<P> Many people think of deserts as consisting of extensive areas of billowing sand dunes because that is the way they are often depicted on TV and in films, but deserts do not always look like this . Across the world, around 20% of desert is sand, varying from only 2% in North America to 30% in Australia and over 45% in Central Asia . Where sand does occur, it is usually in large quantities in the form of sand sheets or extensive areas of dunes . </P> <P> A sand sheet is a near - level, firm expanse of partially consolidated particles in a layer that varies from a few centimeters to a few meters thick . The structure of the sheet consists of thin horizontal layers of coarse silt and very fine to medium grain sand, separated by layers of coarse sand and pea - gravel which are a single grain thick . These larger particles anchor the other particles in place and may also be packed together on the surface so as to form a miniature desert pavement . Small ripples form on the sand sheet when the wind exceeds 24 km / h (15 mph). They form perpendicular to the wind direction and gradually move across the surface as the wind continues to blow . The distance between their crests corresponds to the average length of jumps made by particles during saltation . The ripples are ephemeral and a change in wind direction causes them to reorganise . </P> <P> Sand dunes are accumulations of windblown sand piled up in mounds or ridges . They form downwind of copious sources of dry, loose sand and occur when topographic and climatic conditions cause airborne particles to settle . As the wind blows, saltation and creep take place on the windward side of the dune and individual grains of sand move uphill . When they reach the crest, they cascade down the far side . The upwind slope typically has a gradient of 10 ° to 20 ° while the lee slope is around 32 °, the angle at which loose dry sand will slip . As this wind - induced movement of sand grains takes place, the dune moves slowly across the surface of the ground . Dunes are sometimes solitary, but they are more often grouped together in dune fields . When these are extensive, they are known as sand seas or ergs . </P> <P> The shape of the dune depends on the characteristics of the prevailing wind . Barchan dunes are produced by strong winds blowing across a level surface, and are crescent - shaped with the concave side away from the wind . When there are two directions from which winds regularly blow, a series of long, linear dunes known as seif dunes may form . These also occur parallel to a strong wind that blows in one general direction . Transverse dunes run at a right angle to the prevailing wind direction . Star dunes are formed by variable winds, and have several ridges and slip faces radiating from a central point . They tend to grow vertically; they can reach a height of 500 m (1,600 ft), making them the tallest type of dune . Rounded mounds of sand without a slip face are the rare dome dunes, found on the upwind edges of sand seas . </P>

What type of plant life is in the desert