geology
Natural History of North Carolina by Roger Shew
North Carolina has a long geological history, with the western part of the state having rocks greater than 1 billion years old. The state is divided into three physiographic provinces: from west to east, these are the Blue Ridge, Piedmont and Coastal Plain. The Blue Ridge comprises 10% of the state while the Piedmont and Coastal Plain each cover roughly 45% of the state. North Carolina becomes “younger” as you move east with active sediment movement and deposition in our rivers and on the coast. And with these geological changes from west to east there are also changes in the topography, resources and ecological diversity. The Earth’s surface of continents and ocean basins was much different in the
past compared to what we see today. These continents and ocean basins, termed “plates,” are in constant motion. Plate movement is described by the Theory of Plate Tectonics, the underlying theory in Geology.
The Blue Ridge and Piedmont regions were formed by multiple plate collisions, known as convergent plate boundaries. The buckling and subduction of the plate edges led to the formation of igneous and metamorphic rocks through the deep heating and melting of rock and the pressure and heat of the boundary collisions, respectively. The igneous rocks, or fire rocks, include ancient volcanoes and deep magmas that solidified into hydrothermal veins with gold and granitic batholiths/bodies of rock. These granitic bodies, which are now exposed at the surface because of weathering and erosion of the overlying rock, supply rich resources for North Carolina including feldspar, quartz and even gold. In fact, North Carolina was the original gold rush state in 1799 and the leading producer of gold until 1849.
photo by Valerie Robertson
Roger Shew shares information about two limestone rocks mined from the Martin Marietta Quarry at Castle Hayne. The larger one, on the right, is classic Castle Hayne Limestone with lots of fossils and high porosity. The smaller boulder represents an unconformity, missing time in the geologic record, where 40 million-year-old Eocene Castle Hayne Limestone overlies 70 million-year-old Cretaceous age limestone rocks. This is a classic piece of geological history. Note the milkweed blooming in the rain garden behind Shew.
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During the convergent episodes of continental collisions, sediments and rocks were metamorphosed. (“Meta” means “change” and “morph” refers to “shape” or “form.”) The Piedmont is mostly composed of metamorphic rocks. You might notice from the geologic map below that the colors are mostly oriented NE to SW. This was the angle
Cape Fear’s Going Green • Spring 2022
of continental collisions of island arcs and the Continent of Africa with North America (Africa was part of a much larger continent of Gondwanaland and North America was part of Laurasia). The Blue Ridge and Piedmont were formed in the Proterozoic through the late Paleozoic. Although plates move at slow rates and mountains build and erode at slow rates, the long geologic time of over ½ billion years is adequate to move mountains and continents and to wear them down. The resultant mountains in North Carolina were once higher than 20,000 feet, but Mother Nature has relentlessly worn them away over hundreds of millions of years to the subtle, rounded mountains we know today. We do still have the highest mountain peak east of the Mississippi River: Mount Mitchell stands at 6,684 feet. North America and Africa were joined together at the start of the Mesozoic. But the continents are in constant motion and another phase of continental movement began. North and South America started to move away from Africa and Europe along rift basins formed at a divergent boundary. A divergent boundary is a spreading center. North Carolina was once joined to the African countries of Mauritania and Senegal. Rifting continued with some basins finally opening to form the ancestral Atlantic Ocean. Some of the rift basins ceased opening and formed small NE to SW trending sedimentary-filled basins. These are the Triassic Basins of North Carolina that include the Durham, Sanford and Dan River Basins. The sedimentary rocks include many red beds (source of clay/sand for bricks) of lake and river deposits as well as minor coal deposits, which were important in the Civil War. Numerous red beds are easily seen west of Sanford and at the RDU airport on the west side of Raleigh; this is the Durham Basin. Following this rifting phase, there have been no major tectonic episodes in North Carolina. The last ~100 million years has been a time of weathering and erosion of the mountains with sediments brought to the coast to form rivers, deltas and beaches. Marine limestones and (continued on page 12)
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