Scientists have discovered that deserts used to be lands where water and grasslands ruled. The Sahara used to be a place where ancient humans roamed and lived with many other animals only 10,000 years ago. Cave paintings show animals wandering in grass, yet the Sahara became a barren desert some 5,000 years later. Some scientists believe that slow temperature change probably caused plants to die and the soil to erode.
Water and wind erosion has slowly shaped deserts and will continue to do so. When rare, heavy rains flood the deserts, rocks and sand are swept away. As the rocks and sand are swept through valleys they make them deeper and wider. The water eventually pushes the rocks in low areas of the valley, called playas, or to the foot of slopes. This loose rock can pile up at the foot of these slopes and form large piles of rocks, called alluvial fans or cones. When other alluvial fans collide, together they form bajadas (or bahadas).
Canyons, plateaus, buttes, and mesas are all formed by water erosion. Plateaus are high areas that have not been affected by water erosion, except when canyons slice into them. Rivers that wash away soft rocks and sand slowly form canyons. Parts of plateaus can also be eroded away into large islands called mesas. As water and wind erode mesas, slowly they shrink in size and become a butte.
Winds also work with water to cause erosion. As powerful winds pick up dust, they can create dust and sandstorms that can reach 9,500 feet (3,000 meters) into the air. Winds can even roll large rocks on the ground. Pebbles and sands are usually lifted 5 feet (1.5 meters) above the ground. Most of the erosion in the deserts are caused by the wind. Wind erosion causes different formations in cliffs and large rocks. The wind can wear out small caves in cliffs, form rock pillars, arches, mushroom rocks, and yardangs. Mushroom rocks look like large rocks leaning on skinny pillars. Yardangs are overhanging ridges with steep sides.
The sand itself and rocks are also shaped by the wind. Rocks can be rounded by wind and also form strange shapes called zeugen. Zeugens usually take the shape of a wave when they have been undercut by the wind (Lye,14). The desert surface itself can be changed by the wind. Wind can cause depressions in the desert surface in a process called deflation. As loose dust and sand grains are blown away, basins are formed. Eventually these basins turn into aquifers. Water helps stabilize rocks and sand in the area.
The largest basin formed by the wind can be found in the Quattara depression in Egypt. One-fifth of the world's deserts are covered in sand. The others are covered in rocks or hard, bare rock. The ones covered in sand have ever moving sand dunes. There are three different types of sand dunes, all of which are formed by the wind. Transverse dunes form at right angles to strong, prevailing winds. Crescent dunes form crescent shaped dunes ahead of the prevailing winds. Star shaped dunes are formed when winds frequently change direction.