Boulder deposits and the retreat of mountain slopes, or ' gully gravure' revisited

Observations on mountains composed chiefly of shale and capped with Tuscarora Sandstone in the Valley and Ridge province of southwest Virginia suggest that slopes retreat by a process similar to but different from Bryan's (1940) "gully gravure." The process appears to operate as follows: Bouldery alluvium protects the floors of hollows on the mountain flanks. In each hollow, the junction between the boulder-armored floor and the base of the unprotected shale side slope is the locus of greatest erosion. Runoff from rainstorms that have recurrence intervals of the order of 10 to 10² years incises gullies along the margins of the bouldery deposits, but lacks the competence to disturb the deposits. Periodically, however, runoff from catastrophic rainstorms that have recurrence intervals of the order of perhaps 10³ years mobilizes the bouldery alluvium and deposits it in the gullies. The armored floor thus shifts laterally. Subsequently, new gullies are incised adjacent to the old ones. The result is the lateral migration of one or both walls of the hollow. Eventually, ridges between hollows are destroyed, and valley floors covered with bouldery alluvium become new interfluves in an unending process that lowers mountain flanks. Evidence of this process includes the distribution of surficial deposits that show past topographic inversion of hollows and noses, correlation of surficial clast size with cross-slope variations in topography, and topographic comparisons between mountain flanks mantled and unmantled by Tuscarora boulders. This process probably is common wherever large resistant clasts are transported downslope over less resistant bedrock. Lateral migration of stream valleys has been shown to take place on mountain piedmonts under similar circumstances, but this study suggests that such migration may also take place on the upper slopes of mountains in hollows where water flow may be significant only a few times per century.