HOLOCENE FLOODPLAIN AND TERRACE FORMATION IN PIEDMONT RIVER VALLEYS OF NORTHWESTERN SOUTH CAROLINA

Examination of two sites, one on the North Pacolet River, one on the North Tyger River in the northwest South Carolina Piedmont, helps define the timing of and processes involved in Holocene floodplain and terrace formation. Interpretations of soils and stratigraphic data, supported by magnetic susceptibility, archaeological, and radiocarbon data produce a consistent picture of late Holocene environmental history for this part of the Piedmont. The alluvial floodplains at both sites date to the mid to late Holocene. Also present are remnants of older terraces exhibiting more advanced soil development and colluviation. At the North Pacolet River site, there is evidence for an in-filled scour channel which formed during a large flood, possibly the Pacolet flood of 1903, the largest flood in South Carolina history. Historical colluvium and alluvium overlies a buried soil A-horizon at a depth of 40 to 55 cm across the valleys. Magnetic susceptibility measurements from a soil profile on a terrace of the Pacolet River correlate with major soil-stratigraphic boundaries and support an interpretation of gradual Holocene terrace development. At the site on the North Tyger River recent colluvium and alluvium, post-dating European settlement of the region drapes terrace and floodplain edges, preserving a buried soil A-horizon, which has preserved a terrace containing artifacts dating from Late Archaic to Early Woodland periods (4500 to 1500 BP). The present study supports the hypothesis of Wolman and Leopold, who examined both the Pacolet and the Tyger rivers in their classic 1957 paper, that these floodplains are mostly late Holocene in age and affected episodically by scouring to bedrock during large floods. It also supports the finding of Trimble (1974) documenting the deposition of significant alluvium in Piedmont river valleys over the past 200 years and the burial of relatively stable Holocene landforms.