The Western United States | Page 9

their channels deeper. The
Snake River in its upper course has as yet cut only a very shallow
channel in the hard lava, and the beautiful Shoshone Falls marks a
point where its work is slow. These falls, which are the finest in the
northwest, owe their existence to the fact that at this particular spot
layers of strong resistant lava cover the softer rocks.
There are other cañons in the plateau region which are fully as
remarkable as those which have been mentioned. That of the Des

Chutes River in central Oregon is in places a thousand feet deep, with
almost vertical walls of lava.
We have already seen how mountains have been formed upon the
Columbia plateau, by a bending of the earth upward. Other mountains
of the plateau are due to fractures in the solid rocks, often many miles
long. Upon one side of these fractures the surface has been depressed,
while upon the other it has been raised. The amount of the uplift varies
from a few hundred to thousands of feet. The mountains thus formed
have a long, gentle slope upon one side and a very steep incline upon
the other. They are known as "block mountains," and those upon the
Columbia plateau are the most interesting of their kind in the world.
With the exception of a few large rivers, the greater portion of the
Columbia plateau is remarkable for its lack of surface streams. The
water which reaches the borders of the plateau from the surrounding
mountains often sinks into the gravel between the layers of lava and
forms underground rivers. The deep cañons which have been
mentioned intercept some of these underground rivers, so that their
waters pour out and down over the sides of the cañons in foaming
cascades. The greatest of these cascades is that known as the Thousand
Springs in the Snake River cañon. The waters of the Blue Lakes in the
cañon of the same river below Shoshone Falls also come from
underneath the lava. They are utilized in irrigating the most picturesque
fruit ranch in southern Idaho.
[Illustration: FIG. 11.--CAÑON OF CROOKED RIVER NEAR THE
DES CHUTES RIVER
Eroded in the Columbia plateau]
The climate of the plateau is dry, and its eastern portion is practically a
desert. Toward the west, however, the rainfall is greater, and in central
Washington and northern Oregon the plateau becomes one vast
grain-field. It is difficult to irrigate the plateau because the streams flow
in such deep cañons, but above the point where the cañon of the Snake
River begins there is an extensive system of canals and cultivated fields.
With a sufficient water supply, the lava makes one of the richest and

most productive of soils. Along the Snake and Columbia rivers,
wherever there is a bit of bottom land, orchards have been planted.
Little steamers ply along these rivers between the rapids, gathering the
fruit and delivering it at the nearest railroad point.
Mining is carried on only in the mountains which rise above the lava
flood, for the mineral veins are for the most part older than the lava of
the plateau. We are certain that many very valuable deposits of the
precious metals lie buried beneath the lava fields.
It is thought that the volcanic history of the Columbia plateau has been
completed. Now the streams are at work carrying away the materials of
which it is composed and may in time uncover the old buried land
surface.

THE CAÑONS OF THE SIERRA NEVADA MOUNTAINS
The western half of our country contains the deepest and most
picturesque cañons in the world. Those of the Colorado and Snake
rivers form trenches in a comparatively level but lofty plateau region.
The cañons of the Sierra Nevada Range, on the contrary, take their rise
and extend for much of their length among rugged snowcapped peaks
which include some of the highest mountains in the United States. All
these cañons are the work of erosion. The rivers did not find
depressions formed ready for them to occupy, but had to excavate their
channels by the slow process of grinding away the solid rock.
The streams of the Sierra Nevada mountains begin their course in
steep-walled alcoves under the shadows of the high peaks, where they
are fed by perpetual snow-banks. Soon they bury themselves between
granite walls, which at last tower three thousand feet above their
roaring waters. After many miles the cañons widen, the walls decrease
in height, and the streams come out upon the fertile stretches of the
Great Valley of California.
Nature works in many ways. Her tools are of different kinds, but the

most important one is running water. The forms which she produces are
dependent upon the kind of rock upon which she works. Where the
surface of