Barchan dunes in Tenere Desert (Sahara), Niger. The major requirement for dune-building is a sand supply; in deserts this is transported by wind from areas of erosion. Barchan (crescent-shaped) dunes are formed in desert areas where the pre- vailing wind blows from one direction. In this photograph it would be blowing from left to right. An oval shape forms upwind and behind it a slip face as sand blows over the summit of the dune, and two wings spread out downwind. Such dunes grow at the rate of wind deposition and advance forward at a speed subject to the wind. Barchans typically occur in belts up to 300km long and over 12km wide. Note also the wind ripple effect.

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