Regolith of the Appalachian Plateau

Residuum

Residuum on the southern part of the Cumberland Plateau, where Pennsylvanian Pottsville Sandstone forms the caprock,varies greatly from place to place. Generally, residuum on top of the sandstone is quite thin, often less than a meter, and overlies hard rock. In some locations, however, perhaps owing to weaker cement, the sandstone is decomposed to a depth of many feet, forming an equivolumetric residuum analogous to saprolite on crystalline rocks. In the Cumberland Mountains and the Allegheny Mountains, residuum is generally formed from sandstone and is thin.On the Unglaciated Allegheny Plateau, large areas of shale outcrops occur, and shale residuum is common. Some shales, particularly in parts of Kentucky and West Virginia, are high in swelling clay and are highly prone to landsliding (see mass wasting).

Colluvium

Schematic diagram showing
development of colluvium on
benched slopes in Appalachian
Plateau

In those parts of the Plateau where sandstone caps interfluves, abundant boulder colluvium occurs on escarpments and the walls of gorges. Thicknesses mayreach 10 m on footslopes. The clasts in the colluvium are chiefly sandstone and the fines often include large amounts of material derived from shales that underlie the sandstone cap rocks. In other parts of the Plateau where shales are dominant, fine-grained colluvium is the rule, much of it prone to earthflows. Colluvium generated from shale residuum generally has mechanical properties very similar to those of the residuum.The figureillustrates how colluvium develops on the benched hillslopes typical of this province. Colluvium tends to be generated by weathering higher on the slope, and then to accumulate farther downslope on benches and in shallow downslope-trending swales. Thickness may be as great as 6 ft. Contemporary failure rates indicate that this colluvium is being produced by present-day weathering and slope failure (Jacobson, 1985).

Other colluvial deposits, however, appear to be relics, perhaps from the last interval of glacial climates. In shale-dominated landscapes of the Appalachian Plateau small streamless hollows to be filled with remnant colluvium as thick as 45 ft. This colluvium does not appear to be moving now, and may have been placed by solifluction during the ice age. These deposits are often dissected by more recent drainage.