Page:Popular Science Monthly Volume 7.djvu/691

Rh and thus we may have a series of base-levels of erosion, below which the rocks on either side of the river, though exceedingly friable, cannot be degraded. In these districts of country, the first work of rains and rivers is to cut channels, and divide the country into hills, and perhaps mountains, by many meandering grooves or water-courses, and when these have reached their local base-levels, under the existing conditions, the hills are washed down, but not carried entirely away.

With this explanation I may combine the statements concerning elevation and inclination into this single expression, that the more elevated any district of country is, above its base-level of denudation, the more rapidly it is degraded by rains and rivers.

The second condition in the progress of erosion is the character of the beds to be eroded. Softer beds are acted upon more rapidly than the harder. The districts which are composed of softer rocks are rapidly excavated, so as to become valleys or plains, while the districts composed of harder rocks remain longer as hills and mountains.

Where the beds are of stratified material, so that the change from harder to softer materials is from bed to bed, rather than from district to district, and in a vertical or inclined direction, rather than an horizontal, the topographic features, which I have described as hogbacks and cliffs of erosion, are produced. The difference between hogbacks and cliffs of erosion is chiefly due to the amount of dip or inclination of the beds.

But there is another condition necessary to the production of cliffs and hog-backs in their typical forms. The country must be arid, for, where there is a great amount of rainfall, the water penetrates and permeates the rocks, and breaks them up, or rots them, to use an expression which has been employed with this meaning; and the difference between the durability of the harder beds and that of the softer is, to some extent, compensated for by this agency, though doubtless ridges and cliffs may be produced in less arid climates, as we find them in the Appalachian System, but not so well marked. In a region of country where there is a greater amount of rainfall, the tendency is to produce hills and mountains, rather than plateaus and ridges, with escarpments.

Now let us examine the character of the channels which running streams carve. Where the rocks to be carved are approximately horizontal, and composed of stratified beds of varying thickness, the tendency is to cut channels with escarpments or cliffs ', but if the beds are greatly inclined, or composed of unstratified material, the tendency is to cut channels with more flaring and irregular walls. These tendencies are more clearly defined when the meteorologic conditions are favorable—that is, if a stream cuts through stratified rocks, in an arid region, and carries the waters from a district more plentifully supplied, the cliff character of the walls is increased; and where a