A barchan or barkhan dune,, word origin Russian (bär-ˈkän, -ˈḵän), from Turkistan, credited as introduced in 1881 by Russian naturalist Alexander von Middendorf, for crescent-shaped sand dunes in Turkistan and other inland desert regions. Barchans face the wind, appearing convex and are produced by wind action predominately from one direction. They are highly common, characteristic in sandy deserts all over the world and are arc-shaped, markedly asymmetrical in cross section, with a gentle slope facing toward the wind sand ridge, comprising well-sorted sand. This type of dune possesses two "horns" that face downwind, with the steeper slope known as the slip face, facing away from the wind, downwind, at the angle of repose of sand, approximately 30–35 degrees for medium-fine dry sand. The upwind side is packed by the wind, and stands at about 15 degrees.