centimeter of peel surface area, the type of organ or tissue it represents, and any particulars of preservation, such as charcoal or pyrite. Figure 28 illustrates the specially developed shorthand code used to record these observations. We term this process “grid reading” or “reading peels.” Generally, the coal balls from any given zone of a vertical

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FIGURE 28. Enlargement of overlay grid area in Figure 27, labeled with identification codes for taxa and organs. These codes are transcribed onto a paper data sheet on which each grid is systematically numbered, as on a working acetate overlay sheet. The code can be translated as follows (left to right by row, rows top to bottom): UN = unidentified; SR. PP = Stipitopteris petiole/rachis; ST. RL = Stigmaria rootlet; LM. LL = Lepidophylloides leaf; LC. SG = Lepidocarpon sporangium; M. CO = Medullosa cortical parenchyma; P. RO = Psaronius outer root; M. WO = Medullosa wood; CR WO = Cordaitalean wood; SP. RT = Sphenophyllum root; CR LL = Cordaitalean leaf; S. SG = Scolecopteris sporangia.

Source: Secor VS 2, Zone 5, 26499 DTOP, Westhoff Brothers Enterprise Mine No.1, Haskell County, Oklahoma. Secor Coal.

profile are analyzed as a group. The objective is to quantify not only the content of individual coal balls, but also the groups of coal balls from field-­identified coal ball zones within any given vertical section.

The observations or determinations recorded on the grid sheets then are copied into a suitable form for computer compilation and analysis. The result is a quantitative estimate of taxa, organs, tissues, and preservational states by coal ball and by coal ball zone within a given profile. Quantitative analyses provide the bases for the profile plots and tables in published studies.

COAL BALL STORAGE

Coal ball storage can be a major challenge, particularly if one develops a large collection. The University of Illinois collection is one of the largest collections of coal balls in the world, and for many years was housed in a series of disconnected spaces. In the 1980s, a large steel-­pole barn was built to house the still growing collection, now exceeding 20,000 specimens (Figure 29a); individual coal ball slides are not counted as separate “specimens” in that overall count. Most of the processed coal balls—­those cut, washed, and test peeled—­are placed in wooden flats. Storage flats are labeled and color coded: by plant group if a taxonomic

collection, by vertical section/zone if a profile collection, or by coal bed and locality if a random sample made for quantitative analysis. The wooden flats that contain the coal balls are stacked on open steel shelving (Figure 29b,c). Many coal balls have a very long shelf life, which, based on experience, is likely to exceed a hundred years. Others, however, are “hot,” meaning they can contain iron sulfides, such as pyrite and, particularly, marcasite, that can convert to sulfuric acid upon exposure to humid air, causing the coal ball to decay. This happens to various degrees and often is specific to a coal bed or to a particular locality; “hot” coal balls have a limited shelf life, although that can be extended if they are stored in an area where degassing is possible. In the worst cases, “hot” coal balls not only will self-­destruct but will destroy the storage bags, wooden flats, and even the steel shelves that surround them. Open storage compensates for the lability of coal balls once cut and exposed to air.

Coal ball profiles are often stored in cardboard boxes. The boxes are labeled with the profile and zone information (Figure 29d). Large-­diameter coal balls, or those cut into many slices, are often stored unbagged in wooden flats (Figure 29d–g). In such cases, labels are placed in the wooden flat as well as stapled to the outside (Figure 29d,e) to increase the likelihood that provenance information will be secure. Rolling stock also is necessary in a

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large coal ball storage facility. This may consist of wooden platforms with no sides but to which wheels are attached (Figure 29e) or moving carts of various types (Figure 29f). Because wooden flats are stacked, a nearby flat surface, such as a tabletop, is very helpful in the event that unstacking is needed to get to lower levels in a stack (e.g., Figure 29f). Lastly, some coal balls are stored with peels attached to their surfaces. The example illustrated in Figure 29g is of coal balls intended for a teaching collection. The peels permit the internal components of the coal ball to be recognized readily and also help protect and slow down coal ball decay.

In the University of Illinois coal ball collection, every coal ball is catalogued in a ledger system that includes a 1:1 outline drawing of the coal ball, its specimen number, and comment on its condition or content. The original test peels of the coal ball, those prepared immediately after it was cut, are stored in manila envelopes in a separate collection devoted only to peels. This collection generally would be consulted first, prior to going to the storage facility to obtain the coal ball specimen(s), should further study be called for. These procedures are followed only for the coal balls that have been retained for the collection.

Except for profiles, where the entire collection is retained, regardless of the taxonomic identifiability or taphonomic condition of the peat itself, coal balls are sorted during their initial postpreparation examination. At this time, generally, half or more of the specimens may be discarded as being not of botanical interest or too enriched in iron sulfides to be observed usefully or to have a reasonable shelf life. This is, in effect, a process of coal ball triage. However, the peels of all the discarded coal balls are retained and stored in a separate collection of archival coal ball peels. We have found this collection to be of particular use in the present undertaking because it contains coal ball peat in various states of decay, shrinkage, compression/compaction, or rooting. Specimens such as these generally have been excluded from most previous coal ball studies in the published literature.

Thus, the archival collection provides us with a more realistic view of the preservation of Pennsylvanian-­age peat and the processes that acted on the plant remains as they went from the litter zone of the swamp through the acrotelm and into the catotelm of the peat body. Archival collection peels make up a prominent part of the coal ball plates used in this study.

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An additional aspect of managing a large coal ball collection is making it available to other researchers. As noted above, this will be facilitated by scanning as much of this collection as possible, thus creating the possibility of sharing electronic images with interested researchers. This calls for a formal collections management policy. Such a policy, which most major museums have and will share with those in charge of other professional and private collections, must include several components. The first is a set of standards for tracking physical collections within the storage facility; as described above, it is recommended to have numbers assigned to coal balls or to profile and random collections, and to be able to locate easily and readily the storage site of those specimens or specimen groups within the facility. With regard to coal balls, as mentioned above, examination of specimens is facilitated by a separate curated collection of peels from which coal balls desired for further analysis can be identified. This can be enhanced further by a separate curated collection of electronic-­image scans of specimens in the collection. Next in importance is a policy of collections care that spells out standards, such as periodic evaluation of stability, need for replacement of storage equipment, orderliness of the collection, and condition of labels. Third are policies describing procedures for loans, loan tracking, and loan returns. Finally, it is strongly recommended that a separate collection be made for any and all type specimens and for specimens illustrated in scientific papers; this should include both the specimen of interest and any glass slides or peels made from it. In order to facilitate these procedures, it is necessary to have a designated person or persons responsible for the collections, and to offer them some training in collections management.

SUMMARY: COAL BALL COLLECTION, PREPARATION, AND STORAGE

As should be clear, the science of coal ball paleobotany, in its many different dimensions—­anatomy and morphology, physiology, taxonomy, evolutionary studies, paleoecology, taphonomy, coal geology, and others yet to be imagined—­requires a great deal of up-­front work before one ever gets to look at a specimen.

The management of the collection is, itself, a major commitment,

FIGURE 29. (Opposite) Storage facilities for the coal ball collections at the Paleobotanical Research Center, University of Illinois, Champaign-­ Urbana. (a) Coal ball specimens are stored in wooden flats and cardboard boxes and arranged by major taxonomic categories (color coded) or by profiles/random samples (coals/geographic regions), which make up about 60% of total collections. (b) A diagonal aisle separates the two kinds of collections, with orange labels designating ferns, Marattiales, and small ferns. The west-­end wall (at end of aisle) is full of Upper Pennsylvanian coal balls. (c) Some aisles are blocked by uncut collections on the floor in wooden flats. (d) Many collections remain on four-­ wheel frames during processing, curating, and storage. The exposed coal ball slice shows an aggregate with two different compactional states: the light brown of early coal ball peat permineralization and the very dark brown with severe peat compaction prior to being embedded with calcite. Drill holes across the face and near the top are the locations of drill sampling for stable isotope analyses of the carbonates. (e) Wooden flats with selected coal ball slices utilized in this study. (f) Wooden-­flat storage on open shelving, Flats are labeled to show contents. (g) Teaching collections of coal balls are stored with peels on exposed faces and other surfaces painted.

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but one that all major collections face. Preserving that collection for future use is paramount, especially given scientific technical advances that keep opening to us new ways of analyzing and interpreting specimens of all kinds. As a consequence, there is a great deal of time, money, and human activity behind every cellulose acetate peel, loose or mounted on a glass slide. This should be kept in mind by anyone studying these remarkable, unique specimens of plants from the long-­ago Coal Age.

COAL BALLS IN TIME, SPACE, AND HISTORY Coal ball occurrences throughout the world are concentrated stratigraphically in Pennsylvanian and Permian coal basins.

They are found in more than 125 regional coal beds in the Pennsylvanian of North America (Phillips, 1980; Lyons et al., 1995), Europe (Zaritsky, 1971; Snigirevskaya, 1972; Galtier, 1997), and China, and in the Permian of China (Tian and Wang, 1995; Tian et al., 1996), the Kuznetsk Basin of southwestern Siberia in Russia (Yablokov, 1951), and Australia (Feliciano, 1924). In addition to plant remains, some coal balls may also entomb associated siliciclastic sediment and may include marine invertebrate fossils (“mixed” coal balls of Mamay and Yochelson, 1962).

It is our objective here to give the reader some background on the occurrences of coal balls, their collection in the field, and some of the more important sources of American coal balls that are further described, illustrated, and used in quantitative studies. As noted, the discovery of coal balls throughout the history of work on these important fossils is largely one of serendipity. Most occurrences are in coal mines, which require contacts with mine personnel or with geologists and engineers who work regularly in those mines. The collecting of coal balls is often an adventure and takes both the professional scientist and dedicated avocational scientist into situations not generally imaginable by the public, or even by fellow scientists in other disciplines. We touch on some of these here, as we discuss a selection of the more historically important coal ball sources in the United States, most of which we have worked on ourselves and thus can relate from firsthand experience.

A BRIEF HISTORY OF COAL BALL STUDIES

The discovery of the objects now known as coal balls was first reported by Hooker and Binney in 1855 from England in Lower Pennsylvanian (Langsettian) strata. This publication established clearly the general significance of coal balls and also suggested a possible origin. Hooker and Binney (1855: 149) state that, [Anatomically preserved plants] were found embedded in nodules of limestone, enclosed in a thin seam of bituminous coal, not above six inches thick, in the lower part of the Lancashire coalfield. The origin of these nodules

may probably be ascribed to the presence of mineral matter, held in solution in water and precipitated, or aggregated, around certain centres, in the mass of vegetable matter now for the most part turned to coal.

The coal bed where the nodules were found was overlain directly by black shale containing cephalopods and other mollusks “apparently of marine origin.” Subsequent to their discovery, there were nexuses of coal ball publications in the late 1800s, and in the early (1908–1911), middle (1951–1965), and late (1970–1985) twentieth century, with lesser interest between these times of more intense activity.

The classic monograph of Stopes and Watson (1909) on calcareous coal balls highlighted the Langsettian occurrences in Lancashire, Yorkshire, and Cheshire, England, as well as other reports, including one from Iowa, USA. These authors also established that, with rare exceptions, coal balls formed only where the peat that became coal was buried under marine sediments.

Moreover, Stopes and Watson also firmly established the term “coal ball,” which, according to Stocks (1902), was a local name used by coal miners for the concretions. In-­mine mapping of coal ball distribution patterns in the Katharina seam of North Rhine-­Westphalia in Germany by Kukuk (1909, 1938) provided a key baseline for comparative studies. Discoveries of coal balls in the Donets Basin, Ukraine and Russia, were first reported by ­Zalessky (1910a,b) who also described anatomically preserved Dadoxylon wood from Kuznetsk Basin coal balls (Zalessky, 1911), although, at the time, it was not clear that these upper Permian concretions were coal balls.

Summaries of coal ball occurrences and/or coal swamp floras over the past century are mainly regional in scope. Those from Europe focus on older coal horizons, Langsettian, Bashkirian, and lower Moscovian. North American occurrences are more wide ranging temporally, including the Morrowan and Atokan Stages, but with the greatest concentrations in the Desmoinesian, Missourian, and Virgilian strata. Coal balls from China are of both Pennsylvanian and Permian age, as are those from the Kuznetsk Basin of Siberia. More general studies include the petrographic and geochemical survey of Euramerican coal balls

by Rao (1985), which provides extensive compositional data on the elemental and organic matter content of coal balls, and the review of Carboniferous coal ball formation and its significance

by Scott and Rex (1985), which includes petrographic descriptions of coal ball carbonates and inferred diagenetic alterations.

The so-­called Great Coal Ball Horizon (lower Langsettian) includes the Union seam and splits thereof (Lower Mountain Mine and Upper Foot) in England, the Bouxharmont seam of Belgium, and the Finefrau-­Nebenbank horizon of the Netherlands and Germany (Schopf, 1941, fig. 1; Galtier, 1997, fig. 1). Specific regional summary reports are included in Stopes and Watson (1909), Leclercq (1925, 1934), Koopmans (1928), Hirmer (1928), Holmes and Fairon-­Demaret (1984), and more generally

by Galtier (1997).

