22 



GEOLOGIC WORK OF ATMOSPHERE 



the ripples be watched closely as the wind blows, they are found to 

 shift their position gradually. Sand is blown up the gentler wind- 

 ward slope to the crest of the ridge, and falls down on the other side. 

 Wear on the windward side may be about equal to deposition on 

 the leeward, and the result is the orderly progression of the ripples 

 in the direction in which the wind is blowing, just as in the case of 

 dune ridges. 



Abrasion. While the effect of the wind on sandy and dusty 

 surfaces is considerable, its effect on solid rock is slight, except 



Fig. 14. Wind erosion. 

 (Cross, U. S. Geol. Surv.) 



Cave rocks near Sierra La Sal, in Dry Valley, Utah. 



where sand and dust are driven against it. Rock worn by wind- 

 blown sand acquires a surface peculiar to the agent accomplishing 

 the work. If the rock is made up of laminae of unequal hardness, 

 the blown sand digs out the softer ones, leaving the harder ones to 

 project as ridges. The sculpturing thus effected on projecting 

 masses of rock is picturesque and striking in some cases (Figs. 14 

 and 15), 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, 

 making the loose earth more readily accessible to wind and water. 

 Organisms of various sorts (certain types of seeds, germs, etc.), as 

 well as dust and sand, are transported far by wind. 



