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Desert erosion features

A distinctive topography carved by erosion in regions of low rainfall and high evaporation where vegetation is scanty or absent. Although rainfall is low, it is the most important climatic factor in the formation of desert erosion features. Desert rains commonly occur as torrential downpours of short duration with a consequent high percentage of runoff. As a result of the dryness, wind and mechanical weathering also play an important part in desert erosion. Sedimentology Weathering processes

When storms of the so-called cloudburst type occur in the desert, sudden rushes of water, or flash floods, sweep down the normally dry washes or the narrow canyons in the mountains bordering the basins. The comparatively large volume of water combined with a high velocity due to the steepness of the slopes give the short-lived streams power to carry large amounts of fine and coarse rock fragments. As a result, the streams have great erosive power.

When intermittent streams leave the canyons and spread out at the foot of a desert mountain, they lose velocity and quickly drop the coarsest of the transported material to build an alluvial fan. Some of the water sinks into the fan, and some evaporates, but whatever remains may follow one of the channels on the fan or spread out in the form of a sheetflood, in either case carrying coarse sand, silt, and clay, and perhaps rolling some larger rock fragments along.

When the water reaches the toe of the fan, it spreads still more, dropping all but the finest silt and clay. Any excess water follows shallow washes to the lowest part of the basin, where it may form a playa lake. This evaporates in a few hours or a few days, depositing the silt and clay, mixed perhaps with soluble salts. The flat-surfaced area resulting from the silt and clay deposition is a playa. Playa

The lack of moisture during most of the year and the scanty vegetation make the wind a more potent agent of erosion in deserts than in humid lands. The finest material is blown high in the air and may be carried entirely out of the area, a process known as deflation. The larger sand grains are rolled along the surface, bouncing into the air when they strike an obstacle, knocking more grains into the air as they hit the ground again, until eventually a sheet of sand is moving along in the 3 or 4 ft (1 or 1.3 m) above the surface. This moving sand abrades rocks and other objects with which it comes in contact; at the same time the grains themselves become rounded and frosted. If movement is impeded by vegetation or other obstacles, sand accumulates to form dunes. Dune

Desert landscapes evolve in three stages. In the early, or youthful, stage, alluvial fans are built, washes develop, playas form, and the basins slowly fill with detritus. As this stage progresses, some alluvial fans coalesce to form bajadas or piedmont alluvial plains along the mountain fronts, and individual basins may become deeply filled with waste to form bolsons. Desert flats develop between alluvial fans (or bajadas) and playas, and isolated dunes accumulate on the lee sides of the latter. If the original highlands are flat-topped rather than tilted mountain blocks, mesas develop. As the mountain fronts slowly retreat under the attack of the atmosphere and running water, small bare rock surfaces or pediments form at the canyon mouths, the result of lateral cutting by the intermittent streams. The general tendency during youth is for relief to decrease.

The middle, or mature, stage is initiated by the development of exterior drainage or the capture of higher basins by lower ones as drainage channels erode headward through low divides. The fill deposited during youth undergoes erosion, and pediments become more widely developed. The mountains are worn still lower, and more and more channels extend completely through them, cut by the streams engaged in draining and dissecting the higher basins. Playa deposits or other easily eroded sediments are cut into badlands before being entirely removed, and mesas are reduced to buttes. Undissected remnants of older deposits become covered with desert pavement (flat-lying, interlocking, angular stones left after finer particles are removed by deflation). Where winds are turbulent and large supplies of sand are available, complex dune areas develop. Relief shows some net increase during maturity.

At the late, or old-age, stage of desert evolution, the original mountains are so reduced in elevation that the winds sweep over them with little or no condensation of moisture, and rains become still more infrequent. Great expanses of wind-scoured bare rock, or hammada, are exposed, with here and there a more resistant remnant standing above the general level as an inselberg. Buttes are reduced to smaller bornhardts and finally disappear.Those parts of the flat surface floored by earlier deposits are covered and protected by extensive areas of desert pavement. The rock fragments may be colored brown to black by desert varnish, a coating of manganese and iron oxides. Sand blown from the bare rock surfaces and from the sediments may form large dune areas. If there are no obstacles to obstruct movement or cause wind turbulence, the sand may move as a sheet, forming large expanses of flat or gently undulating sand surfaces. Relief slowly decreases in old age. Desert

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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