22 SOIL COMPOSITION, CLASSES AND TYPES 



The fertile soils in the valley of the Red River of the North 

 and of many large areas bordering on the Great Lakes are of 

 lake origin. 



Loess (lo'&) is soil formed thi'ough the accumulation of dust 

 blown and dropped in favoring localities by wind during long 

 periods of time. These deposits, no doubt, were centuries in forma- 

 tion, beginning after the disappearance of the glaciers. During 

 the Ice Age much fine material was carried many miles south of 

 the ice sheets by rimning water coming from the melting glaciers. 

 Immense quantities were deposited in what is now the Missouri 

 Valley. After the glaciers melted away it is believed a long dry 

 period followed during which time strong westerly winds picked 

 up this fine material and dropped it over thousands of square miles 

 in the central part of the Mississippi Valley. Loess is also found 

 in Washington, Oregon and other parts of the United States. 

 These deposits vary in thickness from a few inches to many feet. 

 In China loess is found covering immense areas, in some places 

 extending in depth to several thousand feet. 



Loess soils are free from stones, and are considered the richest 

 soils in the world. The fact that the famous Corn Belt of 

 the Mississippi Valley includes much loess is indicative of 

 its productiveness. 



CoUuvial soils are found at the bottom of steep slopes — ^they 

 accumulate there because of the force of gravity. Sometimes 

 avalanches bring about the quickest downward movement of soil 

 and rocks from the steep slopes. From the nature of their forma- 

 tion these soils must necessarily consist of more or less mixed mate- 

 rial. Often they are made up largely of fragments of rocks brought 

 down by gravity from the heights above. Generally they are 

 coarse, loose soils and not well adapted for cultivation. 



Soils Easily Described. — ^Now that we have learned how soils 

 may be grouped into classes according to their texture, and how 

 they differ in the manner in which they were formed, we can 

 easily describe them, and in a general way, distinguish one kind 

 from another; for example, we may have a residual silt loam on 

 limestone, or a residual sandy soil on sandstone, or a glacial clay 

 loam, an alluvial sandy loam, a glacial loam, or a loessial silt loam, etc. 



SOIL TYPES 



Soil Mapping.— The United States Bureau of Soil in cooper- 

 ation with the several states and territories has surveyed and 



