<P> A large part of the surface area of the world's deserts consists of flat, stone - covered plains dominated by wind erosion . In "eolian deflation", the wind continually removes fine - grained material, which becomes wind - blown sand . This exposes coarser - grained material, mainly pebbles with some larger stones or cobbles, leaving a desert pavement, an area of land overlaid by closely packed smooth stones forming a tessellated mosaic . Different theories exist as to how exactly the pavement is formed . It may be that after the sand and dust is blown away by the wind the stones jiggle themselves into place; alternatively, stones previously below ground may in some way work themselves to the surface . Very little further erosion takes place after the formation of a pavement, and the ground becomes stable . Evaporation brings moisture to the surface by capillary action and calcium salts may be precipitated, binding particles together to form a desert conglomerate . In time, bacteria that live on the surface of the stones accumulate a film of minerals and clay particles, forming a shiny brown coating known as desert varnish . </P> <P> Other non-sandy deserts consist of exposed outcrops of bedrock, dry soils or aridisols, and a variety of landforms affected by flowing water, such as alluvial fans, sinks or playas, temporary or permanent lakes, and oases . A hamada is a type of desert landscape consisting of a high rocky plateau where the sand has been removed by aeolian processes . Other landforms include plains largely covered by gravels and angular boulders, from which the finer particles have been stripped by the wind . These are called "reg" in the western Sahara, "serir" in the eastern Sahara, "gibber plains" in Australia and "saï" in central Asia . The Tassili Plateau in Algeria is an impressive jumble of eroded sandstone outcrops, canyons, blocks, pinnacles, fissures, slabs and ravines . In some places the wind has carved holes or arches and in others it has created mushroom - like pillars narrower at the base than the top . In the Colorado Plateau it is water that has been the eroding force . Here the Colorado River has cut its way over the millennia through the high desert floor creating a canyon that is over a mile (6,000 feet or 1,800 meters) deep in places, exposing strata that are over two billion year old . </P> <P> One of the driest places on Earth is the Atacama Desert . It is virtually devoid of life because it is blocked from receiving precipitation by the Andes mountains to the east and the Chilean Coast Range to the west . The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama . The average precipitation in the Chilean region of Antofagasta is just 1 mm (0.039 in) per year . Some weather stations in the Atacama have never received rain . Evidence suggests that the Atacama may not have had any significant rainfall from 1570 to 1971 . It is so arid that mountains that reach as high as 6,885 m (22,589 ft) are completely free of glaciers and, in the southern part from 25 ° S to 27 ° S, may have been glacier - free throughout the Quaternary, though permafrost extends down to an altitude of 4,400 m (14,400 ft) and is continuous above 5,600 m (18,400 ft). Nevertheless, there is some plant life in the Atacama, in the form of specialist plants that obtain moisture from dew and the fogs that blow in from the Pacific . </P> <P> When rain falls in deserts, as it occasionally does, it is often with great violence . The desert surface is evidence of this with dry stream channels known as arroyos or wadis meandering across its surface . These can experience flash floods, becoming raging torrents with surprising rapidity after a storm that may be many kilometers away . Most deserts are in basins with no drainage to the sea but some are crossed by exotic rivers sourced in mountain ranges or other high rainfall areas beyond their borders . The River Nile, the Colorado River and the Yellow River do this, losing much of their water through evaporation as they pass through the desert and raising groundwater levels nearby . There may also be underground sources of water in deserts in the form of springs, aquifers, underground rivers or lakes . Where these lie close to the surface, wells can be dug and oases may form where plant and animal life can flourish . The Nubian Sandstone Aquifer System under the Sahara Desert is the largest known accumulation of fossil water . The Great Man - Made River is a scheme launched by Libya's Colonel Gadaffi to tap this aquifer and supply water to coastal cities . Kharga Oasis in Egypt is 150 km (93 mi) long and is the largest oasis in the Libyan Desert . A lake occupied this depression in ancient times and thick deposits of sandy - clay resulted . Wells are dug to extract water from the porous sandstone that lies underneath . Seepages may occur in the walls of canyons and pools may survive in deep shade near the dried up watercourse below . </P>

Where do we find deserts and what causes them to occur