DUNES 



389 



ridged appearance of the rock is due to the fact that the soft 

 parts of it have worn away more rapidly than the harder parts. 



In regions where rainfall is abundant, the wind has few 

 tools, because the soil is damp and the particles cling together 

 and the wind cannot pick them up. But in arid regions, where 

 the soil is dry and loose, the wind never lacks weapons and 

 does considerable damage. In desert regions windows in 

 railroad trains have been destroyed in a single day by sand 

 storms. 



Dunes. Sand particles which are too heavy to be lifted are 

 rolled and dragged along the ground until they meet an obstacle, 

 such as a bush, a log, or a 

 stone, which hinders their 

 progress. There they lodge, 

 forming mounds and hill- 

 ocks (Fig. 257). In deserts 

 and sandy valleys and on 

 sea beaches, the ridges and 

 mounds of wind-deposited 

 sand are large and numerous 

 and are called dunes (Fig. 

 258). In the dry regions of 

 Kansas and Nebraska, on 

 the sandy shores of Lake Michigan, and along the New Jersey 

 coast dunes are seen everywhere. Anything that serves as an 

 obstacle to wind-blown sand causes dunes. Figure 259 shows 

 how a box can be the origin of a dune. 



Some dunes remain small, a foot or two high ; others rise as 

 high as four hundred feet. The size of the dune depends upon 

 the winds which prevail in the region, upon the character of 

 the soil, and upon the obstacles encountered by the winds. 

 Dunes are not stationary, but are slowly shifted by the winds 

 which blow upon them. If unusually fierce winds arise, the 



FIG. 257. Vegetation starts a sand dune 

 and is then often overwhelmed by it. 



