FORMATION OF DRY-FARM SOILS 53 



becomes another physical agency of soil production. 

 Most of the soils covering the great dry-farm terri- 

 tory of the United States and other countries have 

 been formed in this way. 



In places, glaciers moving slowly down the canons 

 crush and grind intcj powder the rock over which 

 they pass and deposit it lower down as soils. In 

 other places, where strong winds blow with frequent 

 regularity, sharp soil grains are picked up by the air 

 and hurled against the rocks, which, untler this 

 action, are carved into fantastic forms. In still other 

 places, the strong winds carry soil over long distances 

 to be mixed with other soils. Finally, on the sea- 

 shore the great waves dashing against the rocks of 

 the coast hne, and rolling the mass of pebbles back 

 and forth, break and pulverize the n^ck until soil is 

 formed. Glaciers, ivinds, and ivaves are also, there- 

 fore, physical agencies of soil formation. 



It may be noted that the result of the action of 

 all these agencies is to form a rock powder, each 

 particle of which preserves the composition that it 

 had while it was a constituent part of the rock. It 

 may further be noted that the chief of these soil- 

 forming agencies act more vigorously in arid than 

 in humid sections. Under the chjudless sky and dry 

 atmosphere of regions of limited rainfall, the daily 

 and seasonal temperature changes are much greater 

 than in sections of greater rainfall. Consequently 

 the pulverization of rocks goes on most rapidly in 