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In the Donets Basin of Eastern Europe, coal balls occur in numerous coal beds. Reports include Zalessky (1910a,b, 1928), Zaritsky (1959, 1971), and Snigirevskaya (1972).

The discovery of coal balls in North America has a long history, partly summarized by Phillips et al. (1973). The earliest account is by Keyes (1894: 278–280), from the Bloomfield Mine at Des Moines, Iowa, where “lenticular masses of ironstone” were abundant and in places formed nearly continuous layers. The first to describe fossil plants from American coal balls was Gresley (1901), using specimens from What Cheer, Iowa (located 95 km east of Des Moines), thus predating A. C. Noé (1925), who has been widely credited with being the pioneer in the search for American coal balls. Later summaries and reviews include those of Darrah (1941), Schopf (1941), Andrews (1951), Phillips et al. (1973), and Phillips (1980). In North America the geochemical and paleontological coal ball monograph of Mamay and Yochelson (1962) provided direct evidence of marine influence by documenting invertebrate fauna in some American coal ball deposits. The study of DeMaris et al. (1983), who mapped coal ball distribution in relation to roof composition in the Herrin Coal of the Illinois Basin, was a key study in the expansion of our understanding of coal ball formation. Similarly, Anderson et al., (1980), also studying the Herrin Coal, reported some of the first stable-­isotope (δC13) and rare-­earth-­elemental (REE) data on coal balls and the surrounding coal, influencing our understanding of coal ball formation. More recent geochemical and petrographic studies of Illinois Basin coal balls include Raymond et al.

(2012) and Siewers and Phillips (2015). Chrpa et al. (2023) have studied carbonate cement chemistry in coal balls from Iowa, in the Western Interior Basin of the United States.

Tian (1985), Tian and Wang (1995), and Tian et al. (1996) provide summaries of coal ball occurrences in the Pennsylvanian and Permian of the North and South China blocks; there also is an additional atlas by Tian and Zhang (1980). At least 13 distinct coal beds in China contain coal balls. The youngest known Permian occurrence of calcareous coal balls was reported at the 1979 Carboniferous Congress by Tian (1985), who described abundant specimens from the uppermost Permian, No. 1 Coal seam, just below the Triassic boundary in Guizhou province, southwest China. Recent studies of Chinese coal balls have revealed many new taxa, some belonging to the same lineages as those from Euramerica, but with subtle differences (e.g., Hilton et al., 2009).

In the upper Permian of the Kuznetsk Basin (Angaran phytogeographical realm), Yablokov (1951) listed at least 29 coal beds with coal balls. Yablokov’s (1951) summary was largely unknown outside of the USSR until cited by Snigirevskaya (1972).

Along with a listing of coal ball occurrences, it includes information on distributional patterns of coal balls within the coals and summarizes accrued research up to that time. Marine influence in the basin is equivocal (Davydov et al., 2021) and the location of the basin in the temperate Angaran paleogeographic region demonstrates that coal balls are not confined to tropical environments, nor may they require marine influence, although

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this would be the only known exception for coal balls formed of calcium carbonate. Coal ball occurrences elsewhere in the world are almost, as far as they are known to the authors, otherwise exclusively from areas of coal beds with roof strata deposited under marine conditions.

SELECTED AMERICAN COAL BALL OCCURRENCES

Coal balls have been encountered and collected in all major Pennsylvanian-­age coal basins of North America. Occurrences are concentrated in the later Middle Pennsylvanian and Late Pennsylvanian (see Figure 1) and are most abundant in the Illinois (Eastern Interior) and Midcontinent (Western Interior) Basins, with fewer reported occurrences from the Appalachian Basin.

Coal balls occur in at least 18 Pennsylvanian-­age coals in the Illinois Basin (Phillips et al., 1985). This number includes most of the commercially important coals and is dominated by specimens from the Springfield and Herrin Coals (Figures 30, 31), which rank first and second, respectively, in terms of basinwide mapped resources and historic mining. Aside from these two, Illinois Basin coal balls range in age from the latest Atokan Minshall Coal to the early Virgilian Watson Coal (Shumway cyclothem).

The Shumway coal balls are unique in the Illinois Basin in being silicified (Scheihing, 1978), whereas all others are preserved as calcium-­magnesium carbonate, often with a substantial admixture of iron sulfide minerals.

We begin our discussion with the prolific collections from the Herrin Coal in southern Illinois. These collections have been among the most important ones for paleobotanical research, from which an understanding of Middle Pennsylvanian plants has emerged. The Herrin is the “flagship” coal for our present study for three reasons: (1) the intense paleobotanical, morphological, and systematic analysis of its plants; (2) the historical significance of the first quantitative analyses of coal ball peat composition having been done on this coal; and (3) the presence of more coal ball collecting sites and data from this coal than any other. Following discussion of the Herrin, we consider some other Illinois and Midcontinent coals from which significant coal ball profiles have been obtained.

Herrin Coal, Illinois Basin The Herrin Coal, also known as the No. 6 Coal in Illinois and No. 11 in western Kentucky, is one of the two most economically important coal beds in the Illinois Basin, widely mined in surface and underground operations throughout Illinois and western Kentucky. The coal exceeds 4 m in thickness in parts of southern Illinois, an area referred to in the mining industry as the “Quality Circle” due to its coal’s thickness and relatively low sulfur content (Figure 32). Extensive mining of this coal has, over many decades, provided great opportunities for geological investigations of various kinds (e.g., Edwards et al., 1979; Johnson, 1979; Kravits and Crelling, 1981; DeMaris et al., 1983; Harvey

FIGURE 30. Map of Herrin Coal mines in Illinois Basin showing coal ball localities.

FIGURE 31. Map of Springfield Coal mines in Illinois Basin showing coal ball localities.

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FIGURE 32. A miner undercutting the Herrin Coal at the Old Ben No. 9 Mine near West Frankfort, Illinois, during the early twentieth century.

Note the miner’s soft cap, carbide lamp, and corncob pipe. Undercutting provided a place for the coal to fall when it was blasted, reducing the amount of powder required. Coal as thick as 4.3 m (14 feet) and of low sulfur content gave this area of the coalfield the name “Quality Circle.”

Photograph courtesy of the Illinois State Geological Survey; photographer unknown.

and Dillon, 1985; Winston, 1986; Burk et al., 1987; Nelson, 1987; Breyer, 1992; O’Keefe et al., 2008; Siewers and Phillips, 2015). Included among these are numerous studies of the plant remains that made up the original peat-­swamp community. Herrin Coal paleobotanical studies also have included analyses of the compression fossils exposed in siliciclastic “roof shales” immediately above the coal bed (e.g., DiMichele et al., 2007), and palynofloras of the coal and roof shales mostly in support of biostratigraphic (e.g., Peppers, 1970, 1996) or ecological (Schopf, 1938; Mahaffy, 1985) analyses. For our particular interests here, Herrin coal balls have been the source of more quantitative study of peat-­swamp ecology and peat taphonomy than those from any other coal (e.g., Phillips and DiMichele, 1981; Winston,

1986, 1988; DiMichele and Phillips, 1988; Siewers and Phillips, 2015; Chrpa et al., 2023).

Mining activities in the Herrin Coal also have encouraged studies of the associated strata in order to understand systematic regional patterns in coal thickness, and of sulfur and ash content as well as roof and floor stratigraphy (Treworgy and Bargh, 1984).

The Herrin Coal is part of a glaciation cycle linked to changes in climate and sea level that is represented by a repetitive sequence of lithological units that group into alternating terrestrial and marine origins (Figure 33a,b). Throughout most of the Herrin Coal’s geographic extent, the terrestrially formed beds generally include floodplain and channel deposits, an underclay paleosol, usually a Vertisol, and the coal bed, above which are beds deposited

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FIGURE 33. The Herrin Coal cyclothem. (a) Herrin Coal subtended by a paleosol and overlain by the marine fissile black Anna Shale and the Brereton Limestone. Knight Hawk Coal Company, Prairie Eagle Mine, Illinois. (b) Herrin Coal overlain by marine Anna Shale, Brereton Limestone, and deltaic gray shales. A shallow, terrestrial paleochannel is cut into the marine strata and flanked by a paleosol (both formed during marine lowstand on the exposed shelf), above which is the Hymera Coal. Wild Boar Mine, Illinois. (c) Eastern edge of the Herrin Coal. Two thin coal benches, between which is a 4 cm thick “blue band” mineral parting. Francisco Mine. (d) Herrin Coal, proximate to the Walshville channel, overlain by Energy Shale. The top surface of the peat was eroded by tidal channel activity. Sulfur staining of the coal bed results from the rapid conversion (during weathering) of sulfide minerals in the coal (pyrite and marcasite) to sulfate minerals (melanterite, rozenite, szomolnokite and others) when coal is exposed to ambient conditions.

under marine conditions: a fissile black shale (the Anna Shale), an open-­marine limestone (the Brereton Limestone), and gray shales formed under deltaic conditions. Above this is another cyclothemic succession that may include paleochannels formed during lowstand and flanking paleosols (Figure 33b). The Herrin Coal is thickest on either flank of, and absent within, a broadly sinuous body of shale and sandstone that runs from the northeastern edge

of the basin through to the southern boundary, over 420 km in length and varying in width, but typically over 2 km. This feature is interpreted as an ancient river channel that existed contemporaneously with peat accumulation and is referred to informally as the Walshville channel (Allgaier and Hopkins, 1975; Treworgy and Bargh, 1984; DiMichele et al., 2007). The rocks that fill this paleochannel are now formally designated as the Walshville

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Member of the Carbondale Formation (Nelson et al., 2020). The Walshville river actively drained the basin beginning while the land surface was exposed during the regressive stages of sea level and at lowstand. The river continued to flow during the period of peat accumulation and ended during the marine incursion that flooded the Herrin peat swamp (Archer and Kvale, 1993). The coal rests on a fossil soil (Figure 33c) of vertic, and locally calcic, character (Rosenau et al., 2013a, b; Rosenau and Tabor, 2013) contemporaneous in origin with the Walshville channel.

The shale bed overlying the Herrin Coal, the “roof shale,” varies regionally. Where the coal is thickest, close to the channel, the roof is primarily the gray siltstone, Energy Shale Member (Figure 33d) that may have a finely laminated texture, reflecting a mudflat-­to-­estuarine origin (Archer and Kvale, 1993). The contact between the coal and roof shale in these areas is most often continuous but may be erosional where tidal channels developed (so-­called “rolls;” Figure 33d). Progressively more distant from the channel, and overlying the coal throughout most of its extent, is a marine, fissile black shale (the Anna Shale) (Figure 33a–c). Regionally, the coal under black shale typically is thinner and higher in sulfur than under gray shale (Allgaier and Hopkins, 1975). Above the black shale is a limestone (the Brereton Limestone) that, where the black shale is absent due to nondeposition or erosion, may lie in direct contact with the coal bed (Figure 33a,b). In these areas of marine roof, the contact between the coal and roof shale is generally erosional, suggesting some degree of ravinement during marine transgression.

A notable feature of the Herrin Coal is the so-­called “blue band,” a siliciclastic parting that occurs in the lower part of the coal bed throughout, and even beyond, the areal extent of the coal, almost certainly indicative of a period of cessation of peat formation and regional flooding, although whether due to short-­ term flooding from the channel or a prolonged interval of intensified rainfall is uncertain. Figure 33c illustrates the Herrin Coal at its eastern edge, consisting of two thin coal benches, each < 1 cm thick, separated by a blue band 4 cm thick.

The Herrin Coal itself is a rich source of coal balls and more have been collected from this coal than any other in the world.

This has resulted, in large part, because of the extensive mining of the Herrin Coal in the Illinois Basin, and also because of a sustained, multigenerational focus on coal ball-­based botanical studies at the University of Illinois, which led to over 75 years of in-­mine collecting beginning in the 1930s. Collections were made from at least 25 mines during this time period, concentrated in the southern portion of the Illinois Basin. Most of these collections are “random samples,” made mainly from mine spoils or tipples, thus not collected from the coalface in a manner that preserves the original orientation and relative positions of the individual coal ball specimens. As part of more recent work, and in contrast to most of the previous studies, we have collected and quantitatively analyzed coal balls from eight of these coal mines, which cover a large area of the southern Illinois Basin. The coal balls from these mines were collected in situ, directly from the coalface, or from isolated pods of coal balls that had been set

aside or bypassed in the course of mining, and from which orientation and relative position of individual coal balls could be established; we refer to these as “profiles,” “vertical profiles,” or “vertical sections” (VS). In addition, a single profile of coal balls was recovered from a core drilled for coal exploration.

