Alluvial plains have level or gently sloping surfaces veneered by loose, rounded rock fragments (gravel- to cobble-size) that are commonly closely packed (see Summary for Desert Pavement). The plains overlie bedrock veneered by alluvium (as on pediments), parts of alluvial fans, and floodplains and terraces of drainage courses. Almost all extensive gravel plains are remnants of broad alluvial valleys. Therefore, the components typical of these plains are smooth and rounded because they were transported by running water, and deposits tend to be thick because in most places they are the upper parts of a valley filling river deposit. Weathering and downslope movement of rock fragments that have not been transported and smoothed by running water produce pavements of rough, jagged rocks (talus slope debris). Slope wash and wind-deposited material fill pores between the stones with fine sediment, commonly producing a zone (accretion mantle) (McFadden, et al.[1]) a few centimeters to about a meter thick of fine soil between the desert pavement, which is one stone thick, and the underlying gravelly alluvium or bedrock. Some old alluvial plains from which streams have disappeared (due to increased aridity) are now covered with windblown sand sheets and are known as sand plains.
Alluvial plains typically occur in areas that, in the past, were subject to running water, and some areas may still receive occasional runoff from precipitation in distant watersheds (See Summarys for Alluvial Features Fans and for Drainage Courses).
Gravel plains, like sand plains, are excellent routes for cross-country movement as illustrated by the Dibdibah Plain of northern Kuwait and southern Iraq during the Gulf War. Their surfaces will also support rotary-wing aircraft operations and, with the exception of areas of sharp-edged rock fragments, fixed-wing operations. Airborne loads of dust and fine sand can be caused by the passage of fast-moving vehicles and by aircraft. Tracks made on gravel plains with desert pavements may persist for decades, whereas tracks on sand plains generally disappear during windstorms. Vehicles crossing gravel plains do not have the problems of bogging down or running into truncated or granule ripples, as on sand plains, although breaking through the surface to the underlying fine, soft accretion mantle sometimes happens on desert pavement. Some gravel plains have sharp-edged rock (talus) fragments that will tear up tires. Gravel plains may have beneath the surface pavement a zone of packed, caliche cemented, gravelly soil many meters thick, which can be trenched only with difficulty.
(common names are in bold): Hammada (North Africa), serir, gibber plains (Australia), desert pavement (some), reg, gobi (China), bahada.
1. McFadden, L.D., S.G. Wells, and M.J. Jercinovich. 1987. Influences of eolian and pedogenic processes on the origin and evolution of desert pavements. Geology, v. 15, pp. 504-508.
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