WORK OF THE WIND 



47 



regions because there only the wind finds sufficiently thick accumula- 

 tions of dry sand and sufficiently extended flat surfaces for effective 

 work. There also the winds blow in the same direction a sufficient 

 length of time. Frequent changes in the direction of winds are as 

 unfavorable to the development of dunes as vegetation or a rough 

 topography. 



If the direction of the wind is constant, typical dunes will have a 

 gentle slope on the windward side and a steep slope on the leeward side 



• 





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, r . i _. : v- ^ • , ; |JV ,-->..-. 











Fig. 25. — Sand dunes. The direction of the wind was from the right to the left. 

 (Photo. D. T. MacDougal.) 



(Fig. 25). This difference between the windward and leeward slopes 

 is due to the fact that the sand is pushed up the former by the wind 

 and dropped over the crest, where it comes to rest at a steeper angle. 

 On the other hand, if the prevailing direction of the wind changes from 

 season to season, the difference between the angles of the slopes will 

 be less marked. The slope is generally steepest on high dunes, but 

 is never greater than io° on the windward and 30 on the opposite 

 side. Since the winds vary greatly in velocity from time to time, the 

 size of the sand particles carried up the dunes differs and usually 

 produces distinct layers, or stratification. The inclination or dip 

 (p. 252) of the stratification also varies widely in direction and steep- 

 ness (Fig. 26), since it depends upon the direction and the force of the 

 wind. The formation of this cross-bedding, as the layers in one 



CLELAND GEOL. — X 



