CLIMATIC FACTORS : THE AIR 



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fine-grained, fertile soil occurs in deposits from a few to fifty feet 

 deep or more over thousands of square miles in the central Missis- 

 sippi Valley region. Our richest farm lands in Iowa, Nebraska, and 

 Kansas are on loess soils. The deposits occur along the Rhine in 

 Europe and in the pampas of Argentina, and reach their greatest 

 extent in Asia, particularly in north-central China. Loess probably 

 originated during the glacial period as dust was swept up from 



Fig. 58. Coastal sand dune moving inland and encroaching on evergreen 

 maritime forest near Kitty Hawk, N. C. Grasses in foreground have been 

 planted.— Photo by C. E Korstian. 



the barren flood plains of glacial rivers and carried high into the 

 air, from which it settled more or less uniformly over wide areas. 

 Sand beaches and desert regions are commonly dry, free of 

 vegetation, and swept by prevailing winds, which carry the soil 

 along near the earth's surface. Any obstacle that checks the veloc- 

 ity of the wind causes some of its load to be deposited and starts 

 a mound or ridge called a dune. Some dunes grow, by the deposit 

 of more sand, to a height of several hundred feet, but usually they 

 are much smaller. Most of the sand is deposited near the crest or 

 on the lee slope; this results in a characteristic gentle windward 

 slope and a sharp drop on the lee slope, the steepness of which is 



