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■15 



such growth cannot go on indefinitely (1) as there is a hmit be- 

 yond which plant life cannot sustain itself under such conditions, 

 and {2) if a dune grows beyond a certain height (varying with 

 the local conditions) the top will be blown oft since the velocity 

 of the wind increases rapidly with altitude. 



It frequently happens that the climate is too arid to support 

 plant life on dunes, or that the dunes themselves are too active. 

 When such conditions exist the dune travels across the country 

 with the prevailing wind, covering everything in its path. Farm 

 lands, buildings, forests, and rivers, offer little resistance (Fig. 

 1). Desolation reigns supreme in its path. Such movement 

 does not occur rapidly as we are w^ont to think of speed, but by 

 the relatively slow process of carrying sand grains up the wind- 

 ward side (a, Fig. 2) and dropping them again on the leeward 

 side. Thousands of dollars are spent in some localities in plant- 

 ing sand reeds upon the drifting dunes to stop this movement. 

 When the wind prevails from one direction the dune acquires 

 the characteristic shape indicated in Fig. 2, viz., gentle windward 



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Figure 3. — Wind ripples. These ripples were formed by a wind blow- 

 ing from right to left. The first and second vegetation zones are shown 

 also. (Caldwell.) 



