64 NATURE AND PROPERTIES OF SOILS 



tlie loess and the great variety of climate and cropping to 

 which it is subject, it may be classed as one of the world's 

 most important soils. In the United States it is the great 

 maize-producing soil of the upper Mississippi Valley. 



36. Other seolian soils. — The term ^ * adobe ' ' is applied to a 

 fine calcareous clay or silt formed in a manner somewhat 

 like loess. It is supposed that, while part of the deposit came 

 from the waste of talus slopes as mountains were weathered 

 under conditions of aridity, the remainder had ^olian origin. 

 Certain characteristics also seem to indicate that the valley 

 adobe might have been deposited almost entirely by water. 

 It appears, therefore, that, while the physical characteristics 

 of all adobe are somewhat similar, its mode of origin and 

 chemical composition may be variable. 



Like the loess, the adobe is an exceedingly rich soil, but 

 it occurs in an arid or a semi-arid region. When irrigated 

 its fertility seems inexhaustible. It is found in Colorado, 

 Utah, southern California, Arizona, New Mexico, and Texas. 

 It has an especially wide distribution in New Mexico. Like 

 loess, its elevation is variable, ranging from sea level in Cali- 

 fornia and Arizona to 6000 feet along the eastern border of 

 the Rocky Mountains. Its maximum thickness cannot be esti- 

 mated, as it is very little eroded and is supposed to be still 

 accumulating. Some valleys are known to be filled to a depth 

 of 3000 feet with this material. Its characteristics are its 

 fine texture, its great depth, its wide distribution, and its 

 great fertility when moisture conditions are suitable for crop 

 growth. 



Sand dunes are the outgrowth of two conditions — a large 

 quantity of sand and a wind that blows in a more or less 

 prevailing direction. Under such conditions the sand and 

 other fine materials are not only blown into heaps, but also 

 tend to move in the direction of the prevailing wind. Sand 

 dunes may often assume gigantic proportions, sometimes be- 

 ing several hundred feet high and twenty or thirty miles 



