Deserts display a great variety of landscape features. Sand dunes, the landforms commonly thought of as typical of deserts, occupy only a relatively small part of the world's deserts. Some deserts, however, have great expanses of dunes, known as sand seas, or ergs. Bare rock surfaces, called yardangs, are found in many deserts; they occur where wind and water have removed the surface particles and left the hard bedrock exposed. Commonly, flat areas are covered by what is known as a desert pavement, a closely compacted layer of rock and gravel. In some places, especially at the foot of a mountain, there are large deposits of gravel and other coarse debris, which are sometimes quite deep.
In many deserts flash floods form temporary rivers that carve steep-sided valleys called dry washes, arroyos, or wadis. Badlands and deep gullies are also common in arid lands.
The water carried by flash floods sometimes accumulates in shallow depressions in the desert floor, called playas. Playa lakes usually disappear quickly and leave behind sand, silt, and dissolved minerals, which may form salt flats. Though most rivers and lakes are intermittent, there can be permanent water features—such as the Great Salt Lake, in Utah, and the Dead Sea in Israel and Jordan.