More than a billion years ago, in the Archaeozoic era, schistose rock of great but unknown thickness extended along what is now the Appalachian and Piedmont Plateau region of North America. The first great upheaval of this land mass, known as the Laurentian revolution, occurred toward the end of the era to produce Appalachia--a highland that separated the Atlantic Ocean from a shallow inland sea that covered the central part of the continent. The older rocks now found in the South Carolina Up Country--the gneisses, schists, slates, and quartzites--were formed by the metamorphism, through intense heat and pressure, of the early sedimentary rocks. Into these older formations granite, diorite, diabase, and some other igneous rocks have, at different times, been intruded. In Greenville and Spartanburg Counties are found lean iron ores of the Preterozoic era, which succeeded the Archaeozoic.
During the next era, the Paleozoic, the high mountains of Appalachia were being worn down. They were steep, rugged, and gaunt, like the newer ranges of the Rockies today. Flowering plants probably did not exist until near the end of the era, and vegetation consisted mainly of huge ferns and club mosses. Aquatic vertebrates and amphibia reached their greatest development during the Devonian and Carboniferous periods, when this region was entirely above the sea-hence fossils of those periods are not found in the State. The coal, iron, and petroleum deposits of the Paleozoic era do not occur in South Carolina, though they are found in Alabama and west of the Blue Ridge. Nor is the Permian period, last of the Paleozoic, represented in the Appalachian region south of West Virginia.
Between the Paleozoic and Mesozoic eras occurred the Appalachian revolution, the second great upheaval of this land mass. The fall line was the shore of the ocean in the Mesozoic era, and from that line to the present coast are marl limestone, and other sedimentary deposits of this era. Successive additions to the coast line were formed by sedimentary material resulting from erosion in the highlands. After the deposition of the Cretaceous sediments the coastal plain was inundated by the sea a number of times, and mantles of clay, sand, and gravel laid down. Seven such terraces, with elevations from 25 to 270 feet, rise successively along the South Carolina coast. At Charleston these beds of clay, sand, and limestone overlie the crystalline rocks to a depth of more than half a mile, but they thin out to nothing at the fall line. The South Carolina Up Country is of very ancient formation, and the Low Country comparatively recent--with little record left of the ages in between. Phosphate beds near Charleston have yielded many sharks' teeth, vertebrae of mammoths, and other fossils; and similar remains have been found around Eutawville and in the swamps of the Pee Dee near Florence. Practically no fossils have been discovered above the fall line--heat or pressure having destroyed them if they ever existed.
The Cenozoic era is represented in the lower part of the State by exposures of sedimentary rocks of the Tertiary period, mostly loose and uncompacted, and usually retaining somewhat their original horizontal positions. The movements that closed the Miocene, third epoch of the Cenozoic era, gave the Atlantic coast nearly its present outline. South Carolina entirely escaped glaciation during the Pleistocene epoch at the end of the Cenozoic era, and as a consequence it has no inland lakes or deposits of glacial debris characteristic of regions farther north.
The soils of the Piedmont Plateau are chiefly residual, having been formed by the weathering of the underlying rocks. The granites and gneisses form loams, while most of the other rocks form clay soils. Organic matter is rapidly destroyed in the warm climate, and the ironbearing minerals become oxidized, giving the red color characteristic of the region. A few hills or small mountains are composed of rocks, such as quartzite, granite, or gneiss, that are extremely resistant to weathering, and stand up above the level of the plateau. Little Mountain, Kings Mountain, Crowder's Ridge, and Henry's Knob are the most prominent of these residual hills or monadnocks. Sugar Loaf and Horseshoe Mountains, in the Sand Hills of Chesterfield County, rouse the visitor's curiosity because they look as if they had been detached from each other by some violent explosion. Much igneous rock is scattered over the Piedmont section.
In the Sand Hills have recently been found fulgurites, long tubes of fused silica formed by the action of lightning on sand; and in the northeastern corner of the State, in Horry County, round pits in the terrain seem to give evidence of a meteoric shower at some remote time in the past.