THE PHYSICAL PROCESSES OF SOIL FORMATION. I5 



washed away to be redeposited as alluvium at a greater or less 

 distance. 



One characteristic of the flood-plain lands of all the larger 

 rivers, and more or less of all streams subject to periodic 

 overflows, is that the land immediately adjoining the banks 

 is both higher and more sandy than are the lands farther back 

 from the stream. The cause of this phenomenon is that as 

 lateral overflow diminishes the velocity of that flow, its coarser 

 portions are deposited near the river banks, while the finer 

 particles are carried farther away, until finally only the finest 

 clay-substance reach the lagoons or lakes filled with the 

 overflow or back-water, and are there in the course of time 

 deposited as heavy clay " swamp " soils. The same occurs 

 where rivers empty into lakes or the sea ; and these slack-water 

 or delta lands are, as a rule, the most productive on the river's 

 course. The continued productiveness of alluvial soils is more- 

 over in many cases assured by the deposition, during overflows, 

 of fresh soil-material brought down from the head waters of 

 the streams. The Nile, and the Colorado river of the West, 

 illustrate this point. 



Lowering of the land-surface by soil formation. It is 

 evident that the soil-forming agencies must in the course of 

 time materially affect both the surface conformation and the 

 absolute level of the land. The sharp pinnacles and crests of 

 rock are abraded into the rounded forms now characterizing 

 our uplands and lower ranges of hills and mountains ; and 

 it is estimated that, e. g., the general level of the drainage basin 

 of the Mississippi river is lowered about one foot in 7.000 

 years, the material being carried into the lowlands and the sea. 



