EOLIAN SAND 



Since dust is carried to a considerable extent in the upper air, its 

 movements and its deposition are affected but little by obstacles 

 on the surface of the land, and when it falls it is spread more or less 

 uniformly over the surface. 



Much of the dust transported by the wind is carried out over 

 seas or lakes and falls into them, causing sedimentation over their 

 bottoms. No determinations of the amount of dust blown into the 

 sea have been made, but it is safe to say that, if such determination 

 were possible, the result would be surprising. 



Sand. Winds do not commonly lift sand far above the surface 

 of the land, and its movement is therefore interfered with seriously 

 by surface obstacles. A 

 shrub, a fence, a building, 

 or even a stone may occa- 

 sion the lodgment of sand 

 in quantity, though it has 

 little effect on dust. If the 

 obstacle which causes the 

 lodgment of sand presents a 

 surface which the wind can- 



Fig. 7. Diagram to illustrate the effect 

 of an obstacle on the transportation 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, caused 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 leeward. At 

 the same time, a depression would be hol- 

 lowed out near the obstacle itself on either 

 side. (After Cornish.) 



not penetrate, such as a wall, 

 sand is dropped abundantly 

 both on its windward and 

 leeward sides (Fig. 7); but 

 if it be penetrable, like an 

 open fence, the lodgment 

 takes place chiefly to lee- 

 ward. In cultivated regions, cases are known where, in a few 

 \vi-i-ks of dry weather, sand has drifted into lanes in the lee of 

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

 vehicles to pass. 



Dunes. 1 In contrast with eolian dust, much eolian sand is aggre- 

 gated into mounds and ridges called dunes. Some dunes are 200 

 or 300 feet high, but many more 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 

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

 the lodgment. The shapes of the cross-sections of dunes are influ- 

 enced by the strength and constancy of the winds. With constant 



1 Geog. Jour., Apr., 1910, p. 379. 



