Barchan
A barchan dune is an arc-shaped sand ridge, comprised of well-sorted sand. This type of dune possesses two "horns" that face downwind, with the slip face (the downwind slope) at the angle of repose, or approximately 32 degrees. The upwind side is packed by the wind, and stands at about 15 degrees. Simple barchan dunes may stretch from meters to a hundred meters or so between the tips of the horns.
Simple barchan dunes may appear as larger, compound barchan or megabarchan dunes, which may migrate with the wind. Barchans and megabarchans may coalesce into ridges that extend for hundreds of kilometers.
The Great Sand Dunes National Monument and Preserve in Colorado has spectacular examples of barchan dunes.
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