Dune motion Sand dunes move due to wind. As air moves rapidly up the slope of a sand dune, it causes sand to saltate, bouncing upward with one grain hitting another until it reaches the crest of the dune. At the crest, sand piles up until the dune becomes too steep â the weight of the sand overcomes the frictional forces that hold the grains together. At that point, a small avalanche takes place â sand heads down the steep side of the dune, restoring the balance between friction and gravity. This shot of Californiaâs Kelso Dunes captures a marvelous set of examples of this process; a series of sand grain avalanches, one after another, bringing material down from the crest of the dune. -JBB Image credit: http://bit.ly/1F6tmPl Read more: http://www.desertusa.com/geofacts/sanddune.html http://www.nps.gov/grsa/learn/nature/dune-types.htm
