Piedmont Regolith

Most of the regolith in the Piedmont Province is saprolite,which is decomposed residual regolith. However, there are also substantial areas of colluvium, which is decomposed and transported regolith.

For a geologist entering the Piedmont for the first time, noted Kerr (1881), "...the most striking and novel feature of the geology is the great deal of earth which almost everywhere mantles and conceals the rocks."Becker (1895, p. 289) coined the term saprolite(from the Greek for "rotten") for this "thoroughly decomposed, earthy, but untransported rock." Saprolite is formed from crystalline rock that has been exposed to chemical weathering sufficiently intense to remove as much as 60 percent of the rock mass but not affect the original volume (Cleaves 1974). Saprolite can be divided into an upper "massive" zone, commonly 1-2 m thick, where structure of the parent rock has been destroyed by root growth and other near-surface processes, and an underlying "structured" zone, where the rock structure is retained. Saprolite thickness varies from 0-100 m in thickness, but typically is 15-20 m (Hack, 1989). To some extent, the thickness of saprolite can be predicted. Costa (1973) developed a model for predicting saprolite thickness in a watershed of the Maryland Piedmont from topographic position, bedrock mineralogy, and bedrock structure.

Pavich (1986) and Pavichand others (1989) found that saprolite production in the Piedmont is controlled by solution flux and by plagioclase feldspar, iron silicate, and iron sulfide reaction kinetics. Mass loss in solution during saprolite formation is limited chiefly by groundwater availability and rate of movement. Pavich (1986) calculated that 1 m of saprolite can be produced in about 250,000 yr, so that a typical saprolite thickness of 15 m need be nomore than several million years old. Rock structure controls ground water movement and may affect the rock weathering rate more than mineral dissolution kinetics.

Eargle's colluvial diagram for S.C. Piedmont

The prominence of saprolite in the Piedmont may lead to the false impression that all the regolith in this province is residual in nature, and that transported regolith, colluvium, is rare. Certainly the modern landscape appears to be very stable. Costa (1974) noted that in Maryland, a rainfall of 311 mm in 24 hours produced no failures in unmodified hillslopes. Parizek and Woodruff (1956) concluded that subsurface creep is practically absent on hillslopes in the Georgia Piedmont. There is evidence, however, that in the past this landscape was somewhat less stable. In some areas, thick colluvial deposits are widespread. Kerr (1881) termed this material "frost drift", and attributed it to the action of frost in the coldest portions of ice ages.

Eargle (1940, 1977)made detailed studies of colluvium in the inner Piedmont of South Carolina, based on more than 200 stratigraphic sections, supplemented by more than 800 borings. In a typical third-order drainage basin studied by Eargle (1977) , colluvial fills generally occur in hollows, with organic deposits commonly underlying the colluvium. As much as 6 m of colluvium may conformably overlie organic deposits as thick as 3 m. The colluvial deposits bear one or more Paleosols, indicating episodic accumulation. In some cases, topographic reversal has resulted in colluvial deposits becoming noses rather than hollows (Profile d-d, Figure). Eargles most significant finding probably is the areal extent of colluvium, which underlies one third of his study area. Colluvium studied in the Piedmont of Maryland (Costa, 1973) and Virginia (Whittecar, 1985)appears to be somewhat thinner and lacking in organic material compared to that studied by Eargle (1940, 1977). Whether this difference reflects regional differences in colluvium is not known.

Piedmont colluvium generallycontains spruce and fir pollen(Cain, 1944; Whitehead and Barghoorn, 1962; Costa and Cleaves, 1984), thus supporting Eargles suggestion that cold climate was responsible for the colluviation (See also: Effect of ice ages).