54 



SOIL PHYSICS AND MANAGEMENT 



and Illinois Rivers, notably in Carrol, Whiteside, Hock Island and 

 St. Clair Counties in Illinois. The conditions necessary for the 

 formation of sand dunes are a supply of sand and a somewhat high 

 and constant wind. Sea and lake shores furnish excellent conditions 

 for the formation of sand dunes. The waves throw the sand upon 

 the beach and the strong winds which so often prevail there carry 

 it landward. Shaler estimates that ninety per cent of the coast line 

 of the world is fringed with sand. The total dune area of Europe 

 is 4,562,000 acres, while the sand wastes add about 9,000,000 acres 

 more. 



Fio. 48. A dust storm in Kansas, May 26, 1912. (Jardine, Jour. Am. Soc. Agronomy, 



Vol. 5, No. 4.) 



Sand is not raised far above the surface as in the case of dust, 

 and a shrub, a tuft of grass or a fence may give lodgment to the 

 sand and origin to a dune. After accumulations have once begun by 

 means of an obstruction, the dune itself will furnish the necessary 

 conditions for growth. In shape, dunes may be either in the form 

 of hillocks, crescents, or ridges, transverse or parallel to the pre- 

 vailing winds. The shape of the individual dune is a steep leeward 

 and a gradual windward slope, especially where the prevailing wind 

 is constantly from the same direction. 



Sand dunes are of two classes, migratory or wandering (Fig. 

 49), and permanent or fixed. With a constant wind, dunes migrate 

 or advance a few feet each year, burying objects in their paths. 



