THE WORK OF THE ATMOSPHERE 91 



in regions draining to the sea, in that the latter cannot be worn 

 below sea level (p. 139) while the former are independent of 

 it. The two are alike in having nearly level surfaces, which 

 are independent of the underlying rock structures. Old 

 desert plains with rock floors are said to cover thousands of 

 square miles in South Africa. 



DEPOSITION 



Formation of dunes. It has already been pointed out 

 (p. 88) that the deposition of dust by the wind rarely gives 

 rise to topographic features of importance, while that of 

 sand frequently does. Elevations of wind-deposited sand 

 are dunes. Figure 79 shows how sand begins to accumulate 



FIG. 79. Sketch from photograph showing how sand accumulates about 

 an obstacle which it cannot penetrate. 



about an obstacle which it cannot penetrate. It is deposited 

 on both the windward and leeward sides, but is prevented 

 from resting directly against the obstacle by the air that is 

 reflected from it, and by wind eddies. In the case of a pene- 

 trable obstacle, such as a hedge or open fence, the sand lodges 

 chiefly on the leeward side (Fig. 80). Figure 81 shows sand be- 

 ginning to gather in and about obstructing vegetation. Once 

 started, a dune causes the lodgment of more sand, and so 

 occasions its own growth. 



Distribution, size, and shape of dunes. Dunes occur 

 chiefly along sea and lake shores, along sandy valleys, and in 

 deserts ; in a word, where quantities of bare, dry sand are ex- 



