Barchan

A barchan, or barkhan dune, is a crescent shaped sand dune that forms in deserts where the wind blows steadily from one dominant direction, giving the dune a convex face pointed into the wind and two horns that trail away downwind. The gentler windward slope rises at around fifteen degrees while the steeper leeward slip face sits at roughly thirty to thirty five degrees for typical medium fine dry sand, and a single barchan can stand nine to thirty meters high and reach up to 370 meters across at its base. Barchans migrate slowly in the direction of the prevailing wind, moving from about one meter to as much as 100 meters a year as wind erodes sand from the windward side and deposits it on the slip face, and they can occur alone, in groups, or merged together into larger ridge systems; the term entered scientific use in 1881 through the Russian naturalist Alexander von Middendorf, who observed the dunes in Turkestan, and comes from a Kazakh word. Barchan-like dunes have also been photographed on Mars and identified underwater on the seabed.