waters
ground against the rocks. Vast were the waters; vast the number of
years; vast the results.

Glaciers were another soil-producing agent. Glaciers are streams "frozen
and moving slowly but irresistibly onwards, down well-defined valleys,
grinding and pulverizing the rock masses detached by the force and
weight of their attack." Where and how were these glaciers formed?

Once a great part of upper North America was a vast sheet of ice.
Whatever moisture fell from the sky fell as snow. No one knows what made
this long winter of snow, but we do know that snows piled on snows until
mountains of white were built up. The lower snow was by the pressure of
that above it packed into ice masses. By and by some change of climate
caused the masses of ice to break up somewhat and to move south and
west. These moving masses, carrying rock and frozen earth, ground them
to powder. King thus describes the stately movement of these snow
mountains: "Beneath the bottom of this slowly moving sheet of ice, which
with more or less difficulty kept itself conformable with the face of
the land over which it was riding, the sharper outstanding points were
cut away and the deeper river canons filled in. Desolate and rugged
rocky wastes were thrown down and spread over with rich soil."

The joint action of air, moisture, and frost was still another agent of
soil-making. This action is called _weathering_. Whenever you have
noticed the outside stones of a spring-house, you have noticed that tiny
bits are crumbling from the face of the stones, and adding little by
little to the soil. This is a slow way of making additions to the soil.
It is estimated that it would take 728,000 years to wear away limestone
rock to a depth of thirty-nine inches. But when you recall the
countless years through which the weather has striven against the rocks,
you can readily understand that its never-wearying activity has added
immensely to the soil.

In the rock soil formed in these various ways, and indeed on the rocks
themselves, tiny plants that live on food taken from the air began to
grow. They grew just as you now see mosses and lichens grow on the
surface of rocks. The decay of these plants added some fertility to the
newly formed soil. The life and death of each succeeding generation of
these lowly plants added to the soil matter accumulating on the rocks.
Slowly but unceasingly the soil increased in depth until higher
vegetable forms could flourish a