pediments

pediments Probably more has been written about the nature and origin of pediments than of any other desert landform, yet to a large extent they remain an enigma. Broadly speaking, pediments are low-angle, more-or-less planar, rock-cut slopes, usually veneered by gravel and separated from fringing hillsides by an angular break of slope. Pediments vary in length from a few hundred metres to several kilometres; their slopes are usually less than 5° and sometimes as little as 0.5°. Individual pediments can coalesce to form extensive surfaces called pediplains, commonly several thousand square kilometres in extent.

Just why and how pediment surfaces develop at such low angles is a matter of much debate. Anglo-American geomorphologists tend to the view that pediments are the result of either lateral planation by ephemeral, migrating streams issuing from bordering mountains, or sheet-flood erosion. Both theories have problems. Sheet floods, for example, require a priori a flat surface and can hardly, therefore, initiate one. And streams issuing from a mountainside on to a pediment simply are not capable of reaching much of the upper pediment edge without defying the laws of gravity.

In contrast, German geomorphologists, especially Johannes Büdel, have developed theories based on deep chemical etching followed by the stripping of the weathered mantle. Such theories are problematic in that they imply that desert pediments were once subject to a much wetter climatic regime. In any case, the idea of deep chemical weathering followed by stripping is difficult to reconcile with detailed measurements from Saudi Arabia, which clearly show that long pediments tend to have lower overall slope angles and are also more rectilinear than short pediments, which are usually concave in profile. In other words, pediment slope curvature declines as pediments lengthen. This leads to the possibility that pediments are modifications of vegetated convexo-concave slope systems typical of contemporary temperate climates and were initiated as a result of aridification in the Quaternary. Very short pediments are thus slightly modified convex foot slopes in this model.

Fossil pediments and pediplains are probably quite widespread but are difficult to recognize as they have been modified by recent processes such as glaciation. In Norway, for example, Professor Just Gjessing believes that the broad intermontane basins forming the Vidda are remnants of preglacial pediplains; and in southern England, Professor G. H. Dury has suggested that the extensive upland surfaces developed on chalk downland are Tertiary pediments modified slightly by periglacial processes.

P. Vincent

Bibliography

Cooke, R.,, Warren, A.,, and and Goudie, A. (1993) Desert geomorphology. UCL Press, London.
Vincent, P. and and Sadah, A. (1996) The profile form of rock-cut pediments in western Saudi Arabia. Journal of Arid Environments, 32, 121–39.