and altered rock. The forces that produce soil from rocks are of two
distinct classes, _physical and chemical. _The physical agencies of
soil production merely cause a pulverization of the rock; the
chemical agencies, on the other hand, so thoroughly change the
essential nature of the soil particles that they are no longer like
the rock from which they were formed.

Of the physical agencies, _temperature changes _are first in order
of time, and perhaps of first importance. As the heat of the day
increases, the rock expands, and as the cold night approaches,
contracts. This alternate expansion and contraction, in time, cracks
the surfaces of the rocks. Into the tiny crevices thus formed water
enters from the falling snow or rain. When winter comes, the water
in these cracks freezes to ice, and in so doing expands and widens
each of the cracks. As these processes are repeated from day to day,
from year to year, and from generation to generation, the surfaces
of the rocks crumble. The smaller rocks so formed are acted upon by
the same agencies, in the same manner, and thus the process of
pulverization goes on.

It is clear, then, that the second great agency of soil formation,
which always acts in conjunction with temperature changes, is
_freezing water. _The rock particles formed in this manner are often
washed down into the mountain valleys, there caught by great rivers,
ground into finer dust, and at length deposited in the lower
valleys. _Moving water _thus becomes another physical agency of soil
production. Most of the soils covering the great dry-farm territory
of the United States and other countries have been formed in this
way.