Regolith of the Blue Ridge

Saprolite

The Blue Ridge is underlain by crystalline rock as is the Piedmont Plateau,andparent material hereis subjected to the same chemical weathering that creates saprolite in the Piedmont. Although thick saprolite does occur in the Blue Ridge, generally saprolite is not as prominent in this Province as it is in the Piedmont.Saprolite tends to be thinner here than in the Piedmont and surfaces in the Blue Ridge are commonly covered by colluvium. Hack (1982) suggested that there is a threshold slope and local relief that determines the retention of a thick blanket of saprolite. When this threshold is reached, a sharp change in relief takes place. In most of the Blue Ridge, this threshold has been exceeded. One place where saprolite-dominated landscape is juxtaposed with colluvium-dominated landscape is the Asheville basin inNorth Carolina. Here,the rolling topography of the basin abuts mountains along the margin of the basin. Saprolite also dominates along the gently-sloping ridge crests of many of the mountains here (Mills 2000b).

Colluvium

Facemaps of the Blue Ridge Province show the presence of extensive colluvium.The red, clayey colluvium in the Piedmont is highly weathered and appears to bea relic ofa time inwhen glacial climates made hillslopes somewhat more mobile than at present In contrastmuch of the colluvium inthe Blue Ridge isbrown, containshard stones, and appears to be either modern or late Pleistocene in age. On gneiss slopes in the Blue Ridge of North Carolina, Ahnert (1970a), using seismic refraction, found that colluvium thickness increases downslope and is thinner where slope is steeper. He found little downslope change in particle-size distribution.