like soil. It has become dark in color and rich in plant food.
Then, many other plants that require a good soil take root there. The
rock has at last completely disappeared under the layer of soil and its
carpet of vegetation.

Suppose, now, that we dig down and find how deep the soil is and what
lies below it. When we have gone down two feet the soil is harder and of
a lighter color, for there are fewer plant remains in it. This poorer,
lighter-colored soil we call _subsoil_. If we dig a little deeper, we
shall find pieces of rock in the subsoil. Below these we come to soft,
crumbling rock and last of all the solid rock.

The soil that is found resting on the rocks from which it was formed is
known as _residual soil_. This name is given to such soil, because it is
what remains after long years of rock decay during which the rains have
washed away a part of the finer material.

What has become of the soft earth that the water washed away? The muddy
rivulet has already told us its interesting story. We have learned that
a part of this earth (or soil) is borne to the distant ocean. There it
is forever lost unless the sea bottom should some day become dry land.
Stranger things than that have happened on this ancient earth of ours.
The part of the soil which the water carried away to form the rich
valley lands and deltas is known as _alluvial soil_.

[Illustration: _U. S. Department of Agriculture_
A flood plain, where alluvial soil has been deposited by the river.]

Long ago the northern part of our country was covered with a sheet of
ice. This ice crept slowly southward, and as it moved along it tore off
all the soil and loose rocks on the surface of the earth over which it
passed. When it melted it left them spread roughly over the country.
Such material forms _glacial soil_. It is often deep but not very rich.

[Illustration: _U. S. Geological Survey_
Soil brought by a glacier and deposited as the ice melted.]

There is another kind of soil, formed by the wind. If you have ever been
in a dust storm you have seen the fine, powdery substance that settles
over everything and creeps into the smallest cracks. In some countries
where there are strong winds and not much rain there is little
vegetation on the surface to hold the soil. Year after year the winds
pick up particles of the dusty soil, whirl them high in the air, and do
not let them down again until they have been carried many miles. In some
far-off la