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 cafions 
crush and grind into 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, under 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 line, and rolling the mass of pebbles back 
and forth, break and pulverize the rock until soil is 
formed. Glaciers, winds, and waves 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 cloudless 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 
