THE ATMOSPHERE AS A GEOLOGICAL AGENT, 



39 



Sand drifted over loose stones lying on the surface often develops 

 flat or flattish faces or facets on them. These facets are likely to be 

 three in number, and the exposed portion of the stone is likely to de- 



FiG. 22. — Wind-carved hillock of cross-bedded sandstone. Missouri River, Montana. 



(Calhoun, U. S. G. S.) 



velop a sort of pyramidal shape, the three flattish surfaces being mutu- 

 ally limited by tolerably well-defined lines (Fig. 24). Thus arise the 

 three-faceted stones (Dreikanter of the Germans) commonly seen 

 where sands have been long in movement. 



Not only does the drifting sand wear the surface over which it 

 passes and against which it strikes, but the grains themselves are 

 worn in the process. They are liable to be broken as they strike rock 

 surfaces, and they are likely to strike one another in the atmosphere. 

 In both cases they are subject to wear, and so to reduction to a finer 

 and finer state. 



The erosion accomplished by the wind is therefore of various sorts. 

 The impact of the wind itself picks up the fine materials which are 

 already loosened, thus wearing down the surface from which they 

 are removed; the materials picked up wear the rock surfaces against 

 which they are blo^vn, and the transported materials themselves suffer 

 reduction in transit. 



Effects of wind on plants. — Another effect of wind work is seen 

 in the uprooting of trees (Fig. 25). The uprooting disturbs the sur- 



