Читать книгу The Circle of Knowledge: A Classified, Simplified, Visualized Book of Answers - Various - Страница 83

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.