16 



SOIL PHYSICS AND MANAGEMENT 



almost entirely covered by an ice sheet. This approaches somewhat 

 the condition that existed over North America and Europe during 

 the glacial period. 



Glacujrs are drainage systems of regions of perpetual snow. The 

 moving ice obeys the same laws as streams and does the same kind 

 of work, but the fact that ice is a solid body gives it great grinding 

 power. Ice exerts a pressure Of forty pounds per square inch for 

 every one hundred feet in thickness, and geologists estimate the ice 

 to have been from a few hundred to five thousand feet or more in 

 thickness during the glacial period. This great pressure gives the 

 ice immense denuding and grinding power. Glaciers move from a 

 few feet to one hundred feet per day, the movement being more rapid 



Fio. 9. The material carried and rolled by streams gives them their great eroding power. 

 (U. S. Reclamation Service.) 



in summer. In their movement large masses of rock become im- 

 bedded in the bottom of the glaciers, grooving and grinding the 

 solid rock over which they pass. It must be remembered, however, 

 that the ice did not hold these rigidly. 



(d) Erosion of Streams. Flowing water doubtless is the most 

 extensive physical agent in the formation of soil material at the 

 present time. The streams with their load of clay, silt, sand, gravel, 

 and even boulders are not only using these tools to deepen and widen 

 their valleys but they also grind the materials into powder fitted for 

 soil formation. The work of moving water varies tus the square of 

 the velocity. If the velocity is doubled the work that the stream is 

 capable of doing will be increased four times, since by doubling the 

 velocity, twice the number of particles will strike an object with 

 double the force. The deepening and widening of the stream chan- 



