Yardangs are streamlined hills carved from bedrock or any consolidated or semiconsolidated material by the dual action of wind abrasion (by dust and sand) and deflation. Unlike most hills carved by running water and mass wasting, yardangs typically are three or more times longer than wide. Individual yardangs range from a few centimeters to several kilometers in length and from a few centimeters to 30 m or more in height; within any one field the dimensions are generally consistent. Along the base of each yardang flank, especially in the larger yardangs, sizable grooves are common. These grooves, cut by abrasion, mark the location of the densest part of the airborne sediment load. Yardangs are typically highest and broadest at the blunt end that faces into the wind; most become characteristically lower and narrower toward the lee end (McCauley, et al.[1]). They may be isolated, or may occur in groups, called fields (or fleets because of the resemblance of the yardangs to inverted boat hulls). Yardangs form the ridges of grooved terrain. Small yardangs (about 2 to 4 m high) are most commonly carved in semiconsolidated playa sediments and other relatively soft, granular materials. Because these features are small, they are usually not visible on satellite images such as Landsat.
In arid regions with strong, unidirectional winds, kilometer-scale yardangs may be carved in siltstone, sandstone, shale, and limestone and rarely in very hard, crystalline rocks such as schist and gneiss [1]. Most of these large yardangs are part of a large area of grooved terrain; but fleets of yardangs without the associated troughs of grooved terrain are also found in isolated bedrock outcrops. Extensive fleets of yardangs occur in siltstone in the Ica Valley of Peru and in sandstone and shale below plateau scarps in the Kharga, Farafra, and Dakhla Depressions in southwestern Egypt. In the Kharga, Farafra, and Dakhla areas, yardangs also occur above the scarps in grooved terrain carved in the limestone cap rock of the plateau. Most yardang fields are in sand-poor areas, but the associated troughs, especially in grooved terrain, may be invaded by sand. This sand can accumulate to build shallow moats around the blunt, forward ends of the yar dangs and to form sand tails streaming to leeward (Breed, et al.[2]).
The development of yardangs requires a very strong, virtually unidirectional winds and an airborne sediment load. The frequency of such winds is unknown. Wind speeds are highest along the flanks and upper surfaces at the highest and broadest part of the yardang. Turbulence increases at the tapering flanks to ward the lee end.
Because yardangs are formed in bedrock or consolidated and semiconsolidated materials, the surfaces between the yardangs are hard and firm. Therefore, vehicular traffic is possible through fleets of isolated, individual yardangs. The problem is primarily one of finding the "right" path through the branching, interconnected corridors between the yardangs. Where yardangs are part of grooved terrain, cross-country movement is generally impossible. Yardangs do not offer good shelter from wind and blowing sediment along their flanks.
(common names are in bold) Mud-lions, sphinxlike hills, mushroom rocks (some), inselbergs (some), koukour (Tunisia), kalut (Iran), cretes et couloirs (Chad)
1. McCauley, J.F., M.J. Grolier, and C.S. Breed. 1977. Yardangs. In Geomorphology in arid regions, edited by D.O. Doehring. Proceedings, 8th Annual Geomorphology Symposium, Binghamton, NY, pp. 233-272.
2. Breed, C.S., J.F. McCauley, and M.I. Whitney. 1989. Wind erosion forms. In Arid zone geomorphology, edited by D.S.G. Thomas. New York: Halstead Press, pp. 284 307.
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