ABSTRACT
The study of the formation of pediments in the southwestern United States has a long evolution. Early geomorphologists envisaged a general model in which the dominant process was backwearing of a mountain scarp accompanied by concurrent development of an erosion surface (pediment) extending outward from its base. Pediments were believed to be surfaces of transport across which detritus produced in the mountains was carried to adjacent basins. Recent work has brought out that pediments vary in character and are formed in a variety of ways over different lengths of time. In south-central Arizona near Phoenix, some granitic pediments are composed principally of residual grus more than 60 feet thick derived from underlying granitic bedrock, whose irregular surface has little relation to erosion by lateral planation, rill action or sheetwash. The model outlined here emphasizes the importance of downwasting by weathering to explain the origin of these granitic pediments. Vertically the pediments can be differentiated into 2 distinct parts; a residual grus upper zone of variable thickness providing the relatively level surface topography of the pediment and a fresh bedrock zone below containing relief either related to the shape of the original granite mass or produced by differential weathering. As the original granite surface decomposed, probably during times of a wetter climate, the increasing thickness of grus accumulated. Mass wastage and minor movements by water caused the grus to fill in the original irregularities of the granite surface. The irregular bedrock surface below the grus is the uneven downward moving front of a weathering zone.52 Residual hills, although in some cases completely “grusified” within, stand up above the pediment due to a protective armor of boulders. Where fresh bedrock is exposed, particularly near the mountains, the grus has been removed and present-day processes are modifying the surface of the pediments.