Thanks to the numerous papers focused on the morphology and taxonomy of the plants, as noted above, the flora of the Herrin Coal is well known. This has permitted the quantitative analyses of specific sites (e.g., Phillips et al., 1977; Phillips and DiMichele, 1981; Winston, 1988; DiMichele and Phillips, 1988), from which a general picture of Herrin peat-­swamp vegetation and paleoecology has emerged. Thus, as a platform for general understanding of Pennsylvanian peat swamps and the ecological processes that operated within them, the Herrin is unrivaled.

That said, its composition is typical of coal balls from those other late Middle Pennsylvanian (late Desmoinesian) coals that have been sampled in the United States. That flora is composed of trees belonging to all five major Pennsylvanian wetland plant groups: lycopsids, marattialean tree ferns, medullosan pteridosperms, calamitaleans, and cordaitaleans. This composition is considerably different both from older, Early and early Middle Pennsylvanian coals (dominated by lycopsids and cordaitaleans, with few tree ferns), and from Late Pennsylvanian coals (dominated by tree ferns and medullosans). These patterns are detailed in Phillips et al. (1985) and reviewed by Montañez (2016).

The later Desmoinesian was a time during which peat-­ forming swamps reached a zenith in the diversity of the evolutionary lineages that contributed to the peat biomass. These groups appear to have been ecologically differentiated (DiMichele and Phillips, 1996a) and, thus, to have constituted the most diverse swamp landscapes of the Pennsylvanian tropics. On an average basis, the dominant plants of the Herrin Coal were a diverse array of tree lycopsids, at least six species, which themselves were microhabitat differentiated (Phillips and Peppers, 1984; DiMichele and Phillips, 1985). Subdominant elements included an array of medullosan pteridosperms, perhaps as many as 12 species of Medullosa and Sutcliffia based on their seed/ovule types (DiMichele and Phillips, 1996a,b), and a variety of marattialean tree ferns, as many as six species based on reproductive organs (Lesnikowska, 1989). Calamitaleans were a minor but widespread part of the arborescent vegetation. Cordaitaleans, among the first or second most abundant plants in peat swamps during the Early through middle Middle Pennsylvanian, were only minor constituents, reduced to a single species, inferred from the single seed/ovule taxon present in late Desmoinesian coal balls (Costanza, 1985).

A diverse array of small vines, epiphytes, and ground cover also has been described, including lycopsids, ferns, pteridosperms, and sphenophyllalean sphenopsids (DiMichele and Phillips, 1996b). Of the plant groups that make up the Herrin peat, we infer that the arborescent lycopsids were the principal biomass contributors, on a landscape average basis. When broken down, however, into a zone-­by-­zone analysis within coal ball profiles, and when compared with certain physical attributes of the coal bed, it would appear that the peat-­forming environment was

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microhabitat differentiated and at different times and places tree ferns and pteridosperms could be locally dominant.

The Herrin peat swamp almost certainly was planar in profile and dependent on a mixture of groundwater and rainfall as the main sources of moisture. In the course of our studies, we observed indications of physical disturbances in this environment such as siliciclastic partings (indicating periods of surface immersion likely accompanied by hiatuses in peat accumulation), charcoal layers (fires), and increasing mineral content of the coal in proximity to the roof shale (onset of flooding by sea-­level-­driven rising water tables). Indications of physical habitat conditions can be deduced further from the condition of the peat itself as preserved in coal balls. Features include several different indicators of peat-­surface exposure and subaerial decay, water-­table fluctuation inferred from patterns of decay and multiple root generations, and periods of rapidly rising water table indicated by thick, nearly unrooted accumulations of aerial plant remains.

Some areas of the swamp appear to have been more subject to fluctuating water table and peat decay than others, which experienced long-­term standing-­water conditions. Fire frequencies may be inferred from abundances and distribution patterns of charcoal, which suggest that some types of vegetation were more prone to fires than others. Even disturbances such as windthrow are suggested by taxa and organ mixtures of particular, recurrent types, indicating that certain kinds of vegetation were especially susceptible to this form of disturbance. On the whole, therefore, the Herrin Coal offers opportunities to study both the original composition of the vegetation, and the taphonomic factors operative between that original vegetation and its record preserved in peat. Reconstructing the original vegetation of the peat swamp requires reference to the basic patterns of peat quantitative composition, but with due consideration for the effects of taphonomic variables on the way in which, and the degree to which, the peat reflects the original swamp vegetation and the conditions under which it grew.

Coal balls were first reported in the Herrin Coal by Gilbert Cady (1936) from the Clarkson Mine, near Nashville, Illinois (Figure 30, location 7; see later discussion in Schopf, 1936). Since that first report, coal ball accumulations have been encountered in more than 25 mines in the Herrin Coal. Coal ball profiles or in situ coal balls have been collected from 10 of these mines, including the following in Illinois: American Coal Company Galatia Mine, Old Ben Coal Company Mine No. 24, Sahara Coal Company Mine No. 6, AMAX Coal Company Delta Mine, Peabody Coal Company Shawneetown Mine (Core), and Peabody Coal Company River King Mine. They also have been collected from the following mines in western Kentucky: J. Smith Coal Company, Peabody Coal Company Camp 11 Mine, Peabody Coal Company Ken Mine, and Peabody Coal Company White City Mine. Random samples have been collected and analyzed quantitatively from many of these same mines, and from other mines throughout the basin. Below we discuss in some detail the collections made at two of these mines, Sahara Coal Company Mine No. 6 and Old Ben Coal Company Mine No. 24. The coal

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balls from these mines have proven particularly important both to descriptions of the Herrin flora (particularly Sahara) and to paleoecological analyses of the Herrin swamp. Numerous coal ball vertical profiles were collected from each and have served to anchor the development of the techniques used to generate the quantitative data reported here.

Sahara Coal Company Mine No. 6, Illinois The Sahara Coal Company Mine No. 6, a large surface mine in the Herrin Coal near Carrier Mills in Saline County, southern Illinois, was visited repeatedly (1950s through 1980s) and became a major source of coal balls for paleobotanical studies.

Evans and Amos (1961) were the first to describe coal balls from this mine and speculate on their origin. This mine was selected as the experimental site at which to develop the quantitative paleoecological methods described here. Included among those ­methods are means to sample coal balls in situ and various methods to quantify peat composition and use that quantification to analyze and characterize the vegetation of Pennsylvanian-­ age peat swamps (Phillips et al, 1977). Coal ball occurrences were very common at the Sahara mine and, importantly, accessibility to them was permitted by the coal company. Locally, coal balls occupied most of the bed thickness, which allowed extensive profiles to be collected. The coal balls also displayed generally good peat preservation, which, over the years preceding the quantitative studies, had led to many publications on the anatomy and morphology of the plants, thus providing a strong baseline from which to identify the plant remains encountered in analyses of vertical sections.

Following the initial studies, Sahara was chosen for a larger-­ scale study of the paleoecology of the Herrin swamp in a substantially larger, but still local, area. In order to accomplish this, sampling of additional coal ball exposures was required. Special attention was given to siliciclastic and charcoal bands in the coal, which enabled both more confident zonation and the potential for correlation of zonation among coal ball exposures. Additionally, in order to have practical means of identifying the bulk of the vegetation at genus and species levels, it also was necessary to develop means to separate the different kinds of lycopsids that occur in coal ball peats. The results of this study were the first detailed quantitative analyses of the paleoecology of Pennsylvanian peat-­swamp vegetation (Phillips and DiMichele, 1981).

This project began when the coal company removed a long strip of Herrin Coal (Figure 34) at the Sahara Mine No.6, including the immediate roof rock, the marine, black-­shale Anna Shale. At that point the coal was ready for loading out to haulage trucks. As it so happened, this strip of coal had numerous podlike areas where nearly the full bed was occupied by coal balls.

We were permitted to collect from these coal ball masses during a lull in mining activities on a Saturday. The largest and most important of the coal ball profiles collected from this enterprise was Sahara Vertical Section (VS) 4, which consisted of 15 coal ball zones (shown in the left side of Figure 34a,b) occupying about

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2 meters of the 222 cm thickness of the full coal bed. Two additional areas of coal balls were exposed laterally to the main area, shown to the right of the meter stick in Figure 34a. These lateral sectors, each a meter wide (vertical white line), were collected where lateral continuity of coal ball zones permitted. The presence of six persistent clastic partings (horizontal white lines in Figure 34a, dotted lines in Figure 34b) aided in tracing coal ball zones laterally, as did similarities in coal ball size, shape, and continuity, and natural benching of the coal. By resisting the compaction that affected the surrounding unpermineralized peat, the coal balls caused the coal bed to bulge upward, especially where large concretions were stacked upon each other (Figure 34b); such thickening of the coal bed in the vicinity of coal balls is typical wherever such concretions occur.

Collection of profiles was done in accordance with the procedures described above. In preparation for extraction by zone, coal ball faces were spray-­painted while still in situ. As the coal balls were removed, newly exposed specimens were repainted to keep the coal ball zones recognizable during the continuing extraction process. Coal balls were removed with chisels and hammers as well as with pick-­mattocks and pry bars. As coal balls were extracted, their top surfaces were spray-­painted with the appropriate zone color in order to maintain their original orientation and to help ensure that their proper zone identity was maintained during subsequent processing (Figure 34c). Collection bags and tags were also color coded according to a preplanned design and all labels bore the profile number, zone number, and color. Based on the initial experimental analyses (Phillips et al., 1977), a coal ball sample permitting approximately 1,500 cm2 of peel surface (using a single peel from the middle of each coal ball) for each zone needed to be collected; based on an average coal ball diameter of 10 cm, this could be fulfilled with a minimum of 15 specimens.

This sampling protocol was applied independently to the reference profile VS 4 (Figure 34a left side) and to the lateral extensions of zones into the meter-­wide sectors, permitting assessment of lateral variation in peat composition.

Coal balls capture a time-­averaged sample of vegetation, the amount of which is dependent on taphonomic processes and the

thickness of the coal ball zone. Thus, coal balls are not necessarily identical to vegetation zones. Natural benches in the coal, resulting from cryptic hiatuses in peat accumulation, permit the coal bed to be divided into a series of compositional layers that on occasion can be further subdivided once detailed analysis is undertaken. There frequently are practical decisions regarding zonation and collection of coal ball zones that must be made on the spot or during processing. For example, in Sahara VS 4 (Figure 34b) the very thick, topmost mass of coal ball material was divided into two zones at the horizontal middle (Zones 14 and 15) during quantitative analysis.

Access to the site of Sahara VS 4 was via a road on the top of the coal bed. From where we were collecting Sahara VS 4, looking westward (Figure 34d) along the “road of coal,” was the large strip shovel that was used for removal of the “overburden” (a term used in the mining industry to refer to the rock between the land surface and the top of the coal bed), which was quiet that Saturday morning. In the opposite direction, to the east (Figure 34e), we had heard some rumblings atop the highwall (the mining term used to describe the still intact surficial overburden rock) but did not realize it was a bulldozer until it pushed a huge modern tree from above onto the far end of the “road of coal,” totally blocking our exit. The tree, composed mostly of wood, was lying on the coal bed, made mostly of nonwoody tissues! This fundamental tissue-­level difference in tree construction exemplifies the great evolutionary gulf between the tropical Pennsylvanian-­age peat swamp forests and those of the post-­Paleozoic.

Old Ben Coal Company Mine No. 24, Illinois An experimental longwall mining project at the underground Old Ben No. 24 Mine near Benton in Franklin County, southern Illinois, in the late 1970s exposed large masses of coal balls in the Herrin Coal. These coal ball masses were so extensive that they seriously impeded the mining operation, prompting the Illinois State Geological Survey (ISGS) to investigate the distribution of coal balls and attempt to forecast their presence ahead of the working face. Although such prediction proved unfeasible,

FIGURE 34. (Opposite) Sahara Coal Company Mine No. 6, a surface mine in the Herrin Coal south of Harrisburg, Illinois. (a) Herrin Coal: in situ coal balls at the Sahara Coal Company Mine No. 6 near Carrier Mills, Illinois. Colored paint layers indicate distinct coal ball zones, which occur at different levels in the coal bed. Horizontal white lines mark siliciclastic bands, which pass between some of the coal ball zones. Vertical white lines divide meter-­wide segments of the coalface from which separate samples were collected to permit tracking of coal ball zone composition laterally. (b) Diagram of Sahara VS 4, analyzed by Phillips and DiMichele (1981), with coal ball zones color coded and numbered in Roman numerals from bottom to top. Dashed lines = shale partings. (c) Coal balls extracted from Sahara VS 4. Balls from each zone rest on a separate canvas bag. Their tops are color coded to match zone designations and to preserve their original orientation. These coal balls were subsequently processed in the lab and curated by zone. (d) Stripping shovel with highwall at left and spoils at right. After the overburden is drilled and blasted, the shovel moves it to the spoil banks, exposing the coal bed. Under modern reclamation laws, spoils are returned to the original contour, then the topsoil is replaced and seeded with cover crops. (e) “Road of coal,” on top of the exposed Herrin Coal bed, with highwall on right, spoil piles on left. A modern tree blocks the road after it was bulldozed off the surface above the highwall.

