water, there begin the attacks of the forces operating to destroy them. Hand in hand there go on growth and destruction. The two may keep an even pace; either may obtain the mastery. In the one case, lack of considerable elevation and flatness result; in the other, great altitudes may be attained. The rivers may cut their valleys downward as fast as the land rises, or the down-cutting may be relatively slower. In any case, after a given land mass has attained its greatest height above the sea, the larger rivers soon cut their channels down as far as river cutting is possible-—namely, to within a few feet of
Fig. 8.—Horizontal Rocks, Grand Canon of the Colorado.
sea level. With relatively rapid elevation, soft rocks, and large rivers, the resultant valley takes the form of a canon, examples of which are found along the courses of the Colorado and the Yellowstone Rivers (see Fig. 8).[1] Valleys of this nature soon lose their steep sides by the action of weathering and all that this implies, and pass into a more open state, like that shown in Fig. 9.
These views have been selected in order that a comparison of this type of mountain structure may be made with that shown in Fig.
- ↑ The bottom of the canon at this point is between four and five thousand feet below the flat surfaces in the foreground—a sheer descent of nearly a mile.
