A sand dune is a hill made of sand, i.e., finely divided rock and mineral particles. Dunes can reach hundreds of feet high -- or, in rare cases, over a thousand feet. Wind plays an important part in creating sand dunes, helping to accumulate a lot of sand in one place. A dune may form because of vegetation blocking wind and allowing sand to accumulate in the the wind shadow of the vegetation. Dunes come in many types due to different wind patterns.
Deserts often include vast, relatively flat and deserted areas. When wind removes all the finer sand particles and leaves a rougher surface behind, scientists call what's left behind "pavement." Desert pavement with loose stones, such as regs in Northern Africa, contain little vegetation. Sand, silt and volcanic fields make up other kinds of desert surfaces. Vegetation arises on surfaces that have some kind of soil, even a thin layer, but of course only plants that need little water will survive on arid desert surfaces.
A desert often hosts steep, rugged mountains with basins called bolsons in between. When rain falls, it creates gullies in these rugged mountains over time, and debris from the mountains will accumulate at the bottom of these gullies, forming alluvial fans. Large areas of rugged desert mountains, gullies and alluvial fans are referred to as badlands.
Temporary, ephemeral streams formed when rains fall in deserts can trickle down to playas, or desert sinks, where rain evaporates or penetrates into the ground. Playas hold water usually only part of the year or after storms and amount to temporary shallow lakes covering an area of clay or silt or a mixture of both. All playas have no outlets, being in closed basins, and usually no vegetation. Plays that hold water year-round are referred to as salinas.
