38 



GEOLOGY. 



light on the origin of the great dune ridges. If the ripples be watched 

 closely during the progress of a wind-storm, they are found gradually to 

 shift their position. Sand is blown up the gentler windward slope 

 to the crest of the ridge and falls down on the other side. The moment 

 it falls below the crest of the ridge to leeward, it is protected against the 

 wind, and is likely to lodge. Wear on the windward side is about 

 equal to deposition on the leeward, and the result is the orderly pro- 

 gression 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 may be considerable, its effect on solid rock is relatively 

 slight and accomplished, not by its o^^m impact, but by that of the 

 material it carries. The effect of blown sand on rock surfaces over and 

 against which it is driven is perhaps best understood by recalling the 

 effects of artificial sand-blasts, by means of which glass is etched. In 

 a region where sand is blowing, exposed surfaces of rock suffer from 

 a multitude of blows struck by the sand grains in transit. The result 

 is that such rock surfaces are worn, and worn in a way peculiar to the 

 agency accomplishing the work. If the rock be made up of laminsB 



Fig. 21. — AVind-carved rock. (Green.) 



which are of unequal hardness, the blown sand digs out the softer ones, 

 leaving the harder 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. 21 and 22), and is most 

 common in arid regions. Details of wind-carving are shown in Fig. 23. 



