Geology

The geology of South Carolina begins with plate tectonics. Heat within the earth drives plates together and apart over millions of years. Current ideas in geology propose that such forces began forming South Carolina about 450 million years ago. A smaller North American continent existed at the time. As tectonic plates moved, a continental fragment and a large island arc approached, welding on to what is now eastern North America, shoving up sediments and volcanic rocks from the continental shelf, which formed the Blue Ridge and the inner Piedmont belt. The island arc became the Piedmont of South Carolina, and it extends along much of eastern North America. Overlapping this terrain are the sediments of the coastal plain, both of which continue offshore far out onto the continental shelf.

Geologists have divided South Carolina into a series of belts, from the northwest Blue Ridge, southwest to the inner Piedmont belt, the Kings Mountain belt, the Charlotte belt, the Carolina slate belt, and the Kiokee and Belair belts. While inexact, the belts framework allows distinctions between rocks to be made and categorized.

The Blue Ridge lies over the ancient Grenville Mountains, a Precambrian, eroded mountain system that existed inland from the present eastern coast of North America. As the east coast of North America rifted apart from another plate during the late Precambrian era, extensive sedimentary deposits and volcanics developed on the continental shelf. Then, in the Ordovician period, a collision occurred with what were perhaps a previously detached continental fragment and an island arc that moved toward and eventually welded onto North America. Over the millions of years that the collision developed, the rocks of the continental shelf were shoved upwards into what must have been a very high mountain range, adding both width and height to the continent. This became the Blue Ridge.

During later continental collisions, lastly with the African plate during the Pennsylvanian to Permian time, the remnants of the eroded Blue Ridge were again thrust upwards, moving more than one hundred miles toward the northwest. The rocks of the Blue Ridge in South Carolina include gneisses, schists, metagreywackes, pegmatites, and amphibolites that were formed from heat and pressure applied to the original sediments and volcanics of the Precambrian continental shelf rocks. The metamorphic intensity seen in the Blue Ridge rocks decreases from west to east, implying that its core lies farther to the northwest in North Carolina. In addition, the Blue Ridge also contains some Jurassic diabase dikes emplaced as the North America and African plates separated in the early Mesozoic.

The inner Piedmont belt is thought to be either a disengaged and reattached North American (Laurentian) continental fragment, or a fragment entirely from offshore. It is a highly eroded thrust sheet that contains metamorphic rocks including schists, gneisses, amphibolites, and metagranites. The collision that created the metamorphic belt also heated rock in some places to melting, and so granites formed at depths of more than twelve miles during the Ordovician and Silurian periods. These were later metamorphosed as North America collided further with Europe (Baltica) and Africa in the Devonian and the Pennsylvanian to Permian periods. The geology of the inner Piedmont provides clear and very accessible examples of the geologic story of the formation of the state. The uplifted remnants of the resistant metagranites, schists, and gneisses now stand at the surface as monadnocks, most forming the Blue Ridge Escarpment. These scenic mountains include Table Rock, Caesars Head, Sassafras, Paris, and Pinnacle.

The Kings Mountain belt is a shear zone, the suture between the inner Piedmont belt and the Charlotte and the Carolina slate belts. It is more highly mineralized than the other belts and it has a lower-grade metamorphism, thus the minerals are more stable at the surface, which causes the rocks to be more resistant to weathering. This causes the remnant ridges to stand taller than the surrounding, eroded Piedmont. The rocks of the Kings Mountain belt include metavolvanics, schists, phyllites, quartzites, granites, and marble.

The Charlotte belt is thought to be the core of the island arc that collided with North America in the Ordovician period. It contains more granite and gabbroic plutons than any other belt, which range in age from 735 million to 235 million years old. The Charlotte belt contains highly altered mafic intrusive rocks formed by ocean crust, including gabbro, amphibolite, serpentine, and greenstone metabasalt. The volcanic island arc created extrusive rocks as well, including ash and tuffs, the eroded remnants of which form many of the rocks seen at the surface today, such as phyllites and schists.

The Carolina slate belt contains the least metamorphosed rocks of all the belts in South Carolina. Island arc sediments formed sands, muds, and ash deposits off its shores, and these in time formed the quartzites, metamudstones, argillites, phyllites, and sericite schists of the slate belt. The slate belt also contains intrusive granites, but most are much shallower and younger than those of the Charlotte belt. They include the granites of Lake Murray, Pageland, Liberty Hill, Winnsboro, and Columbia; and they range in age from 311 million to 286 million years old. In 1982, in metamudstones of the slate belt in Lexington County, new evidence confirmed ideas about the nature of the terrains and the processes that formed South Carolina. Trilobite fossils were discovered in the rocks that were not North American in type. They were from rocks that derived from sediments found in Bohemia, in present-day southern Europe. This could not have been possible except by the rafting of the fragment of land that carried the trilobites onto North America through the movement of the plates.

