Wonderful examples of valleys by erosion occur in the plateaus adjacent
to the Rocky Mountains. The Grand Canon of the Colorado, three hundred
miles long, has a depth of from three thousand to six thousand feet
below the surrounding country. The sides of this tremendous gorge, which
are nearly or quite precipitous, exhibit the successive geological
strata down to the oldest rocks. A similar formation exists in the
upper course of the Yellowstone, one of the main tributaries of the
Missouri, and to a less extent in all the streams flowing through the
high barren plateaus.
Valleys descending the slopes of mountains are formed in the same
manner. The gathering drops make the rill, and the rill its little
furrow; rills combine into rivulets, and rivulets make a gully down the
hill-side; rivulets unite to form torrents, and these work with
accumulating force, and excavate deep gorges in the declivities. Other
torrents form in the same manner about the mountain ridge, and pursue
the same work of erosion until the slopes are a series of valleys and
ridges, and the summit a bold crest overlooking the eroding waters. The
larger part of the valleys of the world are formed entirely by running
water.