2 BULLETIN" 180^ U. S, DEPAETMENT OF AGRICULTURE. 



here is the expansion of the rock material due to changes in tempera- 

 tures, including the expansion of the water contained in the rock at 

 the freezing point. The most important chemical process is solution, 

 while oxidation and hydration are of great importance. 



The layer of resulting material mixed with some organic matter 

 and capable of supporting plant life is kno^vn as soil. Its composi- 

 tion does not differ very markedly from the rocks from which it is 

 derived. At best this process of accumulation is slow, the rate de- 

 pending somewhat on the activity of the agencies producing it. 



TRANSPORTED SOILS. 



Soil materials formed in one place and deposited in another through 

 the agency of wind, water, or ice constitute or develop into trans- 

 ported soils. These, however, are not the eroding soils, but are the 

 results of soil erosion. 



TRANSLOCATION OF SOILS BY WATER. 



The movement of soil material by water is limited by two inherent 

 properties. Water can move only from a higher to a lower level, and 

 it can affect only the surface with which it comes in actual contact. 



Since water moves over the surface of the soil only under the force 

 of gravity, its action is always du'ected toward moving material from 

 the hills and depositing it in the plains. If a stream is arrested in its 

 movement down hill by the construction of an impediment, such as 

 a dam, it produces a lake which acts as a settling basin. The stream 

 gives up its burden of detritus and the lake is gradually filled. 

 Eventually this results in the bottom of the lake reaching the level 

 of the dam, when the stream will then carry its burden to some lower 

 lake or to the ocean and deposit it. 



Streams can erode only those surfaces over which they flow, and 

 this greatly restricts their power in this respect. Thus the larger 

 part of the detritus of streams must be derived from the surface drain- 

 age of adjoining areas. Here the amount of surface drainage is 

 dependent on the absorptive power of the soil and on its drainage. 

 If the soil is more or less loose and porous its absorptive capacity is 

 high, so that it may absorb rain as rapidly as it falls unless the pre- 

 cipitation be extraordinarily heavy. On the other hand, if the soil 

 is close grained and compact the absorption is slow, even though the 

 actual pore space within the soil be greater than in the case of the 

 loose soil. In case of a gentle rain, absorption may be rapid enough 

 to prevent surface drainage, but with any heavy and rapid rainfall 

 the water runs off largely from the surface. 



WATER-TRANSPORTED MATERIAL. 



Physics. — ^The excess of water draining from the surface of a soil 

 carries with it some of the material in suspension. The existing val- 



