ORIGIN OF SOILS 



77 



deposits derived from various kinds of materials has been laid 

 down by the river when in flood. Such soils are usually fertile, 

 though they may not always be profitably cropped. 



Arid and Semi-Arid Division. 8. Great Basin. The soils are 

 derived from a great variety of rocks and consist of colluvial soils 

 of the mountain slopes, lake and shore deposits, stream valley 

 sediments, and river-delta deposits. 



9. Arid Southwest. These soils occupy slopes at the foot of 

 mountains, alluvial plains, sloping or nearly level plains, and 



Fig. 24. Soil provinces of the United States. Bureau of Soils. 



stream valleys. The soils are colluvial, alluvial, and lake deposits. 

 Without irrigation, these soils have little agricultural value. 



10. Residual Soils of the Western Prairie Regions. These 

 soils occupy the unglaciated part of the prairie plains. The rocks 

 from which the soils are derived are of the carboniferious age and 

 consist of sandstones, shales, and limestones. 



11. Northwestern Inter-Mountain Region. The soils of this 

 area consist mostly of residual material derived from basaltic 



