Dunes form wherever there is enough sand and propelling wind, as well as an absence of vegetation, and they acquire one of several characteristic shapes, depending on the velocity and direction of the wind. A barchan, or barkhan, dune is a crescent-shaped sand dune formed in places where the direction of the wind is fairly constant. The windward or stoss slope of the dune is gradual, the leeward side, or slip face, is steep, and the crest slightly horned. Dunes travel as individuals or in groups of varying size, like ocean waves. Here is Edward Abbey in “Desert Images” from The Serpents of Paradise: “Seen from the bird’s point of view, most . . . desert sand dunes have a crescent shape, like the new moon. The horns of the crescent point downwind, with the slip face on the inside of the curve. This type of dune is called a barchan—a Russian term.” According to the Oxford English Dictionary, the term is originally Kazakh, that is, Turkic. In the Great Sand Dunes National Park in southern Colorado there are waves of barchan dunes set against the Sangre de Cristo Mountains.