100 GEOLOGY 



orderly progression of the ripples in the direction in which the 

 wind is blowing, just as in the case of dune ridges. 



Abrasion by the wind. While the effect of the wind on sandy 

 and dusty surfaces is considerable, its effect on solid rock is rela- 

 tively slight and accomplished by the impact of the sand and dust 

 it carries. Rock worn in this way acquires a surface peculiar to 

 the agency accomplishing the work. If the rock is made up of 

 Iamina3 which are of unequal hardness, the blown sand digs out 



Fig. 61. A ripple-marked sand dune in a western valley. (U. S. Geol. 



Surv.) 



the softer ones, leaving the harder ones projecting as ridges between 

 them. Adjacent masses of harder and softer rock of whatever 

 thickness are similarly affected. The sculpturing thus effected 

 on projecting masses of rock is often picturesque and striking 

 (Figs. 62, 63), and is most common in arid regions. 



Effect of wind on plants. Another effect of strong winds is 

 seen in the uprooting of trees. The uprooting disturbs the surface 

 in such a way as to make loose earth more readily accessible to 

 wind and water. The uprooting of trees on steep slopes often 

 causes the descent of considerable quantities of loose rock and soil. 



