ORIGIN OF SOILS 67 



heavy as a river moving at the rate of two miles an hour. While 

 being carried, the particles are ground together and reduced in 

 size. 



Whenever the velocity of a stream is decreased, it deposits a 

 portion of its burden, the heavier particles being deposited first. 

 Thus, when a swollen mountain stream issues from a gorge and 

 spreads out over a plain, it deposits a portion of the material on 

 the surface of the plain. When a river in flood leaves its banks, 

 the velocity of the water is checked on spreading over the plain, 

 and it deposits the coarser particles which it carries near the 

 channel of the river. The finer particles are carried farther, and 

 are deposited in the swamps or low ground at some distance from 

 the river. The tendency of a river bearing rock debris is to build 

 its banks up above the level of the surrounding country. The 

 area over which a river spreads when in flood, is termed its 

 Hood plain, and the soil formed from the particles which it 

 deposits is termed an alluvial soil. Some of the richest soils in 

 the world are alluvial soils. The soils are deep, and as they 

 receive the surface soil washed away from less fortunate regions 

 from time to time, their fertility is maintained. The valleys of 

 the Nile, of the Ganges, the Mississippi, the Red river, the Brazos, 

 and others, contain some very rich soils, which are alluvial in 

 origin. The soils near the river are lighter in texture than those 

 in the low grounds back from the river. The latter are very 

 heavy, and difficult to work, but are often very productive. 



River deposits are stratified; that is, the material is sorted and 

 deposited in layers consisting of material of very nearly the same 

 fineness. Layers of fine and coarse material may alternate accord- 

 ing to river conditions. 



In arid regions, where the streams decrease as they flow from 

 the mountains out upon the dry lowlands and are therefore com- 

 pelled to lay aside a large portion of their burden, mountain 

 streams may form wide-spread alluvial plains, which are called 

 piedmont (meaning foot of the mountain) alluvial plains. The 

 streams which flow eastward from the Rocky Mountains have 

 formed a continuous alluvial plain which stretches hundreds of 