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the ISGS investigation resulted in detailed documentation of coal ball distribution and a proposed model for coal ball formation (Bauer and DeMaris, 1982; DeMaris et al, 1983; DeMaris, 2000).

Further studies of the coal were carried out by Johnson (1979), Harvey and Dillon (1985), and DiMichele and Phillips (1988).

Of the several coal ball profiles analyzed from Old Ben 24, we discuss VS 3 in particular. This profile (Figure 35) was chosen because it was the thickest available in the mine. The coal ball zones in the profile were spray-­painted. Lateral to the colored column in Figure 35, still in situ, are numerous horizontally stacked, tan-­colored coal balls that had been sheared by machinery in the course of mining. Coal layers between the coal balls aided in zonation. Extraction, often difficult from a sheared coalface below ground, was also eased by the coal between the coal balls (more easily removed with hand tools) and by the location of the profile at a rounded corner. Nonetheless, extraction of the coal balls was a challenge, even with 16 lb sledgehammers and chisels onto which long handles had been welded. Although a ladder would have been helpful for the upper zones, swinging a sledgehammer while perched on a ladder is both impractical and unsafe. The uppermost coal balls were lodged in the roof and not fully exposed; they were recovered later, disheveled from a roof fall and agglomerated as the top zone. A drawback to the extraction techniques we use when applied to massive deposits of coal ball material is the shattering of specimens into smaller pieces.

This reduces intact area of any given coal ball specimen, obscures or destroys natural breaks within individual coal balls, and increases processing and handling time and expense. When the coal balls had been collected, spray-­painted, bagged, and tagged, they were carried to the nearest “man trip” vehicle (the term then in use for covered steel cars that traveled on railroad tracks underground, designed to bring miners into and back out of the mine); invariably we received generous help from miners.

In the room and pillar part of Old Ben No. 24, some pillars, such as that illustrated in Figure 36a, revealed the tan-­colored cross sections of coal balls outlined by the coal matrix surrounding them. The curved scratches along the face of the pillar are the marks of tungsten-­carbide teeth of a continuous mining machine. These teeth create a spectacle of sparks when grinding into coal ball carbonates. The crew of coal ball collectors is shown in ­Figure 36b.

Springfield Coal, Illinois Basin The Springfield Coal also is known as the No. 5 Coal in Illinois, the V Coal in Indiana, and the No. 9 Coal in western Kentucky. It is the largest coal resource in the Illinois Basin and is actively mined at the surface and underground in large areas of Illinois, western Indiana, and western Kentucky. The first definitive report of coal balls in the Illinois Basin is from the Springfield Coal; they were collected and reported by A. C. Noé in 1922 from the abandoned O’Gara Mine in southern Illinois (see description of this activity in Phillips et al., 1973). Since that time, numerous coal ball occurrences have been recorded in the

Springfield but collecting and description of them has not been as systematic as studies of those from the Herrin. The palynology of the Springfield, however, has been studied in great detail from a paleoecological perspective (Mahaffy, 1988; Willard, 1993; Willard et al., 1995). Study of the adpression flora in the shales above the Springfield Coal also has received little attention in the literature (e.g., Willard et al., 1995; DiMichele et al., 2017b), and neither the Springfield nor Herrin roof flora has been studied in great depth.

Coal balls have been reported from 19 mines in the Springfield Coal, concentrated in the southeastern portions of the basin (Figure 31), where the coal has been mined most extensively.

Quantitative analyses have been carried out on collections from eight of these mines. In total, we present data from a total of 11 quantitatively analyzed collections. Five of the quantified collections are from areas of the coal bed that formed close to the Galatia paleochannel and other paleochannels contemporaneous with the peat swamps that were active during peat accumulation (Nelson et al., 2020). The coal proximate to the channel is overlain by a gray siltstone deposited under nonmarine to brackish water conditions, based on its invertebrate fauna. No coal balls are known beneath the gray-­shale-­covered part of the coal bed.

Coal balls occur only under black-­shale roof in areas where gray estuarine muds either were not deposited directly upon the peat or where they were eroded during marine transgression, bringing marine muds (black shale) directly into contact with the peat body.

The composition of coal balls from the Springfield Coal is reported by Phillips et al. (1985) in the context of a regional and global summary. Phillips and DiMichele (1998) report coal ball composition through a 1 km-­long transect in the upper bench of split coal of the Peabody Coal Company Eby Pit, east of Evansville in Warrick County, southern Indiana.

Willard et al. (1995) presented a detailed analysis of the Springfield Coal and associated sediments of the Leslie Cemetery channel, also from the Eby Pit. This channel is interpreted as a tributary to the larger Galatia channel, a fluvial feature that existed contemporaneously with Springfield peat formation (Nelson et al., 2020). Palynological studies include those of Mahaffy (1988) and Willard (1993), devoted exclusively to the Springfield Coal, and of Guennel (1952) and Peppers (1996) as part of a larger examination of Pennsylvanian coal palynology. Willard (1993) includes a side-­by-­side comparison of coal ball floras and palynofloras.

As found by Mahaffy (1988), the Springfield shows a large amount of palynological spatial heterogeneity, considerably more than found in the Herrin Coal (Mahaffy, 1985). However, certain generalizations emerge. The Springfield palynoflora is almost always dominated by tree ferns except near the Leslie Cemetery paleochannel, where Lycopora-­producing lycopsids are dominant.

In addition, the overall species richness of the miospore flora is greater near those paleochannel settings where tree fern diversity is higher than it is in areas distant from contemporaneous channel influences. In a separate transect analysis of the Springfield roof-­shale flora proximate to the Galatia channel, DiMichele et

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FIGURE 35. Coal ball profile, Old Ben Mine No. 24. Herrin Coal. Herrin coalface with exposed in situ coal balls at the underground Old Ben Mine No. 24, Benton, Illinois. This is coal ball area “L” of DeMaris et al. (1983) and VS 3 of DiMichele and Phillips (1988). It contained >1,500 m3 of coal balls. Shown here is nearly 4 m of coal containing multiple coal ball layers.

Neither the floor nor the roof units were visible. Color coding identified approximate coal ball zones prior to collecting. The rounded corner was chosen for the location of a vertical section. Coal balls were extracted by digging along coal bands between coal balls. Collectors: Tom Phillips (on left) and Douglas Grierson.

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FIGURE 36. Old Ben Mine No. 24, Benton, Illinois, underground mine coal ball exposures. (a) Herrin coalface in a pillar exposing abundant coal. Coal ball thickness is up to 1.7 m in this mine. Coal ball shapes are variable.

Zones can be identified by tracking them using coal laminae, charcoal layers, or siliciclastic partings in the coal.

Curved scratches in the coalface are gouges created by the rows of tungsten carbide teeth on a continuous mining machine. (b) Coal ball collectors at the Old Ben No. 24 Mine (from left to right): James Mahaffy, Tom Phillips, Richard Harvey, Douglas Grierson, and Bill DiMichele.

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al. (2017b) found that tree fern abundances were greater near, but not immediately adjacent to, the channel and diminished with distance from the channel margin.

Comparison of coal ball biomass (Willard, 1993) to palynoflora percentages reveals the patterns of relative over-­and underrepresentation inherent in both floras. Using the coal balls as a baseline, tree ferns tend to be overrepresented and lycopsids underrepresented in those parts of the Springfield Coal examined

by Willard (1993).

Houchin Creek Coal, Northeastern Illinois The Houchin Creek Coal (the Summum Coal in older literature), also known as the No. 4 Coal in Illinois, is regionally extensive but relatively thin and has been mined in only a few small areas of the Illinois Basin. Coal balls have been collected from a single mine, the Peabody Coal Company’s Northern Illinois Mine, Pit 14, located near Kankakee in northeastern Illinois.

Specimens were collected in the early 1970s during 10 to 12 trips to the mine. Many of the coal balls largely represent single logs or peat nucleated around a log or branch (e.g., Figure 37), rather than masses of purely detrital peat. Over 300 coal balls, found at the breaker (machine used to crush large coal pieces) on the mine site, were examined and it was revealed that most were large logs. The coal balls tended to be disk-­shaped and flattened dorsiventrally.

Where it could be observed, coal balls occurred exclusively in the upper half of the coal bed, above an iron sulfide band in the middle of the coal bed that appears to indicate a change in the chemical conditions of the peat swamp during peat accumulation. Evidence of intermittent exposure of the peat body is suggested by such features as a later-­formed iron sulfide band above a band of charcoal, the latter of which stretches across the coal ball (Figure 38) and is indicative of a fire within the swamp.

Fire horizons indicate periods of lowered water tables, resulting in peat-­surface exposure and drying.

As found in coal balls from some other coals (e.g., from the Murphysboro-­equivalent coal of Indiana, Figure 39) well-­preserved coal ball peat fabric may be surrounded by coal balls that have high concentrations of iron sulfide minerals and are poorly preserved. In other instances, well-­preserved peat may be surrounded by a rim of iron sulfide, as illustrated by coal balls from the Houchin Creek Coal (Figure 40), outside which permineralization continued but with the truncation of some plant material at the iron sulfide rim and with more compaction and poorer overall preservation outside the rim, suggesting a hiatus between the initial episode of permineralization and a somewhat later episode. Factors such as charcoal and iron sulfide, and their differential distributions within the coal bed, or even within a single coal ball or coal ball zone, are indicators of the environmental dynamism under which peat accumulation took place. This dynamism includes flooding (during which iron and sulfur from ground water may have been introduced to the swamp), lower water tables (different degrees of peat decay and shrinkage/compression), fires (charcoal layers), and windstorms

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(reflected in nearly monospecific accumulations of branches and stems associated with limited rooting).

Murphysboro-­Equivalent Coal, Maple Grove Mine, Illinois Basin During the early 1920s, A. C. Noé collected coal balls at the Silver Island strip mine near Cayuga in west-­central Indiana (Feliciano, 1924). Coal balls were rediscovered decades later by Russel A. Peppers and Hermann W. Pfefferkorn in the Maple Grove Mine, near the original discovery location. According to the palynologically based, biostratigraphic work of Peppers (in Eggert and Phillips, 1982), the main coal ball source is an unnamed coal in Indiana, approximately equivalent to the Murphysboro Coal of southern Illinois, which is of early Desmoinesian age. A coal bed two cycles above the Murphysboro (equated by Eggert and Phillips (1982: 23) with the Wise Ridge Coal of southern Illinois on the basis of its palynology and stratigraphic position) was also a minor source of coal balls at the same mine. Interestingly, in southern Illinois, in the type area of the Murphysboro Coal, no coal balls have been reported from the main coal bed, likely because it is under a gray-­shale, nonmarine roof rock, presumably derived from the nearby Oroville paleochannel. Coal balls occur, and a vertical profile has been collected and analyzed (but not reported in the literature) from the presumed Mount Rorah Coal, which is located stratigraphically between the Murphysboro Coal and Wise Ridge in southern Illinois. The Murphysboro-­and Wise Ridge–equivalent coals in western Indiana were very similar to one another in the composition of both their coal balls and palynology. The collection from the Mount Rorah Coal, although with many distinct compositional similarities to the Indiana coals, is more diverse and includes elements not seen in them, possibly reflective of the different locations in the Illinois Basin, the center as opposed to the eastern periphery.

Coal balls in the Maple Grove Mine occurred in masses (e.g., Figure 39), spatially distributed in roughly parallel linear arrays. This was established after the coal ball collections were made by core drilling carried out by the Indiana Geological Survey (Eggert and Phillips, 1982). As the overburden was removed with draglines, these masses of coal balls were revealed.

They occurred in the upper one third to one half of the coal bed, which reached a total thickness of 222 cm. In areas where the bed thinned, however, the proportion of the bed occupied by coal balls increased to up to two-­thirds of the total thickness. Bed thicknesses in areas lacking coal balls, but adjacent to the coal ball masses, were as much as 1.5 m. Individual coal balls were mostly 5–10 cm thick and markedly rounded. They occurred in densely packed aggregates (Figure 41).

Coal ball peat containing marine invertebrates occurred at the top and in several coal ball zones as deep as 30 cm into the bed (Figure 42). In these latter instances, marine sediment was present within the peat, indicating introduction of marine waters/sediments into the peat swamp during and/or after peat accumulation.

In the more typical cases, found in coal balls from a number of

FIGURE 37. Houchin Creek coal ball cross sections. Diaphorodendron phillipsi compressed logs. Log-­sized litter is common in coal balls from this location, perhaps representing storm-­damage blowdown or perhaps log jams in swamp drainages. (a) Log in which xylem cylinder is broken into pieces following decay and compaction. University of Illinois Coal Ball Archival Sheet 0299. (b) Longitudinal section through the periderm, demonstrating its bizonation, divided into an outer, lighter colored phellem and darker, interior phelloderm. This section is turned sideways. University of Illinois Coal Ball Archival Sheet 0340. Houchin Creek Coal, Peabody Northern Illinois Mine, Pit 14, South Wilmington, Ilinois. Upper Middle Pennsylvanian.

