A hill or ridge of sand accumulated and sorted by wind action. Sand will settle on a dune because the friction of the sandy surface is enough to slow the wind, which then sheds some of its load. Dunes formed in the lee of an obstacle are topographic dunes: lunettes form in the lee of a deflation hollow, nebkhas in the lee of bushes, and wind shadow dunes in the lee of hills and plateaux. Crescentic dunes—sand mounds, barchans, barchanoids, and transverse ridges—are restricted to areas with unidirectional high winds and minimal vegetation (Bishop (2001) ESPL 26).

Transverse dunes form when sand supply is abundant, and wind direction constant (Walker and Nickling (2002) PPG 26, 1). As the sand supply is reduced, the dunes transform into parabolic dunes—hairpin-shaped with the bend pointing downwind (Mitasova et al. (2005) Geomorph. 72, 1–4). Where the direction is very changeable, star dunes form. The linear seif dunes form when two prevailing winds alternate, either daily or seasonally; the leeward slope is a zone of deposition, as well as a zone of erosion (Tsoar et al. (2004) Geomorph. 57, 3–4). When sand is limited, barchans form with the horns pointing downwind; see Katsuki et al. (2011) ESPL 36, 3, 372 on a collisional simulation of barchans. The height of the slipface of a barchan is proportional to the width of the horns (Wang et al. (2007) Geomorph. 89, 3–2). Barchans may be changed into dome dunes, and vice versa; completely gradational forms exist between the two (Fryberger et al. (1984) Sedimentol. 31). Barchanoid dunes are undulating, continuous cross-wind dunes which may grade into long transverse dunes; see R. Cooke et al. (1993). Dune fields have recently come to be recognized as self-organizing systems that can be seen progressing from states of disorganization or randomness to uniformity; see Wilkins and Ford (2007) Geomorph. 83, 1–2.

Sand dunes Arrows indicate wind direction