Page:Popular Science Monthly Volume 7.djvu/690

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HE more important topographic features in the valley of the Colorado are mountains, hills, hog-backs, bad-lands, alcove-lands, cliffs, buttes, and canons. The primary agency in the production of these features is upheaval, i. e., upheaval in relation to the level of the sea, though it may possibly be down-throw in relation to the centre of the earth. This movement in portions of the crust of the earth may be by great folds, with anticlinal or synclinal axes, and by monoclinal folds and faults.

The second great agency is erosion, and the action of this agency is conditioned on the character of the displacements above mentioned, the texture and constitution of the rocks, and the amount and relative distribution of the rains.

In a district of country, the different portions of which lie at different altitudes above the sea, the higher the region the greater the amount of rainfall, and hence the eroding agency increases in some well-observed but not accurately-defined ratio, from the low to the high lands. The power of running water, in corrading channels and transporting the products of erosion, increases with the velocity of the stream in geometric ratio, and hence the degradation of the rocks increases with the inclination of the slopes. Thus altitude and inclination both are important elements in the problem.

Let me state this in another way. We may consider the level of the sea to be a grand base-level, below which the dry lands cannot be eroded; but we may also have, for local and temporary purposes, other base levels of erosion, which are the levels of the beds of the principal streams which carry away the products of erosion. (I take some liberty in using the term level in this connection, as the action of a running stream in wearing its channel ceases, for all practical purposes, before its bed has quite reached the level of the lower end of the stream. What I have called the base-level would, in fact, be an imaginary surface, inclining slightly in all its parts toward the lower end of the principal stream draining the area through which the level is supposed to extend, or having the inclination of its parts varied in direction as determined by tributary streams.) Where such a stream crosses a series of rocks in its course, some of which are hard, and others soft, the harder beds form a series of temporary dams, above which the corrasion of the channel through the softer beds is checked,