Sand covers only about 20 percent of the Earth's deserts. 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.
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. Soils Soils that form in arid climates are predominantly mineral soils with low organic content. The repeated accumulation of water in some soils causes distinct salt layers to form. Caliche is a reddish-brown to white layer found in many desert soils.