The coastal plain is South Carolina’s largest landform region, forming two-thirds of the state and encompassing approximately twenty thousand square miles. It includes the land from the Sandhills to the coast. The coastal plain continues into nearby states and is found from New Jersey to Texas. The eastern coastal plain formed through the erosion of mountain ranges to the north and west—the Blue Ridge, the Piedmont ranges, and the present Appalachian Mountains. In addition, the sea rose and fell many times over millions of years, forming extensive deposits of off-shore limestone that is visible today at the surface in many areas of the coastal plain. Other unconsolidated sediments are found on the surface and at depth. These include sands, clays, and silt that eroded from the highlands and settled out onto the ocean floor. These sediments later were exposed as sea levels fell. Over time the sediments were eroded and redeposited repeatedly by rivers and estuaries, thus constructing the complex geomorphology of the coastal plain. In South Carolina the coastal plain is divided into three sections according to elevation and topography: upper, middle, and lower.
The upper coastal plain is that section lying between the Sandhills and the Orangeburg Scarp. The region is composed largely of sands and clays, the oldest formation of which is the Middendorf, which is a one-hundred-million-year-old Cretaceous-period clay that in places underlies the Sandhills and is exposed in eroded locations. A second distinctive feature of the upper coastal plain is the Aiken Plateau, which crosses the center of the state from Aiken to the Orangeburg Scarp west of Sumter. The Congaree and Wateree Rivers cut the plateau such that only small, eroded remnants remain beyond Aiken and Lexington Counties. These remnants, the Richland Red Hills and the High Hills of Santee, form small but interesting and scenic ridges. Also on the Aiken Plateau is Peachtree Rock, a small but fascinating sandstone remnant in Lexington County.
The middle coastal plain begins at the base of the Orangeburg Scarp and continues only a few miles to the Surry Scarp, where the lower coastal plain begins. Six escarpments and seven terraces run from the Orangeburg Scarp to the coast, which represent six ocean limits and seven temporary ocean floors of their day, from the Pliocene to the Holocene epochs. The largest of these escarpments is the Surry Scarp, which is a younger and much smaller escarpment than the Orangeburg, rising only to 130 feet. It formed 85,000 years ago during the last advancement of the ocean during the Pleistocene epoch. The Surry Scarp illustrates the complex erosional history of the area. As it traverses the coastal plain, the Surry Scarp shows evidence of beach ridges, a barrier island shoreline, and portions of a river valley.
The geologic features found in the coastal plain are many. They include floodplains of the large rivers that traverse the area, including the Congaree, Wateree, Pee Dee, and Santee. There are near- pristine river bottomlands and swamps, estuaries, and other wetlands, including those found at Congaree Swamp National Monument, Four-Holes Swamp, and the ACE Basin. Dark coastal plain rivers run gently and clearly from the base of the Sandhills to the coast. The coastal plain is the only region in the state that contains Carolina bays, unique elliptical depressions that dot the region. The coastal plain also contains a unique area of karst topography along the western edge of Lake Marion in Orangeburg County. These caves and sinkholes formed over millions of years as acidic groundwater reacted with the Santee limestone of the area.
The geology of the coastal plain provides the state with important fossil sites. The Santee limestone contains important Eocene-epoch marine fossils found in quarries of Dorchester and Berkeley Counties. Many other formations throughout the coastal plain contain fossils ranging in age from the Cretaceous period to the Holocene epoch. In addition, Pleistocene plant fossils have been recovered from sand and gravel quarries located along the Pee Dee, Wateree, and Pocotaligo Rivers. Of particular interest was the 1987 discovery in Williamsburg County of the first dinosaur fossils ever located in the state.
The coastal plain also includes the Coastal Zone, the land approximately ten miles inland from the coast. The coast is divided into three sections: the Grand Strand, which was created by currents into a welded barrier island terrain; the Santee River Delta, the largest delta on the eastern seaboard; and the Barrier and Sea Island Complex, which runs from the Santee Delta south of Georgetown into Georgia and north Florida. Located on the Georgia Embayment, the Sea Islands formed as sea levels rose and drowned the coastline. The barrier islands formed as sea levels slowed about six thousand years ago. The barrier islands are eroding in some instances and growing in others, but there is generally less sediment in place due in part to the damming of the Santee River and subsequent loss of sediment at the delta. Three examples of large-scale erosion can be seen at Hunting, Bull, and North Islands.
The resources of the coastal plain include limestone, clay, sand, and gravel dug from the many open-pit quarries located across the region. Clay is mined primarily in Aiken County, but deposits are also found in Orangeburg and Dorchester Counties, among others. Sand is quarried extensively in Lexington County and from smaller local quarries in many sites across the region. The coastal plain is the most productive and largest agricultural area in the state. The sandy soils, while not rich in organic matter, with artificial fertilization support tobacco, corn, cotton, soybeans, wheat, and other crops. The coastal plain also supports a large pulp industry with large stands of fast-growing loblolly pine trees.
The beauty of the coastal plain is perhaps its greatest resource, not only because it fuels South Carolina’s tourist economy, but also because it reflects the successful preservation of irreplaceable ecosystems. Nearly forty percent of the coastal lands are held in trust, either as preserves or as public parks and other recreational areas. Some of these trust sites include the ACE Basin Preserve, Cape Romain National Wildlife Refuge, Pinckney Island National Wildlife Refuge, Francis Marion National Forest, and many state parks, including those at Edisto Beach, Hunting Island, Huntington Beach, and Myrtle Beach.
Kovacik, Charles F., and John J. Winberry. South Carolina: The Making of a Landscape. 1987. Reprint, Columbia: University of South Carolina Press, 1989.
Murphy, Carolyn H. Carolina Rocks! The Geology of South Carolina. Orangeburg, S.C.: Sandlapper, 1995.