THE FEATURES IN DESERT LANDSCAPES 



211 



sand and with moderate winds, sickle-shaped dunes known as 

 bar charts (Fig. 221) are formed, whose convex and flatter slopes 

 are toward the wind and whose steep concave leeward slopes are 



FIG. 221. View of desert barchans (after Haug). 



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 6) . 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 beau- 

 tiful ripples in the sand, 

 which, seen from a little 

 distance, may give the ap- 

 pearance of watered silk 

 (plate 7 A). 



Under normal condi- 

 tions dunes are not sta- 

 tionary but continue to 

 wander with the prevail- 

 ing winds until they have 

 reached the outer edge of 

 the zone of vegetation 

 near the base of the foot- 

 hills 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 



FIG. 222. Diagrams to show the relationships 

 in form and in orientation of dunes to the sup- 

 ply of sand and to the strength of the wind, 

 a, barchans formed by small supplies of sand 

 and moderate winds ; 6, 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). 