FIGURE 38. Houchin Creek coal ball aggregate. (a) Three compositionally and preservationally distinct zones, numbered I, II, and IIII. Zone II contains abundant charcoal, likely indicative of fire during a dry season or interval. Charcoal is well preserved. Charred remains include the lycopsid Diaphorodendron phillipsii, enlarged in (b). Above this fire layer is a dark zone rich in iron sulfides, which was first noticed on the coalface as a distinct midbed band. Houchin Creek Coal, Peabody Northern Illinois Mine, Pit 14, South Wilmington, Illinois. Upper Middle Pennsylvanian. University of Illinois Coal Ball Archival Sheet 0466.

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FIGURE 39. Murphysboro-­equivalent coal ball aggregate. This example demonstrates the relative timing of coal ball carbonate precipitation and the formation of the iron sulfides pyrite and marcasite. Most of the iron sulfide here appears to have formed around the carbonate concretions. This suggests that the coal balls had formed prior to most of the iron sulfide formation, which occurred in areas of as-­yet unpermineralized peat. The aggregate also preserves two visually distinct coal ball layers. Coal balls in the lower half are compositionally similar, relatively detrital, and weakly compacted. Those in the upper half are composed only of Diaphorodendron periderm and wood, are poorly preserved, and the iron-sulfide-­rich peat flanks the better preserved carbonate coal balls. University of Illinois, Coal Ball Archival Sheet 0229, Maple Grove Mine, Cayuga, Indiana. Murphysboro-­equivalent coal, Middle Pennsylvanian.

different coal beds, marine mud in the peat is the result of post-­ accumulation burrowing by marine organisms following marine flooding of the peat-­swamp surface. Such burrows indicate that the peat was still soft and penetrable at that time. Iron sulfides are high in this high-­sulfur coal. As reflected in coal balls, some of this sulfur may have originated during the influx of marine waters, both during and after the cessation of peat accumulation.

Iron sulfides surround many well-­preserved coal balls from the Murphysboro-­equivalent coal but are only rarely found within the peat fabric (Figure 43); this suggests that waters enriched in iron,

and with sufficient sulfur to form pyrite and/or marcasite, infiltrated the peat bed around coal balls that were already present. In such instances, it appears that coal ball formation may have been interrupted and stopped by iron sulfide precipitation. In other, less common instances, iron sulfides are dispersed within the coal ball peat fabric. Peat is generally poorly preserved in coal balls with dispersed iron sulfides; iron sulfides may be found surrounding coal balls of nonpyritic/marcasitic character with excellent preservation of the peat fabric. This latter pattern indicates two distinct intervals of coal ball formation; the initial preservation is good but

FIGURE 40. Houchin Creek coal ball aggregate. Coal balls are surrounded by iron sulfide rims (white arrows), which seem to have interrupted their formation, meaning a temporary cessation of carbonate precipitation within the uncompressed peat fabric. Less concentrated iron sulfide is present in what appear to have been voids in the peat fabric; its formation is likely to have been independent of the iron sulfide rims surrounding the spheroidal coal balls in the upper right of the image. Two or perhaps three layers of coal balls appear to be present. Each is preserved differently with regard to decay and the amount of iron sulfide that infused the peat, possibly at different times during permineralization. Diaphorodendron phillipsii remains are relatively well preserved here; three small twigs can be seen (labeled at right, unlabeled on the left) surrounded by lycopsid leaves in a manner suggesting attachment. University of Illinois, Coal Ball Archival Sheet 0521. Houchin Creek Coal, Peabody Northern Illinois Mine, Pit 14, South Wilmington, Illinois. Upper Middle Pennsylvanian.

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FIGURE 41. Murphysboro-­equivalent coal ball aggregate. Note the difference in coal ball shape between the top, platelike layer and the multiple layers of rounded coal balls. Note also the rotated coal ball, second from left, showing that permineralization occurred while peat was still soft.

A question arises regarding the timing of peat permineralization: Was the coal ball rotated after formation, or was the peat fabric disrupted prior to coal ball formation, perhaps by upward fluid flow? Middle Pennsylvanian age. Indiana. University of Illinois coal ball specimen 40708A.

in later-­formed iron-­sulfide-­rich coal balls, preservation is generally poor, and organic matter may even be lacking entirely.

Quantitative analyses of coal balls (Phillips et al., 1985; Di­ Michele and Phillips, 1995) from both the Murphysboro-­equivalent and Wise Ridge–equivalent coals showed the peat to be dominated by the lycopsid tree Diaphorodendron vasculare. Almost no other lycopsid tree remains were identified during the analyses (in the Mount Rorah Coal in southern Illinois, Lepidophloios sp. also was common). There also was a subdominant element of cordaitalean gymnosperms, which added significant volumes of wood to the peat. As with the lycopsids, the cordaitaleans were entirely of a single type that produced ovules attributable to Cardiocarpus spinatus, which Costanza (1985) assigned to the genus Pennsylvanioxylon.

Mineral and Fleming Coals, Kansas, U.S. Midcontinent Sporadic masses of coal balls occurred in the middle Desmoinesian-­ age Mineral Coal and especially the overlying Fleming Coal, at the Pittsburg and Midway Coal Company Mine No.19 near West Mineral in Cherokee County near the southeastern corner of Kansas. Coal balls were so abundant that the mine stockpiled them for use as road ballast (Figure 44a). The “West Mineral” locality became well known among paleobotanists as a major source of material for studies of coal swamp plants from the Midcontinent (Western Interior) coal region, especially by scientists at the University of Kansas (see Baxter and Hornbaker, 1965). These coal balls could be collected only as random samples; consequently, their location within the coal bed or relative to one

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FIGURE 42. Murphysboro-­equivalent coal ball aggregate from top of coal. Several distinct zones of coal balls are intersected by peat with marine organisms and highly degraded plant remains. Marine element: ostracod valves, gastropods, and brachiopod shells and spines. “Normal” coal balls contain no marine animal remains and no iron sulfides. The sharp boundary (dashed white line) between normal coal balls and mixed marine peat suggests formation at different times; normal coal ball formation likely had already occurred when marine material was introduced into the peat via burrowing, after flooding of the peat swamp by normal marine water. Plant litter is mostly Psaronius tree fern remains including frond parts, Scolecopteris fertile pinnules, and outer roots. University of Illinois, Coal Ball Archival Sheet 0274, Maple Grove Mine, Cayuga, Indiana. Murphysboro-­equivalent coal, Middle Pennsylvanian.

another in profile context is unknown. The availability of West Mineral coal balls attracted people with a variety of purposes, including avocational paleontologists as well as professionals. A memorable sight at the West Mineral coal ball stockpile involved college students and teachers hefting coal balls in order to separate the heaviest (pyritic/marcasitic) from the ostensibly “normal” coal balls. Others searched over the sun-­bleached coal-­free surfaces to find evidence of invertebrate fossils.

Lenticular shapes of many of the coal balls (Figure 44b) are evident, along with mostly coal-­covered exterior surfaces. Their

lenticular shape is assumed to result from vertical compaction, reflecting peat decay before permineralization, resulting in flattening parallel to the horizontal banding of the coal. The laminae in the coal then follow the contours of the concretions (Mamay and Yochelson, 1962: 203). Broken coal ball faces from mining and haulage often have iron stains from the weathering of iron sulfides. Many of these coal balls have a particularly short shelf life, especially after they have been cut and processed, which leads to rapid oxidation of the pyrite/marcasite, formation of sulfuric acid, and degeneration of the coal ball (and any storage

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FIGURE 43. Murphysboro-­equivalent coal ball cross section. Differential timing of the formation of iron sulfides and calcite. Iron sulfides are differentially concentrated in an extended “pocket.” The iron sulfide, although dispersed through some denser tissues, is also concentrated on surfaces surrounding cavities, interior surfaces of stigmarian-­rootlet cortex, or around outer surfaces of the same rootlets. This coal is characterized by high abundances of the lycopsid Diaphorodendron and the cordaitalean Pennsylvanianioxylon. University of Illinois, Coal Ball Archival Sheet 0520, Maple Grove Mine, Cayuga, Indiana. Murphysboro-­equivalent coal, Middle Pennsylvanian.

materials in contact with it); for extended storage, we preserve processed iron-­sulfide-­rich coal balls by applying several coats of marine spar varnish, which itself has limited shelf life.

In a study of marine animal fossils in American coal balls, Mamay and Yochelson (1962) documented direct evidence of marine sediment mixed with permineralized coal ball peat, which they termed a “mixed coal ball.” Mixed coal balls have similar sizes and shapes to normal coal balls and thus often cannot be identified from their external features. A census of coal balls spread about on the grounds of the Pittsburg and Midway No. 19 Mine (Mamay and Yochelson, 1962:201) concluded that

1–2% of the concretions were mixed. Although lack of access to in situ coal balls prevented observations of exactly how the mixed specimens were distributed in the coal bed, it is likely they occurred at or near the top of the coal bed, based on where this kind of mixed-­marine coal ball is found in other coal beds (Eggert and Phillips, 1982). It is almost certain that the marine component of some (if not most) of these “mixed marine” coal balls was actually introduced to the peat by burrowing invertebrates that lived above the peat surface in marine waters that had flooded the peat swamp; Mamay and Yochelson (1962) argued against this interpretation. The Mamay and Yochelson collection

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FIGURE 44. Pittsburg and Midway Coal Co. Mine No. 19 near West Mineral in Cherokee County, southeastern Kansas. (a) A huge black pile of coal balls can be seen at the top center and right. These were being saved for use as road ballast. Robert W. Baxter (then professor at the University of Kansas, Lawrence) stands on top of the pile. The Mineral (older) and Fleming coals were often mined in the same mines. Coal balls were found in both coals and were concentrated in the same piles. Because most of the collecting came from these piles, coal balls from the two beds could not be discriminated. (b) A closer view of coal balls from the ballast pile, mostly still covered with a thin coal rind. Broken faces are the result of mining and haulage, and orange iron stains are from iron sulfide weathering. Mineral-­Fleming coal balls are generally very rich in iron sulfides.

They store well until processed (cut), after which they often undergo rapid oxidation and the iron sulfides form sulfuric acid, resulting in degradation.

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is housed at the National Museum of Natural History, Smithsonian Institution, in Washington, D.C. Mixed marine coal balls were considered by Mamay and Yochelson (1962: 216) to be relevant to the matter of coal ball formation. They envisioned the periodic invasion of the peat swamp by marine waters on a transitory basis, such as storm surges, accompanied by percolation of those waters downward through the peat, bringing along calcareous fluids. In their model, these intermittent incursions of marine water are pictured to have stimulated normal coal ball development before final marine inundation. Mamay and Yochelson (1962: 216) concluded, “The presence of mixed coal balls within the seams constitutes the only physical evidence of the marine invasion.” However, they had no evidence of the actual location of these mixed coal balls within the coal bed. Their research suggests that there ought to be some direct evidence of marine influence from mixed coal balls near the top of the coal bed if marine burial of the swamp has caused coal ball formation.

Collecting coal balls at West Mineral depended very much on the collector’s purpose—­ours was the assembly of a random sample. Although the exact provenance of the coal balls is in doubt, field observations and reports by mine employees indicate that most West Mineral specimens are from the Fleming Coal.

There, the coal balls were more abundant in the upper half of the bed but also occurred throughout the bed in some places (Mamay and Yochelson, 1962). According to mine personnel, the Fleming Coal undulated; the coal thinned on rises and coal balls tended to be concentrated there.

With the above caveats, the West Mineral locality is notable because a random sample of coal balls indicates that woody cordaitalean remains exceed in abundance the remains of lycopsid trees, even though the latter are notable for their abundantly produced, decay-­resistant periderm tissues. This observation is noteworthy because during the Pennsylvanian there were dramatic changes in the vegetation that dominated Middle and Late Pennsylvanian peat swamps (Phillips et al., 1974; Montañez, 2016; DiMichele et al., 2023b), particularly at the transition from the Middle to the Late Pennsylvanian. This latter vegetational change was first detected by White (in White and Thiessen, 1913) on the basis of adpression floras, and later by Kosanke (1947)

from palynomorphs. It involved a major reduction, likely a complete extinction west of the Appalachian Mountains, of the previously dominant lycopsid trees (except Sigillaria) and many other plants. After a transition interval, tree ferns became dominant in peat-­swamp habitats (Peppers, 1985, 1996, 1997; Phillips et al., 1985; Kosanke and Cecil, 1996; DiMichele and Phillips, 1996b; Falcon-­Lang et al., 2011a). Concurrently, based on the relative sizes of fossils, the trees of many groups became larger in the Late Pennsylvanian. Coal balls, and compression–impression floras indicate another, temporally extended turnover through the early Desmoinesian within the Middle Pennsylvanian, characterized particularly by the great diminishment of cordaitaleans and rise in abundance and diversity of tree lycopsids and marattialean tree ferns (Pfefferkorn and Thomson, 1982; Phillips and Peppers, 1984; Phillips et al., 1985; van Hoof et al., 2013; Bashforth et al., 2016; Montañez, 2016; DiMichele et al., 2023b). The Mineral and Fleming coals fall within this transition period.

