THE SOURCES OF THE SEDIMENT 



17 



several feet. Some are formed in fresh and others in salt water 

 (Fig. 8). They are not confined entirely to lowlands, but may occur 

 on hill tops and even on hill sides in depressions kept wet the year 

 round. On many of them, in their natural state, wild grasses 

 grow — on others trees and shrubs. When artificially well drained 

 most of them can be converted into valuable agricultural lands. 



' Alluvial soils are found along streams, and are built of the 

 alluvium carried and deposited by them during flood flow. When 

 muddy streams overflow their banks, the flow over the flooded land 

 is retarded, consequently the sand settles out, then the silt, and 



Fig. 8. — A bed of peat four feet deep underlaid by a 3-foot bed of marl, which in turn is 



underlaid by sand and then clay. 



finally the clay — if the flow is nearly or completely checked. 

 Through this deposition, therefore, low level areas are gradually^ 

 brought to higher levels. Many streams during the past ages have 

 greatly subsided, leaving high and dry many level and productive^ 

 expanses of these water-formed soils. 



The sources of the sediment carried by streams are mainly the 

 uplands drained by them. Heavy rains and melting snows are 

 responsible for the land erosion so commonly seen in hilly or ^ ^roll- 

 ing'' sections. The sand, silt and clay particles carried into streams 

 may be gathered from many kinds of soils and which in turn form 

 new kinds. Geologists have estimated that the United States 

 is being planed down at an average rate of one inch in 760 years, 

 2 



