GEOLOGICAL WORK OF THE ATMOSPHERE 93 



ward side as well as on its leeward side (Fig. 55); but if it be 

 penetrable, like an open fence, the lodgment takes place chiefly 

 on its leeward side. In cultivated regions, cases are known where, 



Fig. 55. Diagram to illustrate the effect of an obstacle on the transporta- 

 tion and deposition of sand. The direction of the wind is indicated by 

 the upper arrow. The lower arrows represent the direction of eddies 

 in the air occasioned by the obstruction. If the surface in which the 

 obstacle was set was originally flat (dotted line), the sand would tend 

 to be piled up. on either side at a little distance from it, but more to lee- 

 ward. At the same time, a depression would be hollowed out near the 

 obstacle itself (full line). (After Cornish.) 



in a few weeks of dry weather, sand has been drifted into lanes in 

 the lee of hedges to the depth of two or three feet, making it difficult 

 for vehicles to pass. 



Dunes. In contrast with dust deposited from the atmosphere, 

 wind-blown sand is often aggregated into mounds and ridges 

 called dunes. Dunes sometimes reach heights of 200 or 300 feet, 

 but much more commonly they are no more than 10 or 20 feet in 

 height. The shape of dunes depends, among other things, on the 

 extent and form of the area furnishing the sand, the strength and 

 direction of the wind, and the shape of the obstacles which occasion 

 the lodgment. 



The shapes of the cross-sections of dunes are influenced by the 

 strength and constancy of the winds. With constant winds and 



Fig. 56. Section of a dune showing, by the dotted line, the steep leeward 

 (be) and gentler windward (ab) slope. By reversal of the wind, the cross- 

 section may be altered to the form shown by the line adc. (Cornish.) 



abundant drifting sand, dunes are steep on the lee side (6c, Fig. 56), 

 where the angle of slope is the angle of rest for the sand. It rarely 

 exceeds 25. Under the same conditions, the windward slope is 



