A pediment is a broad, gently sloping surface of low relief eroded in bedrock at the base of a much steeper mountain slope. It is commonly covered by a thin, patchy veneer of alluvial sand and gravel that thickens downslope to merge with the alluvial fill of the valley. Pediments are prominent landscape features in arid and semiarid regions. In contrast to depositional piedmont plains, pediments are erosional, although the processes that formed them are poorly understood and remain controversial. Parallel to the mountain front, the pediment profile is commonly smooth. The gradient of the slope toward the valley is 1/2° or so to as steep as about 10°; average gradient is about 3° to 6°.
In some areas of the Sahara and central Australia, pediments form extensive plains of very low gradient--they appear nearly flat. Such coalesced pediments of regional extent are termed pediplains. Pediment surfaces typically are cut across rocks of different types and ages, and the same erosional surface may thus be traced across several different localities within a region. Pediments commonly contain inselbergs (isolated hills or mountains), drainage channels, and well developed soil profiles in the alluvial veneer. The inclined bedrock surfaces of the lower pediment slopes commonly continue at depth beneath the adjoining alluvial plains. Upper slopes abut the adjacent mountain at what is typically an abrupt break in slope (piedmont angle); occasionally the abutment is gradual or is masked by alluvial fans. Where pediments merge between two side mountains or two inselbergs, they form a pediment pass. Terrace pediments occur in sequences of stair step topography along many major rivers in desert areas.
Unknown and controversial.
In basin-and-range deserts, pediments are excellent routes for vehicular and foot travel, because they do not have the hazards of the lower basin floors (see Summarys for Playas, Drainage Courses, and Depressions), and because pediment passes may be routes to, and at places across, mountain ranges. Air photos can be used to identify pediment surfaces and particularly areas of coalesced pediments (pediplains) along mountain fronts, where the surface is not too deeply or intricately dissected by drainage channels. The surface typically is a gravel plain (see Summarys for Alluvial Deposits - Fans, Gravel Plains, and for Desert Pavement). The alluvial veneer on pediments is generally better consolidated and more trafficable than on alluvial fans. Unlike fans, where depositional processes are generally more active, pediment alluvial surfaces are generally stable: they commonly have a relatively dense cover of grasses, shrubs, and a few trees. Pediments generally lack sources of surface water, and bedrock occurs at shallow depths, shallowest at the upslope margin. Pediments are, therefore, difficult areas to trench, and they make poor defensive positions.
(common names are in bold) Bench, terrace (some)
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Description