Iron Post Coal, Peabody Rogers County Mine No. 2, Vinita, Oklahoma, U.S. Midcontinent The Peabody Coal Company’s Rogers County Mine No. 2 (Pit 291) near Vinita, Oklahoma, was the largest surface coal mine in the state during the early 1980s. Coal balls were collected from a large mass in the Iron Post Coal bed, which belongs to the minor Breezy Hill cyclothem overlying the Bevier and underlying the Lower Fort Scott cyclothems (Heckel, 2013), of Desmoinesian age (Perkins, 1976). The correlative position in Illinois is above the Survant Coal and beneath the Houchin Creek Coal, possibly correlative to the highly lenticular, channel-­form Cardiff–Roodhouse–Kerton Creek coal complex of western Illinois (Calder et al., 2023). The Rogers County Mine was extracting coal from a relatively thin Iron Post Coal, under relatively shallow surface cover (overburden), across a large area (Figure 45).

After driving through much of the mining operation with the superintendent, we got out and asked if there was an outcrop of the coal that could be examined. The superintendent quipped, “You’re standing on it” and it happened to have coal balls (Figure 45a, within dashed-­line box, b). The area in which coal balls occurred in this mine, fortunately, was out of the way of active

FIGURE 45. (Opposite) Western Interior coal mines extracting coals of Middle Pennsylvanian age. (a) “Paul’s Pride” shovel at the Peabody Coal Company Rogers County No.2 Mine, Pit 291, Vinita, Oklahoma. Iron Post Coal, location of Iron Post VS 2. The use of gigantic electric stripping shovels and draglines in strip-­mining peaked between the 1950s and 1980s. In more recent decades, fleets of more versatile scrapers and bulldozers have taken their place. (b) Enlargement detail from dashed-­line box in part (a): in situ coal ball zones of Iron Post VS2, which extends through the entire bed thickness. Coal balls were first reported from the Iron Post Coal by Aureal T. Cross (1969) and have been studied by Thomas Perkins (1976). (c) University of Iowa Star Mine near Oskaloosa, Iowa. The overburden has been removed from the top of the Buffaloville-­equivalent coal by the shovel (upper right), revealing coal balls. Pictured (from left to right) are Tom Phillips, Matthew Avcin, James Mahaffy, and Penelope Avcin. (d) James Mahaffy. (e) ICO Mine, Oskaloosa area, Iowa. A massive thickness of highly compacted and permineralized peat occupies most of the bed thickness (1, 2) over most of the exposed pit area. Coal balls may be more abundant in Iowa than any other American state. Several of the Coal Project cores drilled by the Iowa Geological Survey during the 1970s encountered coal balls.

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FIGURE 46. Coal ball cross section with an oblique section through a marine-­sediment-­filled burrow within a cordaitalean leaf mat. The burrow crosscuts layers of Cordaites leaves and is infilled with marine sediment. Note the sharp boundary between the burrow wall and Cordaites leaves. This coal ball was collected from the uppermost coal ball zone of a vertical profile. It is likely, therefore, that the sediment, and its accompanying marine shelly debris, were introduced after marine waters had flooded the peat swamp following sea-­level rise during transition from a glacial to an interglacial interval. Iron Post VS 2, Zone 3, 24304, Peabody Rogers County Mine No. 2, Welch, Oklahoma. Iron Post Coal, upper Middle Pennsylvanian.

mine machinery. Thus, we were permitted to stay in the mine after the normal shift change. In an effort to finish the collecting while we had permission to carry out the work, and while the weather was cooperative, we continued to collect coal balls well into the evening, after sunset. This required that we move the vehicle, a University of Illinois motorpool Chevrolet Suburban, to within a few meters of the coal ball exposure, to illuminate the outcrop with the headlights. Successful fieldwork, especially in an area with changeable weather and access conditions, often requires that one follow the maxim “don’t put off until tomorrow what you can do today”—­or tonight!

The Iron Post coal ball collections were made in only one area of the mine. Because of the thinness of the coal bed, the profiles comprised only three to four zones (Figure 45b). These zones are interesting compositionally, with animal burrows in the upper peat deposits (Figure 46). Despite its stratigraphic position below the Houchin Creek Coal, coal ball peat from the Iron Post Coal has many compositional similarities to the Springfield and Herrin Coals, leading us, initially, to place it stratigraphically above the Houchin Creek (DiMichele and Phillips, 1995). As can be seen in the figures, however, the Iron Post peat is enriched in cordaitaleans, which makes it similar to other coals in the same

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time interval, illustrated by Phillips and Peppers (1984) as their second cordaitalean-­rich interval, based on coal balls and coal palynology.

Cliffland/Blackoak Coal, Oskaloosa, Iowa, U.S. Midcontinent Many coal mines formerly operated in the area of Oskaloosa, Iowa. The coal beds occur stratigraphically very near to the Atokan-­Desmoinesian boundary and have been tentatively identified as either the Blackoak or Cliffland Coals (Raymond et al., 2010). Thus, these coals are within the early Desmoinesian transitional interval from cordaitalean enriched coal swamp floras to swamps dominated by lycopsids, tree ferns, and medullosans, with cordaitaleans reduced to a miniscule presence.

We collected coal ball profiles from the University of Iowa’s Star Mine; this collecting was particularly difficult because it required digging straight down through the coal ball deposit from the top (Figure 45c,d). Compositionally, Star Mine coal balls proved to be dominated by a diversity of cordaitaleans, mixed with a variety of lycopsids.

At the ICO Mine in the Oskaloosa area, we encountered an exceptional permineralized peat deposit, one that had replaced virtually the entire coal bed. Drilling by the Iowa Geological Survey had extracted core from such a deposit, but this was our first look at this kind of permineralization in the field.

It proved, however, not to be a typical early-­peat-­stage coal ball layer. Rather, permineralization had not occurred until after considerable diagenesis and alteration of the peat had taken place, including advanced decay and compaction of the organic matter.

Consequently, the permineralized peat appeared to be in a lignitic diagenetic stage. We had earlier found broken pieces of this compacted, permineralized lignite at a gob pile (waste material resulting from coal mining) near Lovilia, Iowa, thus not in situ.

However, one specimen captured our attention because a typical early-­peat-­stage coal ball was embedded within the larger, later compacted and permineralized lignitic peat (Figure 45e). This mass of permineralized peat thus preserved two quite separate stages in the process of peat diagenesis and documents widely separated times of permineralization.

Friendsville Coal, Allendale, Illinois, Illinois Basin The palynological characterization of vegetational turnover between the Middle and Late Pennsylvanian led to a search for Upper Pennsylvanian coal ball occurrences in order to test the hypothesis of a Middle–Late Pennsylvanian biological turnover event. Hermann Pfefferkorn and Roger Nance, then of the Illinois State Geological Survey, brought in several boxes of coal ball material from the Upper Pennsylvanian Friendsville Coal.

This came from the closed Allendale Coal Co. Mine near Mount Carmel, southeastern Illinois, where a 20 m2 mass of in situ coal ball material about 1 m thick had been discovered (Figure 47a).

The coal ball mass rested directly on the underclay and had no

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coal above it; consequently, it was unclear if coal had been removed or never was present above the coal ball mass—­the upper surface of the exposure was weathered. Upon cleaning up the outcrop, the surface was marked off with a square-­meter grid.

Some huge Psaronius tree-­fern trunks were visible in upright position (Figure 47b), each occupying adjoining square-­meter grid cells. Our sampling began with an attempt to dry-­core drill each grid cell for a profile, which failed due to structural weaknesses in the coal ball mass, mainly along coal partings between successive coal ball zones. We then decided to collect as much of the outcrop as possible. In the course of this collecting, not only were the large Psaronius trunks further revealed, but a permineralized Calamites stem, 45 cm in diameter, was found; this stem was, at the time, the largest known Pennsylvanian Calamities, based on reports in the literature (a still larger calamitalean stem, 60 cm in diameter, has since been reported from the lower Permian of Germany [Rößler and Noll, 2006]). The Allendale specimen slanted downward to the base of the coal ball mass. Quantitative analysis of the Friendsville coal ball assemblage was consistent with the palynological findings of major differences between Middle and Upper Pennsylvanian coal composition (Willard and Phillips, 1993).

The Allendale coal ball mass presents several challenges to our models of peat accumulation. Most obvious of these are when and how the mass of Allendale peat was permineralized, entombing the large tree ferns and calamitalean tree upright in situ. The coal has a marine roof, which is consistent with other coal ball occurrences. The presence of large, upright trees suggests that permineralization occurred after peat formation had ceased. However, we do not know the extent of the original peat bed that was permineralized, or whether the coal ball mass represents all, most, or only a portion of the original peat body.

If permineralization occurred during peat accumulation, even if restricted to a brief episode, the matter of the chemical environment of the peat body also must be considered, given that CaCO3 cannot precipitate under low pH conditions.

More problematic is the large size of the tree ferns and calamitalean and, even more importantly, that they were upright and extended through the entire coal ball mass to the bottom of the coal bed. These observations have important implications for plant life-­spans and/or rates of peat accumulation at this site. Even if Psaronius and calamitaleans had life-­spans in the hundreds of years, peat accumulation rates must have been very high, particularly given indications of normal processes such as decay, fire, and compaction revealed within the other layers of coal ball peat extracted at this site from areas immediately surrounding these large tree stumps. Furthermore, the standing plants must have survived the various disturbances revealed by the composition of the surrounding peat, especially the swamp fires indicated by the presence of charcoal. Could rates of Pennsylvanian peat accumulation have been an order of magnitude higher than imagined, in the hundreds of years per meter, which might lead to conclusions about extreme rates of plant productivity (e.g., Cleal and Thomas, 2005)? Was this a low spot in the

FIGURE 47. The Allendale Mine coal ball locality in the Upper Pennsylvanian Friendsville Coal near Mount Carmel, southeastern Illinois. (a) Tom Phillips standing on weathered coal ball mass (brown) exposed from mining in the Friendsville Coal, Allendale Mine in 1976. This coal ball occurrence was discovered by Roger Nance and first collected by Hermann W. Pfefferkorn, then later passed along to Phillips. This proved to be a good test for the proposed absence of lepidodendrids in Late Pennsylvanian coal swamps. The rutted, near-­vertical path to the left was a track used by dirt bike riders. (b) Allendale Mine coal ball exposure prepared for coring analysis. Meter-­square grids were outlined and numbered for dry coring (device at left with unidentified individual) and extraction. Grid cells 6 and 7 were each fully occupied by a Psaronius tree fern trunk. Because of the larger sizes of many Late Pennsylvanian coal swamp trees, especially the tree ferns, ecological sampling required collection of a large portion of the outcrop. Collector: James F. Mahaffy (kneeling).

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peat swamp, attracting floating litter, and accumulating peat at a more rapid rate than elsewhere in the swamp? Or might these plants have had enormously longer life-­spans than have been imagined? Given that we have encountered no other coal ball occurrence similar to this one, with large upright trees extending through an entire coal ball mass, perhaps the Allendale Mine occurrence represents some sort of unusual conditions, and its puzzles are not relevant to more typical coal ball occurrences where the coal clearly evidences hiatuses in accumulation (such as mineral bands).

An alternative possibility, given that the bases of these trees rested on the coal underclay, is that they were dead, upright, and perhaps even partially permineralized at the time peat accumulation began around them. This explanation may seem unlikely, but it is no more unlikely than fantastically rapid rates of peat accumulation or exceptionally long life for the tree ferns and calamitalean preserved in the Allendale coal ball mass. Although rare, Pennsylvanian-­age coniferalean trees, plants with woody trunks, have been found buried upright in carbonate/gypsum dunes of Late Pennsylvanian age in New Mexico (Falcon-­Lang et al. 2011b, 2016; Elrick et al., 2017b); these tree remains were silicified and rooted in limey mud beneath the dunes, likely a coastal deposit, possibly a lagoon transgressed by landward-­ migrating dunes. In another instance, from the Middle Pennsylvanian of Iowa, cordaitalean stems were found rooted in the paleosol below a coal bed, but projecting into the overlying coal (Nelson et al., 2022); these woody trunks were permineralized by CaCO3, yet resisted dissolution when peat accumulated around them.

The Friendsville coal ball outcrop was at the bottom of a pit, bordered by a highwall (cliff) on one side and a small stream on the other. We had most of the flat dry land covered with collections, bags and equipment strewn around the outcrop. You can imagine our surprise one morning when a group of off-­road motorcyclists suddenly roared down the cliff face, splashed across the water, and zoomed out of sight! You can see the rutted pathway to the left of the outcrop in Figure 47.

et al. (2007). The coal ball exposures occur along creeks in Lawrence (Berryville locality) and Richland (New Calhoun locality) Counties. As with the Allendale locality, the analysis of both vertical profiles and extensive random samples from the Calhoun Coal is consistent with the findings of a major plant turnover between the Middle and Late Pennsylvanian (Willard et al., 2007), between which is a short, floristically transitional interval identified only palynologically (Peppers, 1985, 1996). The Calhoun Coal flora is dominated by tree ferns and pteridosperms, with some zones in some profiles dominated by Sigillaria.

