Mass Wasting and Periglacial Features

The colluvium on sandstone-capped ridges is coarse, like that in the Blue Ridge.As a result,debris slides and flows are more common than types of mass wasting associated with fine-grained debris. Some very large debris flows have been reported from the Ridge and Valley Province (Jacobson, 1993; Cenderelli and Kite, 1998).

Sorted Patterned Ground

Periglacial features, although reported from other Appalachian provinces, have been most studied in the Ridge and Valley and are therefore discussed here. Features most diagnostic of former periglacial conditions include ice-wedge casts and sorted patterned ground. The former are indicative of mean annual air temperatures of < -6^o C, and have been reported only within about 45 km of the former ice margin. Large-scale sorted patterned ground, in the form of sorted stripes, nets, and polygons, however, is found as far south as the Great Smoky Mountains. The minimum elevation of sorted patterned ground increases with decreasing latitude , suggesting a paleoclimatic temperature gradient. Patterned sorted ground is the only widespread feature in the Appalachians that is considered definitely diagnostic of past periglacial conditions, but the temperature implication is not well defined, other than a mean annual temperature below 0^o C. Other features may be periglacial relicts, but because they are poorly dated and because they conceivably could be produced by currently active processes, they should be regarded only as possible periglacial features. Talus is one example. Hack (1965) argued that the only conditions required for the production of talus are resistant rock, sufficient relief, and climate sufficiently cold to allow mechanical weathering. Hupp (1983) demonstrated that talus (at least steep talus) in Virginia is currently moving. Hack also noted, however, that talus derived from quartzite is more abundant in Pennsylvania than in Alabama, suggesting a climatic effect.

A second example is block or boulder streams or fields. These are collections of matrix-free boulders (at least at the surface) that are too far from cliffs or on insufficiently steep slopes to have been emplaced by the action of gravity alone. The largest such features occur in Pennsylvania just south of the Wisconsinan glacial border, and may have very low gradients. Blue Rocks Block Stream, which is 800 m long, 180 m wide, a minimum of 9 m thick, with a surface gradient ranging from 15^o to 3^o,is a good example (Potter and Moss, 1968). The emplacement mechanisms for such features are unknown, but the size and structure of the Blue Rocks Block Stream seem to preclude transportation by debris flow. Block streams farther south are much smaller, and often steeper, so that transport by debris flow set off by catastrophic rainfalls becomes a more plausible alternative method of emplacement.