THE WORK OF THE ATMOSPHERE 



89 



depend on the strength 

 and structure of the rocks. 



in a single day in passing through severe sand storms. In 

 many places telegraph poles must be protected, or the wind 

 cuts them down in a comparatively short time (Fig. 76). 

 Wind wear (abrasion), like wind transportation, is most 

 important in dry regions, 

 for in such places slopes 

 are often bare and unpro- 

 tected, and the winds, fre- 

 quently strong, are likely 

 to be abundantly supplied 

 with tools. 



Characteristics Of Wind- FIG. 77. Wind-worn surface in Wyo- 



WOin surfaces. The de- ming. The protruding masses are harder 



. than the rock which surrounds them, 



tails Ot Wind-worn Surfaces Why are these rocks not etched like 



those of Figure 78 ? 



Rocks of varying strength wear 

 unequally (Fig. 77), and often in the case of stratified rocks 

 the more rapid removal of the weaker layers or laminae 

 leaves the stronger ones in relief (Fig. 78). Horizontal beds 



_. _ in deserts may be eroded 



into extensive flat-topped 

 elevations, which are later 

 cut up into abrupt conical 

 hills, and finally destroyed. 

 Inclined beds (whether 

 worn by wind or water) 

 tend to develop hills hav- 

 ing a relatively long and 

 gentle slope in the direction 

 in which the beds dip, and 

 a shorter and steeper op- 

 posite facing slope. Wind- 

 abraded elevations generally suffer most rapid wear near 

 the bottom, in spite of the fact that the wind is retarded 

 by friction with the surface of the ground and its blows 

 thereby weakened, for many more tools are carried in the low- 



FIG. 78. Cross-bedded sandstone 

 etched by the wind. 



