Professional Contributions

The State Rock of Illinois: Dolostone

Liviu Iordache, PG, AEG Chicago Chapter

n June 6, 2022, at The Morton

Arboretum, Governor J. B. Pritzker signed House Bill 4261, which made dolostone “the official State rock of the State of Illinois.” Adding dolostone to the Illinois state geological symbols (fluorite is the State mineral!) was the culmination of a more than a year-long relentless campaign spearheaded by fifth grade and younger students of Pleasantdale School in Burr Ridge and Maplebrook Ele mentary School in Naperville.

Quarried extensively all over Illinois for crushed stone aggregates, dolostone occasionally crops out in the Chicago area from underneath the glacigenic cover as the upper sections of mile-large carbonate mounds that rise tens of feet above the surrounding low-lying bedrock areas. Benchmark studies have certified these exhumed Silurian-age reefs of Illinois as classic geological models.

For the more recent history or architecture buffs, one may still count the 3,300 dolostone blocks used to build the outside walls of the Old State Capitol in Springfield in 1837. As for the outdoor enthusiast willing to explore the Illinois landscape, the breathtaking rock walls of Sagawau Canyon along the Des Plaines River or the awe-inspiring Mississippi River bluffs north of Savanna provide scenic exposures of dolostone formations.

A French medical officer and engineer eventually-turnedgeologist, Deodat de Dolomieu, is given credit for the first description of a new calcareous rock visually alike to limestone yet less effervescent in acid. Dolomieu made his observations in the Stubaier Alps, Austria, and published them in 1791. A year later, the rock was named “dolomie” in his honor. About a decade later it became clear the main mineral making up Dolomieu’s rock was a double carbonate of Ca and Mg and thus different from calcite, limestone’s dominant mineral. And dolomite (English for “dolomie”) has since been used, some would say in a rather confusing manner, for both the rock and the mineral.

In 1946, Robert R. Shrock, an MIT professor and eminent scholar of Silurian reefs, presented a new classification of sedimentary rock before the Chicago meeting of the Geological Society of America. Shrock proposed the name “dolostone” for the rock made up the mineral dolomite, so that the dolomite-

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dolostone pair meaningfully parallels the calcite-limestone one. In Shrock’s own words, published two years later in the Journal of Geology, “ The term ’dolostone’ is proposed for those sedimentary rocks consisting largely of the mineral dolomite, such as the Niagaran dolomitic rocks of Illinois.” It seems only fitting then that dolostone has become the rock symbol of Illinois.

Silurian dolostone bluff near Mississippi Palisades State Park. Geologists prepare the outcrop for mapping of rock mass discontinuities via the linescanning method.

The Thornton Quarry, located just south of Chicago, produces crushed aggregate from Silurian dolostone and exposed one of the largest Niagaran reefs found in the Chicago region.