Accumulations of sediment blown by the wind into a mound or ridge, dunes have gentle upwind slopes on the wind-facing side. The downwind portion of the dune, the lee slope, is commonly a steep avalanche slope referred to as a _slipface_. Dunes may have more than one slipface. The minimum height of a slipface is about 30 centimeters. Sand grains move up the dune’s gentle upwind slope by saltation and creep. When particles at the brink of the dune exceed the angle of repose, they spill over in a tiny landslide or avalanche that reforms the slipface. As the avalanching continues, the dune moves in the direction of the wind. Some of the most significant experimental measurements on eolian sand movement were performed by Ralph Bagnold, a British engineer who worked in Egypt prior to World War II. Bagnold investigated the physics of particles moving through the atmosphere and deposited by wind. He recognized two basic dune types, the crescentic dune, which he called “barchan,” and the linear dune, which he called longitudinal or “sief” (Arabic for “sword”). [Illustration: Sand dunes in Death Valley, California (photograph by Richard Frear).] [Illustration: Ripples on a dune in Eureka Valley, California (photograph by Terrence Moore).] Types of Dunes A worldwide inventory of deserts has been developed using images from the Landsat satellites and from space and aerial photography. It defines five basic types of dunes: _crescentic_, _linear_, _star_, _dome_, and _parabolic_. [Illustration: These crescentic dunes of coastal Peru are migrating toward the left (photograph by John McCauley).] The most common dune form on Earth and on Mars is the _crescentic_. Crescent-shaped mounds generally are wider than long. The slipface is on the dune’s concave side. These dunes form under winds that blow from one direction, and they also are known as barchans, or transverse dunes. Some types of crescentic dunes move faster over desert surfaces than any other type of dune. A group of dunes moved more than 100 meters per year between 1954 and 1959 in China’s Ningxia Province; similar rates have been recorded in the Western Desert of Egypt. The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than 3 kilometers, are in China’s Taklimakan Desert. Straight or slightly sinuous sand ridges typically much longer than they are wide are known as _linear dunes_. They may be more than 160 kilometers long. Linear dunes may occur as isolated ridges, but they generally form sets of parallel ridges separated by miles of sand, gravel, or rocky interdune corridors. Some linear dunes merge to form Y-shaped compound dunes. Many form in bidirectional wind regimes. The long axes of these dunes extend in the resultant direction of sand movement. [Illustration: Star dunes, such as these of the Namib, indicate the winds that formed them blew from many directions (photograph by Georg Gerster).]