Part 2 (1/2)
[Footnote 2: _lapilli_, volcanic ashes, consisting of small, angular, stony fragments.]
[Ill.u.s.tration: FIG. 9.--PITT RIVER CAnON, NORTHERN CALIFORNIA
The plateau is built of layers of lava]
The volcanoes at last ceased to grow any higher, for the lava, if the eruptions continued, formed new craters at their bases.
It is probable that all these great peaks have been extinct for several thousand years, although some are much older and more worn away than others. One of these volcanoes has completely disappeared, and in its place lies that wonderful sheet of water known as Crater Lake. It is thought that the interior of this mountain was melted away during a period of activity, and that the outer portion fell in, leaving a crater five miles across and nearly a mile deep.
The streams of lava, as they flowed here and there building up the plateau, frequently broke up the rivers and turned them into new channels. As time went on the eruptions were less violent, and the rivers became established in the channels which they occupy to-day. The Columbia River, winding about over the plateau, sought the easiest path to the sea. It soon began to dig a channel, and now has hidden itself between dark walls of lava.
But other forces besides the streams were now at work in this volcanic region. The lava plateau began slowly to bend upward along the line of the great volcanoes, lifting them upward with it. In this manner the Cascade Range was formed. The Columbia River, instead of seeking another way to the sea, continued cutting its channel deeper and deeper into the growing mountain range, and so has given us that picturesque canon which forms a most convenient highway from the interior of Was.h.i.+ngton and Oregon to the coast.
Take a sheet of writing paper, lay it upon an even surface, then slowly push the opposite edges toward each other. This simple experiment will aid one in understanding one of the ways in which mountain ranges are made. Besides the upward fold of the plateau which made the Cascade Range, another was formed between the Blue Mountains in eastern Oregon and a spur of the Rocky Mountains in northern Idaho. This fold lay across the path of the Snake River, but its movement was so slow that the river kept its former channel and in this rising land excavated a canon which to-day is more than a mile deep. The upper twenty-five hundred feet of the canon are cut into the lava of the plateau, and the lower three thousand into the underlying granite. The canon is not so picturesque as the Colorado, for it has no rocks with variegated coloring or castellated walls. Its sides are, however, exceedingly precipitous and it is difficult to enter.
[Ill.u.s.tration: FIG. 10.--SHOSHONE FALLS, SNAKE RIVER, IDAHO]
Along portions of the lower Columbia and Snake rivers, navigation is obstructed by rapids and waterfalls. The presence of these falls teaches us that these streams are still at work cutting 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 canons 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 canons which have been mentioned intercept some of these underground rivers, so that their waters pour out and down over the sides of the canons in foaming cascades.
The greatest of these cascades is that known as the Thousand Springs in the Snake River canon. The waters of the Blue Lakes in the canon 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.
[Ill.u.s.tration: FIG. 11.--CAnON 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 Was.h.i.+ngton and northern Oregon the plateau becomes one vast grain-field. It is difficult to irrigate the plateau because the streams flow in such deep canons, but above the point where the canon of the Snake River begins there is an extensive system of ca.n.a.ls 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 CAnONS OF THE SIERRA NEVADA MOUNTAINS
The western half of our country contains the deepest and most picturesque canons in the world. Those of the Colorado and Snake rivers form trenches in a comparatively level but lofty plateau region. The canons 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 canons 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 canons 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 the earth is soft the results of her labor are not very interesting, but if the crust is hard the forms which she produces are often so remarkable that they arouse our wonder and admiration.
[Ill.u.s.tration: FIG. 12.--SAN JOAQUIN RIVER EMERGING UPON THE PLAIN OF THE GREAT VALLEY]
In shaping the Sierra Nevada mountains Nature had a grand opportunity.
Here she produced the Yosemite Valley, which has a setting of cliffs and waterfalls that attract people from all over the world. Hetch-Hetchy Valley at the north of the Yosemite, and Tehipite and King's River canons at the south, are interesting places, but not so majestic and inspiring as the Yosemite.
Nature never seems satisfied with her work. After she has created a piece of wonderful scenery she proceeds to destroy it. The great cliffs of the Yosemite will sometime lose their grandeur and be replaced by gentle slopes down which the streams will flow quietly.
The mountains of the Laurentian highlands in the northeastern portion of the continent undoubtedly were once lofty and picturesque, but there were no people upon the earth at that time to enjoy this scenery. Now these mountains have become old and are nearly worn down.