Pediments and platforms: problems and solutions

Pediments have been regarded as smooth, gently inclined, surfaces that front receding escarpments, are epigene forms shaped by running water, and are well represented in arid and semi-arid lands. Covered pediments carrying a cover of coarse allochthonous detritus largely conform to this model, but are of limited distribution. The more common mantled and rock forms however do not. Many of the mantled pediments developed in sedimentary terrains are associated with scarps that have been worn back, but the mantle comprises a surficial wash over a weathered mantle in situ: they are first-stage etch forms. Many mantled pediments shaped in granite front scarps that demonstrably recede only slowly, and others lack a backing upland and scarp. Though many are drained by stream networks some lack any surface drainage, and in any event, though diffuse drainage can erode friable materials, they do not greatly affect coherent rocks such as fresh granite. The mantle consists of a veneer of introduced material over weathered granite in situ: the bedrock is shaped by subsurface moisture attack or mantle-controlled planation. They too are first-stage etch forms. Diffuse wash can remove friable grus and where the mantle has been stripped the weathering front is exposed in rock pediments and platforms, both of which are etch forms.