WIND EROSION 135 
fact that the wind blows with variable velocity. When 
violent, coarse particles are removed and deposited, 
while in more quiet weather only the finer fragments 
can be moved. Besides this, the layers are tilted 
at various angles according to the direction of the 
Fig. 64. 
A forest being destroyed by the encroachment of sand dunes near the shore 
of Lake Michigan. 
wind, which is also variable. Therefore the wind-drift 
structure in a deposit of blown sand is very com- 
plex, with layers of different texture set at various 
angles (Fig. 65). 
Erosive Action. —Not only is sand moved by the 
air, but even rocks are destroyed when they stand in 
