EROSION AND SEDIMENTATION 



221 



240. Erosion by wind. We have but to recall the dust 

 clouds of windy days to see clearly that the air has a soil- 

 carrying power, and if we will keep a moistened surface, as 

 of a glass or a towel, in the dust-laden air for a few minutes 

 we shall be able to get some notion of the quantity of this 

 dust. Snow soon becomes colored by dust carried by the 

 wind from places where the soil is exposed. The air is never 



FIG. 109. Erosion by wind 



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

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



entirely free from dust. This air-borne dust is deposited 

 everywhere, and in some places it has accumulated in im- 

 mense banks known as loess. 



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

 the strongest air currents carry the largest pieces. Along the 

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

 readily seen (fig. 109). Sometimes it cuts the bark from trees 

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

 sand may be carried great distances. 



