Sand covers only about 20 percent of the Earth’s deserts. Most of the
sand is in sand sheets and sand seas—vast regions of undulating dunes
resembling ocean waves “frozen” in an instant of time.
Nearly 50 percent of desert surfaces are plains where eolian
deflation—removal of fine-grained material by the wind—has exposed loose
gravels consisting predominantly of pebbles but with occasional cobbles.
The remaining surfaces of arid lands are composed of exposed bedrock
outcrops, desert soils, and fluvial deposits including alluvial fans,
playas, desert lakes, and oases. Bedrock outcrops commonly occur as
small mountains surrounded by extensive erosional plains.
Oases are vegetated areas moistened by springs, wells, or by irrigation.
Many are artificial. Oases are often the only places in deserts that
support crops and permanent habitation.
[Illustration: Underground channels carry water from nearby mountains
into the Turpan Depression of China. If the channels were not covered,
the water would evaporate quickly when it reached the hot, dry desert
land.]