The coal ball aggregates at Berryville (Figure 49) were massive with few thin coal partings between zones. At New Calhoun, the exposures were more extensive and variable in the degree of massiveness. We collected vertical profiles at both sites.

The coal ball mass in the center of Figure 49a (Berryville VS 2) has been zoned with spray paint and is ready to be collected.

The massiveness of the coal ball exposures at Berryville (like the exposure at Allendale mentioned above) challenged collectors to remove and disaggregate them, a process that, at times, involved dynamite, jackhammers, and bulldozers, as well as the usual hand tools. During one visit to the Berryville locality, before the days of profile collecting, a huge coal ball aggregate was uncovered by a bulldozer and then tied with a winch line to drag it away from the creek (Figure 49b). The mass was so heavy that at first, the bulldozer was pulled toward the outcrop.

It had to be braced behind the nearest large tree to move the coal ball aggregate across the creek. Early in the process of collecting coal balls at Berryville, one paleobotanist offered to pay the property owner for access, setting an unfortunate precedent that lasted for many years. During our last visit, in the 1990s, this requirement no longer applied, due to the generations of both property owners and coal ball collectors having turned over sufficiently, in addition to a declining interest in collecting at this site.

Calhoun Coal, Berryville, and New Calhoun Locations, Illinois, Illinois Basin

We have examined here the nature of an unusual form of fossil-­ plant preservation, and occasionally animal preservation: objects generally referred to as “coal balls” in the scientific literature, because of their shape and occurrence in beds of coal. This form of preservation is unique to the late Paleozoic. Coal balls have been, for nearly 200 years, a principal source of an entire field of study—­that of the anatomy, morphology, developmental biology, physiology, and phylogenetics of late Paleozoic wetland plants.

In more recent years, they have been foundational in the study of paleoecology, both of individual plant taxa and of whole ecosystems, the latter particularly linked to the glacial–­interglacial cyclicity and accompanying climatic changes that occurred during the Late Paleozoic Ice Age. The combination of systematics and paleoecology has provided many insights into ecosystem responses to changing climate and other large-­scale disturbances to terrestrial environments.

The Calhoun Coal is of youngest Virgilian age and is the youngest coal in the Illinois Basin known to have CaCO3 coal balls (Virgilian coal balls occur at several localities in Effingham County, Illinois, but these are composed of silica). Coal balls from the Calhoun Coal are among the best studied American specimens and are the best known from the Upper Pennsylvanian. Collecting and studying of the site began in the 1930s (Figure 48), with publications soon following (e.g., Schopf, 1941).

There is an extensive literature examining taxonomy and morphology of plants from the two main collection sites, Berryville and New Calhoun (also known as Calhoun North), much of which is summarized by Phillips (1980) and in a quantitative study of coal balls and palynomorphs from the coal by Willard

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Study of coal balls is multifaceted, requiring familiarity not only with botanical sciences, but also with several aspects of geology, such as stratigraphy, sedimentology, and geochemistry. This study begins with fieldwork and specialized methods of collecting specimens, followed by laboratory preparation of the specimens, from lapidary methods to the microscope. This is followed by the need to store and retrieve original specimens or their accompanying information, basically a system of collections

management. These various aspects have been covered here in some detail both to facilitate understanding and advise those seeking to undertake coal ball study.

The summaries of coal ball occurrences and a brief look at some of the adventures that accompanied them provide a general sense of what such work entails. There are, of course, many additional stories to be told, some unremarkable, some hair-­raising.

They include such things as standing on the roof of the vehicle

FIGURE 48. Berryville, Illinois, coal ball locality, ?1930s, about 20 km southeast of Olney and 20 km west-­northwest of Allendale. Coal balls cropped out along a stream bank on what was then the Ralph Bryan farm. From left to right are collectors James M. Schopf, Robert W. Baxter, and an unidentified individual.

FIGURE 49. (Opposite) Berryville locality, Calhoun Coal, Illinois, Upper Pennsylvanian. (a) Photograph from 1990 of an outcrop of coal ball mass in a stream bed. Zones have been spray-­painted. Profile collecting here required careful extraction because there was often only a very thin coal layer between adjacent zones and preventing overlap required caution. Collecting, therefore, was carried out with small hand tools, mainly rock hammers, crack hammers, and chisels. (b) Collecting coal balls from the creek-­bed locality near Berryville, Illinois. At the lower left is a massive coal ball aggregate being winched away from the outcrop by a bulldozer.

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looking for diesel plumes from bulldozers to locate mines, to being surprised underground by blasting in mines still employing so-­called “conventional” mining methods (undercutting the bed with a chainsaw-­like device, followed by fracturing the coal with compressed air). Challenging working conditions are common, such as cramped working spaces underground created by thin coal beds (“low coal”), surface-­mine mud that one steps in to find oneself without warning at midcalf depth, stuck vehicles, or needing gang labor to haul bags of coal balls up a mine highwall from an abandoned pit below.

We have encountered and collected coal balls where and when we find them. Recent collections were made from the Mount Rorah and Pirtle coal beds in Indiana and Illinois respectively, both encountered by happenstance during studies of those coals and associated adpression fossil floras. The Pirtle is the youngest Middle Pennsylvanian coal bed so far known to have coal balls, and thus important in evaluating the lead-­up to the massive Middle–Late Pennsylvnaian floristic turnover. Findings such as these, from coal beds never before known to have had coal balls, suggest that there is still much more out there to find and much left to learn. Only continued fieldwork can reveal such things, however. If the past is any indication, studies of coal ball paleobotany will continue to have their ups and downs, with new ideas bringing successive reawakenings to the potential held by these unique fossils to inform our understanding of the dynamics of the Pennsylvanian world and our world in general.

ACKNOWLEDGMENTS Our work has been materially assisted by classes of undergraduate students, coal company employees, private property owners, scientists from geological surveys, and university faculty and graduate students, from organizations across the midwestern and eastern United States. Without such expertise, access to the mines and natural exposures would have been and would still be impossible—­ this is an important part of collecting and processing that must always be kept in mind. Without the crucial assistance from the geologists who actively study the stratigraphy and sedimentology of the strata that include the coals of interest, we would, in many instances, have only limited understanding of the stratigraphic provenance of our samples. Although we have not emphasized this kind of collaboration in the descriptions and stories above, it should be taken as given that many other people assisted us in getting to and in interpreting the exposures we investigated.

We would especially like to thank James F. Mahaffy, Debra A. Willard, Suzanne H. Costanza, Richard B. Winston, Philip J. DeMaris, Donald L. Eggert, Mark H. Scheihing, Hermann W. Pfefferkorn, Russel A. Peppers, James M. Schopf, and Aureal T. Cross for their insights, fellowship, and material assistance at various times over the past 60 years.

This paper has been substantially improved by the comments and suggestions of reviewer Hermann W. Pfefferkorn and an anonymous reviewer. We thank them sincerely.

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Index

Air Quality Mine, Indiana, 23 Alabama, 13 Alleghenian orogeny, 12–13, 16, 17 Allegheny Formation, 12 Allendale Coal Co., Friendsville Coal, Illinois, 63–65 AMAX Chinook Mine, Survant Coal, ­Indiana, 21 AMAX Coal Company Delta Mine, 40, 45 American Coal Company Galatia Mine, 8, 19, 25, 45 analysis of coal balls objectives of, 18, 32 quantitative analysis of coal balls, 32–35 Ancestral Rocky Mountains (ARM), 3, 16 Anthracite coalfields and region, 13, 14, 15 Appalachian coalfield location and geology of, 10, 12–16 marine elements and strata, 15–16 Appalachian Mountains, 12–13, 60 Appalachian orogen, 16, 17 Arkansas, 10, 16–17 Arkoma Basin, 17 Atokan Minshall Coal, 39 Atokan Stage/age, 2, 10, 12, 13, 14, 16, 18, 19, 38, 63 Australia, 38 Avcin, Matthew, 60–61 Avcin, Penelope, 60–61

Baxter, Robert W., 66 Belgium, 38 Berryville, Calhoun Coal, Illinois, 19, 21, 65, 66–67 bituminous coal, 16, 17, 38 Blackoak/Cliffland Coal, Oskaloosa, Iowa, 60–61, 63 Black Warrior Basin, 13 Bloomfield Mine, Iowa, 39

Calhoun Coal, Berryville, and New Calhoun Locations, Illinois, 19, 21, 65, 66–67 Canada, 2 Cantabrian Stage, 2 Carboniferous Period, 1, 10, 38 Cardiff-Roodhouse-Kerton Creek coal ­complex, 60 Cathaysia, 2, 3, 4 Cedrum Mine, Alabama, 13 charcoal layers, 9, 19, 24, 45, 50, 51, 53, 63 Cherokee Group coal, 17 Cherokee Platform, 17 China, coal ball occurrences and collection in, 10, 38, 39 Cliffland/Blackoak Coal, Oskaloosa, Iowa, 60–61, 63 climate drier climates and coal and coal ball formation, 14–15, 16 fluctuations and changes in, 2, 3, 4, 18, 42, 65 coal composition of, 1 geology of American coalfields, 10, 12–18 Industrial Revolution and use of as fuel, 1 nonmarine strata in coal beds, 10 origins of, debate around, 1 peat swamps and lowlands and formation of, 3 position and density in coal balls in, 6, 8, 10 preservation of flora in, 4–6 role of coal balls in development of, 1 Coal Age, 1–2 coal balls areas with no coal balls reported, 13, 14, 18, 48, 51 coal development, role in, 1 composition of, 9–11 decay, compaction, and formation of, 9–11 description and characteristics of, 1, 6

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coal balls (continued) discovery of and history of coal ball studies, 38–39 first observations and descriptions of, 1 focus of field of study of, 65–66 formation and timing of formation of, 6, 39 future collection and study of, 1, 68 geochemical and petrographic data and studies of, 6, 38, 39, 66 in situ occurrences, value of information from, 18–19, 20 iron-sulfide-rich coal balls, 35, 51, 54, 56, 57–58, 59 lenticular shapes, 39, 57, 59 peat-formation process preservation in, 6, 8–11 position and density in coal beds, 6, 8, 10 preservation of flora in, 4–6, 7, 10 preservation of marine elements in, 10 quality of peat preservation in, 9–11 resurgence of interest in, 59 coal mining apparel and equipment for miners, 22, 23, 42 safety equipment and precautions in, 22, 23 types of mines, 23 undercutting before blasting, 42, 68 collection of coal balls background on collection techniques, 19–24 bags and tags for samples, 22, 24, 26, 46 color-coded zonation and position of coal balls in the bed, 18–19, 20, 24 color-coding for Old Ben No. 24 Vertical Section (VS) 3 study, 48, 49 color-coding for Sahara Vertical Section (VS) 4 study, 20, 46–47 ex situ and mine spoils occurrences, collection from, 19–20, 21, 22, 24, 44 locating and field work to collect coal balls, 18–19, 20, 38, 65, 66–68 processes and methods for, 18–24, 63–65, 66 random samples, 21, 56–57, 59, 60 in situ occurrences, collection from, 18–19, 20, 21–22 stories about and adventures encountered during, 38, 62, 66, 68 tools for, 21, 22, 24, 65, 66–67 vertical disposition of coal balls, collecting from, 20–21, 24, 25, 26 working conditions for, 68 Colorado, 18 Conemaugh Formation, 12 Corinth Coal, Illinois, 5 Creek Paum Mine, Mount Rorah Coal, ­Illinois, 19 Cross, Aureal, 60

Darrah, William, 19–20 Desmoinesian Stage/age, 2, 10, 12, 14, 16, 17, 38, 44, 51, 56, 60, 63 DiMichele, Bill, 32–33, 50 Donets Basin, 10, 38, 39 drift mines, 23 Dunkard Basin, 13, 15

Euramerica and Euramerican coal balls, 2, 3, 4, 38, 39 Europe coal ball occurrences and collection in, 10, 38–39 wetland regions of the Carboniferous Period, 2

Fleming Coal, Pittsburg and Midway Coal Company Mine No.19, West Mineral, Kansas, 56–60 Forest City Basin, 17 Friendsville Coal, Allendale, Illinois, 63–65

Galatia channel, Springfield Coal, 48, 51 Galatia Mine, Herrin Coal, 8, 19, 25, 45 geochemical and petrographic data and ­studies, 6, 38, 39, 66 geology of American coalfields, 10, 12–18 Germany, 2, 38, 63 Gibson County Mine, Indiana, 23 glaciers and glaciation cycles, 2, 3, 14, 42, 65 graphite (meta-anthracite) deposits, 17–18 Great Britain/British Isles coal ball occurrences and collection in, 1, 10, 38 coal mining in, 1 discovery of coal balls in England, 38 wetland regions of the Carboniferous Period, 2 Great Coal Ball Horizon, 38 Grierson, Douglas, 49, 50

