Page:Popular Science Monthly Volume 7.djvu/414

398 the sea; not by an extravagant and violent use of power, but by the slow agencies which may be observed generally throughout the world, still acting in the same slow, patient manner.

There are yet some interesting facts to be observed concerning these inter-hog-back valleys. Their floors are usually lower than the general surface farther away from the mountains. There seem to be two causes for this: The great fold having been lifted and truncated prior to the exposure of the rocks farther away from the mountains, its strata present their edges, instead of their upper surfaces, to the down-falling rain, and the softer beds are not so well shielded by the harder. Erosion hence progresses more rapidly than where the beds are approximately horizontal.

Again, the mountains, with peaks among the clouds, condense their moisture, and a greater quantity of rain falls on them, or in their vicinity. The region of country adjacent to the mountains receives



a portion of this extra rainfall, so that this dynamic agency increases from the plains to the summits of the mountains, probably in some direct ratio. This increase of the eroding agency, and the greater exposure of the soft beds, probably account for the fact that the lowest country is at the foot of the mountains.

There is a limit to the effect of these conditions, for it should be observed that no valley can be eroded below the level of the principal stream, which carries away the products of its surface degradation; and where the floor of such a valley has been cut down nearly to the level of such a stream, it receives the débris of the adjacent cliffs and mountains, and in this way the rocks composing the floor are usually masked, to a greater or lesser extent. The same topographic facts under like conditions, are found on the eastern slope of the Rocky Mountains, in Colorado Territory, and the valleys which run into the South Platte from the south, between the hog-backs, are lower than