dune

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Mound or ridge of wind-drifted sand, common on coasts and in deserts. Loose sand is blown and bounced along by the wind, up the windward side of a dune (saltation). The sand particles then fall to rest on the lee side, while more are blown up from the windward side. In this way a dune moves gradually downwind. Dunes occur in areas where there is a large supply of sand, strong winds, low rainfall, and some vegetation or obstructions to trap the sand.

In sandy deserts, the typical crescent-shaped dune is called a barchan. Seif dunes are longitudinal and lie parallel to the wind direction, and star-shaped dunes are formed by irregular winds.

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