SAND DUNES 



387 



to another, but it uses these particles to cut and erode ob- 

 stacles in its path. The artificial sand blast is in common 

 use. In it a stream of sand is driven with great velocity 

 upon an object which it 

 is desired to etch. In 

 nature the same kind of 

 etching is done by the 

 wind-blown sand. 



The glasses in the 

 windows of lighthouses 

 along sandy coasts are 

 sometimes so etched as 

 to lose their transpar- 

 ency. Rocks exposed to 

 the winds are carved 

 and polished; the softer TRER BK[Na Duo up BY THp W[ND 

 parts are worn away 



more rapidly than the harder parts, just as in all other 

 forms of erosion. In certain regions where the prevailing 

 winds are in one direction, one side of exposed rocks is 

 found to be polished, while the other sides remain rough. 



178. Wind Burying and Exhuming. In exposed sandy 

 regions where there are strong winds, objects which ob- 

 struct the movement of the air cause deposition of the 

 transported sand just as obstructions in flowing water 

 cause sediment to be deposited. And just as sand bars 

 may be deposited by>a river and then carried away again, 

 owing to a change in the condition of the river's load, so 

 forests and houses in sandy regions are sometimes buried, 

 to be uncovered again perhaps by a change in the load 

 carried by the wind. 



179. Sand Dunes. Sand-laden wind generally deposits 

 its burden in mounds and ridges called sand dunes (page 

 302). When once a deposition pile begins, it acts as a 



