Desert pavement

Coastal dunes occur along beaches or sand-flats that form part of a non-erosional sandy or deltaic coast. The source areas of the sand will eventually lose all sand, silt and clay particles some become wet (groundwater) depressions whereas others acquire a rocky or boulder-strewn surface known as a desert pavement . 

Reg soils are closely associated with desertic regions. They have developed on stable surfaces where coarse, gravelly desert alluvium is exposed, and are characterized by a well-developed desert pavement and exhibit some well-defined soil horizons. They occur mostly on depositional surfaces where stones and gravels have been deposited since Neogene times. The surfaces commonly consist of stony, unconsolidated sedimentary deposits in which limestone, dolomite, chalk, flint and marl predominate, together with some fines (silt and clay). Sandstone and granite debris have also been reported to contribute to Reg formation. Less frequently, they form on sedimentary bedrock (Fig. 1.5). 

Most of the desert pavement stones are covered with a brown-black and shiny crust. When the stones are composed of limestone, the dark crust contrasts strongly with the much lighter inside color exposed on fracture surfaces. The crust forms on various stones, both sedimentary and igneous and is also known under the name of desert varnish , or desert patina . The varnish is less common on non-resistant rocks such as a soft limestone. These, apparently, disintegrate before the crust has time to develop. 

The result in terms of soil formation is a loss of soil. In desert areas covered by physically weathered shallow soils, deflation removes mainly the fine and medium-sized particles - clay and silt first, the somewhat coarser sand afterwards - and leaves behind a desert pavement, variously called reg (Sahara), serir (Libya) ox gibber plains (Australia). 

Abrasion and erosion. The dust-loaded wind has an erosive action and contributes to a physical disintegration of rock surfaces or to a polishing of the components of the desert pavement, giving them a characteristic patina desert varnish) and shape (ventifacts). 

Field observations have resulted in a number of informal classifications. Early field geochemists recognised that varnish on stones in deserts differs from varnish on intermittently flooded rock surfaces (Lucas, 1905). Another example of differentiating varnish involves position on a desert pavement clast (Mabbutt, 1979) black varnish rests on the upper parts of a pavement clast, a shiny ground-line band of varnish occurs at the soil-rock-atmosphere interface, and an orange coating is found on the underside of... 

Surface stability Since cosmogenic nuclide analyses show erosion of rock surfaces (Nishiizumi et al., 1993), and geomorphological surfaces hosting varnish such as desert pavements experience instability (Peterson, 1981), how can varnish exist for 105 years (Liu and Broecker, 2000 Liu, 2003) Rock surface erosion rarely occurs in a linear fashion. Larger clasts can host zero erosion rates, while adjacent clasts experience much more rapid erosion. Linear rates of boulder erosion remain unusual. Similarly, landforms can exhibit signs of instability such as erosion of fines, even though particular rock faces can remain stable... 

In some cases, the volcanic ash in Arena valley was mixed into lobate achalanche deposits and into Quartermain I till. Ash also occurs in thin layers within colluvium and was deposited on the desert pavement of the Monastery colluvium in the central... 

When the wind is armed with grains of sand, it possesses great erosive force, the effects of which are best displayed in rock deserts. Accordingly, any surface subjected to prolonged attack by wind-blown sand is polished, etched or fluted. Abrasion has a selective action, picking out the weaknesses in rocks. For example, discontinuities are opened and rock pinnacles developed. Since the heaviest rock particles are transported near to the ground, abrasion is there at its maximum and rock pedestals may be formed. In deserts, flat smoothed surfaces produced by wind erosion are termed desert pavements. 

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