In places, glaciers moving slowly down the canons 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
seashore 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, therefore, 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 dry-farm districts. Constant heavy
winds, which as soil formers are second only to temperature changes
and freezing water, are also usually more common in arid than in
humid countries. This is strikingly shown, for instance, on the
Colorado desert and the Great Plains.

The rock powder formed by the processes above described is
continually being acted upon by agencies, the effect of which is to
change its chemical composition. Chief of these agencies is _water,
_which exerts a solvent action on all known substances. Pure water