Erosion

The landscape of South Carolina is highly eroded. The state, and indeed most of eastern North America, was formed by collision with other land that thrust up high mountain ranges. The eroded roots of these mountains are exposed as the Blue Ridge and Piedmont terrains, which form the upper one-third of the state. The coastal plain makes up the lower two-thirds of the state, formed largely from the sedimentary products of the erosion of upstate mountains. This erosion has provided many of the state’s valuable mineral and rock products, including clay, granite, sand, and top-soil. In contrast, some human activities adversely affected erosion, which caused a tremendous loss of topsoil during the late nineteenth and early twentieth centuries that still affects the environment today.

Erosion is the natural, geologic process whereby rock and other earth materials are loosened or dissolved from their original positions and redeposited elsewhere. The high mountains that were uplifted as South Carolina formed 505 million to 245 million years ago have almost completely worn down. The rocks of the Blue Ridge and Piedmont are the remnant cores of mountains that are gone. The sands and clays that were produced by the erosion of these mountains were carried downstream. They formed an immense wedge of sediments that begin thinly at the Sandhills in the upper coastal plain and continue thickening seaward until, at the coast and on the lower coastal plain, this wedge is many thousands of feet thick. The sediments continue off the coast and make up huge deposits that overlie Piedmont basement rock on the continental shelf. They are composed of clays, shales, sands, and sandstones and are interlayered with limestones that give evidence for intermittent changes in sea level over time.

South Carolina contains many interesting erosional features, among which are caves. On the western edge of Lake Marion at and near Santee State Park lies the only site of karst topography in the state–caves and sinkholes cut into the limestone on the coastal plain. Caves form through weathering as acidic water percolates through basic limestone, initiating a chemical reaction that displaces the limestone. At Santee, as the acidic groundwater moved through and hollowed out the limestone in the Santee caves, some of the roofs collapsed to form sinkholes, some retained water to become small ponds, and some remained dry. The caves contain an underground river system that is visible in places through holes in the rock called “chimneys.”

Other erosional structures include the Blue Ridge Escarpment and the towering sheer rock faces of Table Rock, Caesars Head, and other prominent monadnocks. These are exposed through the erosional process brought on by the rise and fall of the sea level, uplift of the land through isostatic rebound, and through heating and cooling of the land during and after collision and disengagement. A closer look at the rocks shows erosional features characteristic of granite. Because granites are formed deep underground under tremendous pressure, the minerals that compose them are less stable at the surface. Over time, the granite reacts to the release of pressure by exfoliating, or popping off in large slabs to form its characteristic rounded surfaces.

To the south, the Orangeburg Scarp is a steep grade formed as a wave-cut ridge by the sea. Adjacent to the scarp are the Sandhills, discontinuous ridges of sand and clay that cut the state diagonally from northeast to southwest. The sediments that make up the Sand- hills eroded over millions of years from the mountains to the north and northwest. They were carried by streams and rivers downslope, where they were transported further by wind and ocean currents and shaped into huge deposits of sand. In the Piedmont, the consistent elevation of the ridges shows that what was once the rock of high mountains wore away, and then the terrain was again uplifted and dissected anew by the many streams of the region.

The erosion of the soils in South Carolina is a chapter in soil management that has had an impact on the entire country. In the nineteenth and twentieth centuries, many of the state’s farmers were careless, even cavalier, about soil management. The demand for cot- ton led South Carolina farmers to plant on the highly sloped land of the upstate with little thought to the ultimate effects on the soil and the long-term health of agriculture. Consequently, it is estimated that nearly forty percent of South Carolina’s arable topsoil was lost to erosion. This, in turn, affected the rivers as sediments moved downslope, killing fish and permanently altering the ecology of the rivers. To address the problem of erosion in South Carolina and elsewhere, in 1933 the federal government created the United States Soil Conservation Service to help manage and preserve the soils throughout the country.

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. Orange- burg, S.C.: Sandlapper, 1995.