310 



ELEMENTS OF GENERAL SCIENCE 



Sand is carried by air currents, and, as in the case of 

 water, the strongest currents carry the largest pieces. Along 

 the shores of lakes and oceans the action of wind-blown sand 

 is readily seen (fig. 150). Sometimes it cuts the bark from 

 trees and carves out the softer portions of large rocks. The 

 finest sand may be carried great distances. 



. 150. Erosion by wind 



The hill to the right, a sand dune, is being blown away in spite of the protecting 

 cover of vegetation. At the left, sand is burying a group of trees 



In some regions the winds build dunes, or hills, of sand, 

 those of coarsest sand near -the shore and those of finest 

 sand farthest away. Some of these dunes may keep moving 

 inland as sand from the windward side is carried over and 

 dropped on the leeward side. A dune may thus migrate far 

 inland, and with a change of prevailing winds it might 

 migrate back to the shore or even into the lake or ocean. 

 Sometimes vegetation gains a foothold on a dune, and plants 

 may live upon it for ages ; then winds may again bring sand 

 to the dune, cover the vegetation beyond the tops of the 