Hartshorne coal bed, 17 Harvey, Richard, 50 Henryetta Coal, Oklahoma, 5 Herrin Coal, Illinois Basin coal ball occurrences and collection in, 16, 39–40, 42–48 coal ball research in and understanding of coal ball formation, 39 coal mining in, 16 cyclothem and features of coal in, 42–44 importance of for research and “flagship” coal for study, 39, 42, 44 location of coal mines, 39, 40 “Quality Circle” coal, 39, 42 Herrin Coal, Illinois Basin, mines AMAX Coal Company Delta Mine, 40, 45 Clarkson Mine, 40, 45 Galatia Mine, American Coal Company, 8, 19, 25, 45 J. Smith Coal Company, 40, 45 Old Ben Coal Company No. 24 Mine, 8, 40, 45, 46, 48, 49–50 Old Ben No. 9 Mine, 42 Peabody Coal Company Camp 11 Mine, 40, 45 Peabody Coal Company Ken Mine, 40, 45 Peabody Coal Company River King Mine, 40, 45

Peabody Coal Company Shawneetown Mine (Core), 45 Peabody Coal Company White City Mine, 40, 45 Peabody Cottage Grove Mine, 20, 23 Sahara Coal Company Mine No. 6, I­ llinois, 20, 24, 40, 45–47 Houchin Creek Coal (Peabody Coal Company, Northern Illinois Mine, Pit 14), 10–11, 20, 51, 52–53, 55, 60, 62

ICO Mine, Oskaloosa area, Iowa, 60–61, 63 Illinois Basin coalfield age of coal balls in, 39 coal ball occurrences and collection in, 39–56, 57, 58 coal mining in, 16 location and geology of, 10, 12, 16 See also specific mines Illinois State Geological Survey (ISGS), 18, 46, 48, 63 Indiana, 16. See also Illinois Basin coalfield Indiana Geological Survey, 51 Industrial Revolution, 1 Iowa abundance of coal balls in, 60 discovery of coal balls in, 39 Midcontinent coal deposits in, 10, 16–17 Iowa Geological Survey, 60–61, 63 Iowa-Missouri Platform, 17 Iron Post Coal, Peabody Rogers County Mine No. 2, Vinita, Oklahoma, U.S. Midcontinent, 60–63 iron-sulfide-rich coal balls, 35, 51, 54, 56, 57–58, 59

J. Smith Coal Company, 40, 45

Kansas coal ball occurrences and collection in, 18 Midcontinent coal deposits in, 10, 12, 16–17 Kentucky, 13, 14, 16. See also Illinois Basin coalfield Kuznetsk Basin, 10, 38, 39

laboratory preparation of coal balls cutting, shelf life of iron-sulfide-rich coal balls after, 57–58, 59 cutting coal balls, 26–27, 28–29 equipment for, 24, 26–27 peel preparation and liquid-peel technique, 24, 26–27, 28, 29–32 processes and methods for, 18, 24, 26–32, 66 saws for cutting coal balls, 26–27, 28–29 siliceous coal ball preparation, 32 surface preparation, 26–27, 29 thin-section method, 24, 28, 32 late Paleozoic age/Late Paleozoic Ice Age (LPIA), 2, 3, 17, 65 Lemmon Brothers Mine, Springfield Coal, Indiana, 21, 41

NUMBER 111

lenticular-shaped coal balls, 39, 57, 59 limestone Brereton Limestone, Herrin Coal, 43, 44 coal ball occurrences near, 10, 16, 17, 43 Lomax, James, 24, 28, 32

Mahaffy, James, 50, 60–61, 64 Maple Grove Mine, Murphysboro-Equivalent Coal, Illinois Basin, 5, 8, 51, 54, 56, 57, 58 marine elements and organisms Appalachian coalfields and coal balls, 15–16 coal ball formation beneath rocks containing marine fossils, 10 discovery of marine influences in American coal balls, 39 Iron Post Coal plants, burrow walls, and marine elements, 62–63 Midcontinent coalfields and coal balls, 17 Mineral and Fleming plants and mixed marine elements, 56–60 mixed marine coal balls, 58, 60 Murphysboro-equivalent and Wide Ridge– equivalent coal balls, 51, 54, 56, 57 preservation of in coal balls, 10 Marmaton Group coal, 17 Massachusetts, 17–18 meta-anthracite (graphite) deposits, 17–18 Midcontinent coalfield, location and geology of, 10, 12, 16–17 Midcontinent time scale, 2, 10, 12 Mineral and Fleming Coals, Kansas, U.S. Midcontinent, 56–60 Mine Safety and Health Administration (MSHA), 22 Mississippian Subperiod and coal basins, 13, 15 Missouri, 10 Missourian Stage/age, 2, 10, 12, 14, 16, 17, 38 Morrowan Stage/age, 2, 10, 12, 13, 14, 16, 19, 38 Mount Rorah Coal, Illinois, 19, 51, 56, 68 Murphysboro-Equivalent Coal, Maple Grove Mine, Illinois Basin, 5, 8, 51, 54, 56, 57, 58

Nance, Roger, 63, 64 Narragansett Basin, 12, 17–18 Nebraska, 16–17 Netherlands, 38 New Calhoun, Calhoun Coal, Illinois, 65 New Mexico, Pecos River, 12, 18, 19 New River Formation, 12 North America, discovery of coal balls in, 39

O’Gara Mine, Illinois, 41, 48 Ohio, 14–15 Oklahoma coal ball occurrences and collection in, 17, 18 Midcontinent coal deposits in, 10, 16–17

Old Ben Coal Company No. 24 Mine, Illinois, 8, 40, 45, 46, 48, 49–50 Old Ben No. 9 Mine, Herrin Coal, 42 Origin of Coal, The (Stutzer), 1 Ouachita orogenic, 12, 13, 17 Ouachita orogeny, 12 Ozark Dome, 16

paleobotanical research Herrin Coal studies, 39, 40, 42, 44 role of coal balls in, 6, 37–38, 65–66 paleoecology role of coal balls in study of, 65–66 in situ occurrences of coal balls, value of information for, 18–19, 20 paleosol, 14, 42–43, 65 Paleozoic time, 16, 65. See also late Paleozoic age/Late Paleozoic Ice Age (LPIA) Pangea and Pangean supercontinent, 2–3, 4, 12–13, 18 “Paul’s Pride” shovel, 60–61 Peabody Coal Company, Northern Illinois Mine, Pit 14 (Houchin Creek Coal), 10–11, 20, 51, 52–53, 55, 60, 62 Peabody Coal Company Camp 11 Mine, 40, 45 Peabody Coal Company Eby Pit, 41, 48 Peabody Coal Company Ken Mine, 40, 45 Peabody Coal Company River King Mine, 40, 45 Peabody Coal Company Rogers County Mine No. 2, Pit 291, and “Paul’s Pride” shovel, 60–63 Peabody Coal Company Shawneetown Mine (Core), 45 Peabody Coal Company White City Mine, 40, 45 Peabody Cottage Grove Mine, Herrin Coal, Illinois, 20, 23 peat swamps and lowlands glacial, climate, and sea-level oscillations and formation of, 2, 3, 4 locations of, 1 peat-formation process preservation in coal balls, 6, 8–11 See also Pennsylvanian peat-forming swamps Pecos River, New Mexico, 12, 18, 19 Pennsylvania Anthracite coalfields and region, 13, 14, 15 coal ball occurrences and collection in, 14–15 Pennsylvanian peat-forming swamps Friendsville Coal, Allendale, plants, 63–65 glacial, climate, and sea-level oscillations and formation of, 2, 3, 4 Herrin Coal plants, 40, 44–45 Houchin Creek plants, 51, 52, 53, 55 Mineral and Fleming plants and mixed marine elements, 56–60 Murphysboro-equivalent and Wide Ridge– equivalent plants and marine organisms, 51, 54, 56, 57, 58 Peabody Coal Company Rogers County Mine No. 2, Iron Post Coal plants,

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­ urrow walls, and marine elements, b 62–63 preservation of flora from in coal balls and fossils, 4–6, 7, 10 Springfield Coal plants, 41, 48, 51 types and diversity of plants in, 2, 4–6, 18 vegetation changes at transition from Middle to Late Pennsylvanian, 60, 63–65, 68 Pennsylvanian Subperiod/Coal Age American chronostratigraphic subdivisions, 10, 12 coal ball discovery from and study and understanding of, 1–2 coal ball occurrences in basins from, 38 land surface and continental configuration during, 2–3 location of coalfields, 12–18 Midcontinent time scale, 2, 10, 12 rocks of Pennsylvanian age devoid of coal, 19 time period of, 2 USGS time scale, 10, 12 Peppers, Russel A., 51 Perkins, Thomas, 60 Permian Period, 2, 3, 4, 10, 14, 15, 38, 39 Pfefferkorn, Hermann W., 51, 63, 64 Phillips, Tom, 19–20, 21, 32–33, 49, 50, 60–61, 64 Pinkerton Trail Formation, 18 Pirtle coal beds, Illinois, 68 Pittsburg and Midway Coal Company Mine No.19, Fleming Coal, West Mineral, Kansas, 56–60 Pittsburgh Coal bed, 15 plants/flora coal ball discovery and research and knowledge about, 1–2 discovery of fossil plants in American coal balls, 39 Friendsville Coal, Allendale, plants, 63–65 Herrin Coal plants, 40, 44–45 Houchin Creek plants, 51, 52, 53, 55 Mineral and Fleming plants, 56–60 Murphysboro-equivalent and Wide Ridge– equivalent plants, 56, 57, 58 Peabody Coal Company Rogers County Mine No. 2, Iron Post Coal plants, ­burrow walls, and marine elements, 62–63 preservation of in coal balls and fossils, 4–6, 7, 10 Springfield Coal plants, 41, 48, 51 studies of development of and resurgence of interest in coal balls, 18 types and diversity of, 2, 4–6, 18 Pocahontas Basin, 13, 14, 17 Prairie Eagle Mine, Illinois, 23

“Quality Circle” coal, 39, 42

Redstone coal bed, 15 Rhode Island, 17–18

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road ballast, 56, 59 Russia and Kuznetsk Basin, Siberia, 1, 2, 10, 18, 38, 39

Sahara Coal Company Mine No. 6, Illinois, 20, 24, 40, 45–47 Sandia Formation, 18 Schopf, James M., 66 sea level, fluctuations and changes in, 2, 3, 42 Sewickley coal bed, 15 shaft mines, 23 shales black shale (Anna Shale, black fissile shale), 10, 43, 44 Corinth Coal, 5 Energy Shale, 43, 44 Henryetta Coal, 5 Herrin Coal, 43, 44 marine shale, coal ball occurrences near, 16, 17 preservation of flora in, 4–6 roof shale, 44 Shuler Mine, Iowa, 19–20 Shumway coal balls, 39 Silver Island strip mine, 51 Sorby, Henry C., 32 Springfield Coal, Illinois Basin, 16, 21, 39, 41, 48, 51 Star Mine, University of Iowa, 21, 60–61, 63 storage and retrieval of coal balls collections management system and policy for, 37–38, 66

marine spar varnish use for, 58 oxidation and degeneration of iron-sulfiderich coal balls, 57–58, 59 processes and methods for, 35–37, 66 shelf life of coal balls, 35, 37, 57–58, 59 stratigraphy American chronostratigraphy, 10, 12, 38 beginning of science of, 1 coal ball collection and research and under­standing of, 18, 66 coal ball occurrences throughout the world, 38–39 time and subdivisions used, 2 strip mines and strip-mining shovels, 46–47, 51, 60–61 Summum Coal, 51. See also Houchin Creek Coal (Peabody Coal Company, Northern Illinois Mine, Pit 14) surface mines, 23 Survant Coal, 21, 60, 62

Tennessee, 14 Texas coalfield, 12, 16, 17 Thompson, F. O., 19–20 Transcontinental Arch, 16 Triassic Period, 12–13

Ukraine, 2, 38 United States coal basins and coal ball occurrences and collection in, 39–65

discovery of coal balls in, 39 geology of American coalfields, 10, 12–18 wetland regions of the Carboniferous Period, 2 United States Bureau of Mines, 18 United States Geological Survey (USGS) coal ball studies role of, 18 time scale, 10, 12 University of Illinois coal ball processing laboratory and equipment at, 26–27 coal ball studies at, 18 storage of coal ball collection at, 35–37 University of Iowa Star Mine, 21, 60–61, 63

Variscan-Appalachian belt, 2 Variscan/Hercynian orogeny, 12–13, 17 Virgilian Stage/age, 2, 10, 12, 14–15, 16, 17, 38, 65 Virgilian Watson Coal, 39 Virginia, 13, 14

Walshville Channel, 43–44 West Mineral locality, Mineral and Fleming coals, Pittsburg and Midway Coal Company Mine No.19, Kansas, 56–60 West Virginia, 12, 13, 14 What Cheer, Iowa, 39 Wise Ridge Coal, 51, 56