The Carolina slate belt is also the site of the Crowburg Basin, the only Triassic period redbed in South Carolina. Located just west of Pageland in Chesterfield County, the six-mile-long Crowburg Basin represents the rifting of the North American plate from the African (Gondwana) plate as massive landmass called Pangaea began to break apart early in the Mesozoic era. The crust thinned and faulted downwards all along the eastern North American coast as the plates separated. These Triassic basins then filled with sediments, which today are visible at Crowburg as red fanglomerates. Lava also flowed in much of the state during this period and diabase dikes cut through the rocks, extending for hundreds of miles. These are visible in various road cuts and quarries from the slate belt to the Blue Ridge, and they extend from well offshore on the continental shelf and continue into North Carolina. The Carolina slate belt contains the major South Carolina gold deposits, which produced millions of dollars in gold during the nineteenth and twentieth centuries. The geology of the goldfields is a product of the volcanic nature of the Carolina slate belt and the later movement of minerals as the land was reheated during the orogenies (mountain-building episodes) that formed and transformed the state.

The Kiokee and Belair belts lie at the southwest edge of the Carolina slate belt. They are small in area and are thought to represent a higher-grade metamorphic environment than the slate belt, and may have formed through subduction, when one plate slid beneath another. They are bounded on the north by the Modoc Fault, which runs from Lake Murray to Georgia.

The final and largest section of South Carolina is the coastal plain, which makes up two-thirds of its landmass. The unconsolidated sediments and rocks of this region overlie the hard, crystalline rocks of the Piedmont terrain and they both extend out beyond the coastline as part of the continental shelf. The uppermost section of the coastal plain consists of sands, sandstone, and clays that formed from river-deposited sediments eroded from the Blue Ridge and other high mountains that once stood on the Piedmont. These rocks weathered over time into the clays and sands, which were then reworked by rivers and ocean currents to become back beach ridges and wind-blown dunes of the Miocene and Pliocene epochs. Known today as the Sandhills, the region contains many interesting geologic sites that lie either directly on the sediments, or cut down through and below them to reveal the geologic history of the region. These sites include Peachtree Rock in Lexington County and Sugarloaf Mountain in Chesterfield County. The mineral resources of the upper coastal plain include sand and clay mined in many counties including Lexington and Aiken.

The upper coastal plain becomes the middle coastal plain at the Orangeburg Scarp, the high sea level boundary at the Pliocene epoch. The land then becomes much more level, gently sloping over younger scarps and terraces toward the sea. The sediments that overlie the land on the coastal plain include limestones deposited as the sea levels rose during the Eocene epoch. The Santee limestone is a source of lime and cement and it is the site of the only karst (cave) topography in South Carolina. Eroded by acidic water, the limestone caves still contain active underground rivers, and they contain erosional features including chimneys and sinkholes. The sandy soils of the coastal plain are interrupted by thousands of Carolina bays, elliptical depressions that dot the region. It is thought that the bays were formed as the prevailing southwesterly winds sculpted water-filled depressions that existed near the coast during the Pleistocene epoch.

The lower coastal plain surfaces date from 1.8 million years ago to the present. The coast is divided into three sections: the Grand Strand, the Santee River Delta, and the Barrier and Sea Island Complex. At the Grand Strand the barrier islands have welded onto the shore; at the Santee Delta sediment has been deposited from the immense Santee River system; and the Barrier and Sea Island Complex formed as sea levels rose as glacial ice melted beginning about 15,000 years ago. The Sea Islands are products of sea level rise that drowned river valleys and formed islands cut off from the mainland. The barrier islands attached both to the Sea Islands and directly to the mainland. These islands continue to erode and accumulate at various rates on different islands as their sediment supply changes and as global sea levels rise.

South Carolina geology is currently in a passive, erosional phase. Collision and mountain building have ended for now. The state has developed from processes over a billion years in the making and it has contributed greatly to the field of geology by providing evidence of the universal processes of plate tectonics, a process that gives rise to the dynamic, if slow, transformation of the surface of the earth.

Murphy, Carolyn H. Carolina Rocks! The Geology of South Carolina. Orangeburg, S.C.: Sandlapper, 1995.