ks are attacked. If these rocks
are soft and easily worn away, the channel deepens rapidly. One after
another the alternating layers are excavated, and the river flows in a
canyon which deepens more and more. As the level is lowered, the current
of the stream becomes slower and the cutting away of its bed less rapid.
The stream is content to flow gently, for it has almost reached the old
level, on which it flowed before the valley became a ridge or
table-land.

The rivers that flow in canyons have been thousands of years in carving
out their channels, yet they are newer, geologically speaking, than the
streams that drain the level prairie country. The earth has risen, and
the canyons have been carved since the prairies became rolling, level
ground.

[Illustration: This little pond is a basin hollowed by the same glacier
that scattered the stones and rounded the hills]

[Illustration: Every stream is wearing away its banks, while trees and
grass blades are holding on to the soil with all their roots]

The Colorado River flows through a canyon with walls that in places
present sheer vertical faces a mile in depth, and so smooth that no
trail can be found by which to reach from top to bottom. The region has
but slight erosion by wind, and practically none by rain. The local
rainfall is very slight. So the river is the one force that has acted to
cut down the rocks, and its force is all expended in the narrow area of
its own bed. Had frequent rains been the rule on the Colorado plateau,
the angles of the mesas would have been rounded into hills of the
familiar kind so constantly a part of the landscape in the eastern half
of the continent.

The Colorado is an ancient river which has to carry away the store of
moisture that comes from the Pacific Ocean and falls as snow on the high
peaks of the Rocky Mountains. Similar river gorges with similar stories
to tell are the Arkansas, the Platte, and the Yellowstone. All cut their
channels unaided through regions of little rain.

When the earth's crust is thrown up in mountain folds, and between them
valleys are formed, the level of rivers is sometimes lowered and the
rapidity of their flow is checked. A stream which has torn down its
walls at a rapid rate becomes a sluggish water-course, its current
clogged with sediment, which it has no power to carry farther. When such
a river begins to build and obstruct its own waters it bars its progress
and may form a lake as the outl