Wind keeps sand constantly on the move. When one grain is kicked up and falls back to the ground, its force knocks the next grain of sand into the air.
Meanwhile the wind keeps blowing and more and more sand is launched. Dunes tend to form around some barrier, such as a tree, boulder, or fence. When the wind pushes the sand against the obstruction, the grains fall and are blocked from further travel. More sand builds up until a mound, or dune, is created.
As the wind continues, sand is pushed up the face of the mound and just over the crest at the top. The dune then serves as a shelter from the wind, so piles of sand build up steeply on the windless, or leeward, side of the dune. These dunes can reach hundreds of feet (meters) high.
If the wind doesn’t stop, the dune actually travels across the desert, or beach, as grains continually blow from one side, up over the crest, to the other. Dunes can travel more than 80 feet (24 meters) in a year in the desert.