Page:Popular Science Monthly Volume 7.djvu/411

Rh by minor oblique and transverse flexures, while the general direction of these ridges is as described, they are turned back and forth from these lines in gentle or abrupt curves. These ridges are sometimes low mountain-ranges.

So, if we approach these mountains from either direction, north or south, we first meet with ridges, or, as they are usually called in the Western country, hog-backs. In many places these are so steep as to form a complete barrier to progress.

Usually the slope away from the side of the mountain corresponds above with the dip of the rock, and is gentle or steep, as the dip is lesser or greater. The side of the hog-back, next to the mountain, is composed of the cut edges of the strata, and varies greatly with the texture of the rocks; but usually it is steep or broken, sometimes but tressed, sometimes terraced, sometimes columned and fluted.



On the south side of the Yampa Plateau, near the head of Cliff-Creek Valley, there is an abrupt, oblique flexure, on the side of the great fold, by which the rocks are turned up, so as to stand vertically. In the rocks at this place there are two very hard conglomerates; the intervening strata are soft sandstones and marls, and have been carried away, and the conglomerates stand as vertical walls, 30 or 40 feet in thickness, 50 to 300 feet in height, and several miles in length, and between these is a broad avenue, or narrow valley, beset with ragged bowlders of conglomerate.

The drainage of these narrow valleys between the hog-backs is not always along their lengths, but the water is sometimes carried by channels crossing them and cuttings through intervening; ridges; hence