=The forms of dunes.=—The forms assumed by dunes are dependent to a very large extent upon the strength of the wind and the available supply of sand. With small quantities of sand and with moderate winds, sickle-shaped dunes known as _barchans_ (Fig. 221) are formed, whose convex and flatter slopes are toward the wind and whose steep concave leeward slopes are maintained at the angle of repose. The barchan is shaped by the wind going both over and around the dune, constantly removing sand from the windward side and depositing it to leeward. With larger supplies of sand and winds which are not too violent a series of barchans is built up, and these are arranged transversely to the wind direction (Fig. 222 _b_). If the winds are more violent, the minor depressions in the crests of the dunes become wind channels, and the sand is then trailed out along them until the arrangement of the ridges is parallel to the wind (Fig. 222 _c_). The surfaces of dunes are generally marked by beautiful ripples in the sand, which, seen from a little distance, may give the appearance of watered silk (plate 7 A). [Illustration: FIG. 221.—View of desert barchans (after Haug).] [Illustration: FIG. 222.—Diagrams to show the relationships in form and in orientation of dunes to the supply of sand and to the strength of the wind. _a_, barchans formed by small supplies of sand and moderate winds; _b_, transverse dune ridges, formed when supply of sand is large and winds are moderate; _c_, dune ridges formed with large sand supply and violent winds (after Walther and Cornish).] Under normal conditions dunes are not stationary but continue to wander with the prevailing winds until they have reached the outer edge of the zone of vegetation near the base of the foothills at the margin of the desert. Here the grasses and other desert plants arrest the first sand grains that reach them, and they continue to grow higher as the sands accumulate. Some of the desert plants, like the yuccas, have so adapted themselves to desert conditions that they may grow upward with the sand for many feet and so keep their crowns above its surface.