Physics of the Soil. 



tion in one place or re- 

 moval to another. All 

 are familiar with the 

 creeping of soils from 

 the brows of steep hill- 

 sides toward their bases 

 and out upon the more 

 level plains which 

 stretch away from them. 

 These downward move- 

 ments are caused by sev- 

 eral agencies : ( 1 ) The 

 beating of falling rain- 

 drops and the carrying 

 power of the streamlets 

 which form as these 

 gather together ; ( 2 ) the 

 expansion and contrac- 

 tion of the soil due to 

 the alternate wetting 



and drying, there being FIG. 12. Giant's Castle, near Camp Douglas, 

 , . , Wis., showing cliffs of rock crumbling into 



leSS resistance to expan- soil. (After Chamberlin.) 



sion downward than upward against gravity. These 

 movements are analogous to those of the steel rails of 

 the railroad which tend to creep down grade under the 

 influence of changing temperature, which causes them to 

 first lengthen and push down hill and then shorten and 

 again draw downward because of less resistance in that 

 direction. (3) Then, again, every disturbance of the 

 soil produced by animals burrowing or walking up or down 

 the hillside, tends usually to work the soil from higher to 

 lower levels. Even the action of the wind is on the whole 

 downward. 



72. Soils Produced by Running Water. Rivers and 

 streams are continually at work at this double process of 

 soil building and soil removal. When one watches the bed 

 of a stream as the water ripples over the uneven surface 



