of the
moisture that falls on the earth falls as snow. This snow has piled up
until it has become very deep and very heavy. The great weight has
packed the bottom of this great snow bank to ice. On the mountains
where the land was not level the masses of snow and ice, centuries
ago, began to slide down the slopes and finally formed great rivers of
solid water or moving ice.

The geologists tell us that at one time a great river of ice extended
from the Arctic region as far south as central Pennsylvania and from
New England to the Rocky Mountains. This vast river was very deep and
very heavy and into its under surface were frozen sand, pebbles,
larger stones and even great rocks. Thus it acted as a great rasp or
file and did an immense amount of work grinding rocks and making
soils. It ground down mountains and carried great beds of soil from
one place to another. When this great ice river melted, it dropped its
load of rocks and soils, and as a result we find in that region of the
country great boulders and beds of sand and clay scattered over the
land.

_Work of the Air._--The air has helped in the work of wearing down the
rocks and making soils. If a piece of iron be exposed to moist air a
part of the air unites with part of the iron and forms iron rust. In
the same way when moist air comes in contact with some rocks part of
the air unites with part of the rock and forms rock rust which
crumbles off or is washed away by water. Thus the air helps to break
down the rocks. Moving air or wind picks up dust particles and carries
them from one field to another. On sandy beaches the wind often blows
the sand along like snow and piles it into drifts. The entire surface
of sandy regions is sometimes changed in this way. Sands blown from
deserts sometimes bury forests which with their foliage sift the fatal
winding sheet from the dust-laden winds.

_The Work of Plants._--Living plants sometimes send their roots into
rock crevices; there they grow, expand, and split off rock fragments.
Certain kinds of plants live on the surface of rocks. They feed on
the rocks and when they die and decay they keep the surface of the
rocks moist and also produce carbonic acid which dissolves the rocks
slowly just as the vinegar dissolved the limestone in our experiment.

Dead decaying roots, stems, and leaves of plants form largely the
organic matter of the soil. When organic matter has undergone a
certain amount of decay it is called humus