46 THE PRINCIPLES OF SOIL MANAGEMENT 



stream is strewn with material which has been dropped 

 by the water. If the bed of the stream is steep, it is 

 paved almost entirely with large stones and boulders. 

 If the bed is very flat and the flow slow, the bottom is 

 formed by sand or silt. These variations are well illus- 

 trated by the ripples and quiet pools of almost any 

 stream, the former being stony, the latter more fine- 

 textured. This principle of the varying carrying power 

 of flowing water is of great agricultural importance. 

 It results in sorting the material which comes into the 

 water, and the particles of one size are deposited to- 

 gether. In this way is accumulated a fine pure clay 

 in one place, a sand at another place, and gravel at 

 still another. These formations are strikingly different 

 in their relations to plant-growth because of their dif- 

 ferent physical and chemical properties, as will be shown 

 in the further discussion of these matters. 



The character of such soil depends upon two factors: 



(1) The character of the rocks from which it is derived. 



(2) The conditions under which it is deposited. 



The soils of this group are by far the most important, 

 agriculturally, of any which will be discussed, on account 

 of both their relative area and crop relations. In a 

 general way, they may be divided into three sub-groups; 

 but it is impossible to draw any sharp line of distinction 

 between these groups. These are: (1) Marine soils. 

 (2) Lake and pond deposits, or lacustrine soils. (3) 

 Stream-laid, or alluvial soils. 



17. Marine soils. The marine soils occupy large 

 areas in the United States and many other countries. 

 They consist of stratified gravels, sands, silts and clays 



