The rate at which an ocean bed is raised to form a mountain range is, no doubt, a variable one; always slow, often interrupted, but seldom or never violent. During this time the strata usually undergo crushing and folding; stretching takes place, and displacements of the rocks, or faulting, are not uncommon. As an example of the wrinkling that the strata may suffer under these conditions, the reader is referred to the beautiful symmetrical fold shown on the side of a mountain in the Appalachians (Fig. 7). Similar folding is the rule, but often immense areas are raised to great heights above the ocean without disturbing the horizontal position of the beds (see Fig. 8). Coincident with the emergence of the rocks from beneath the 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 sea level. With relatively rapid elevation, soft rocks, and large rivers, the resultant valley takes the form of a cañon, examples of which are found along the courses of the Colorado and the Yellowstone Rivers (see Fig. 8).[11] 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. [Illustration: FIG. 8.--HORIZONTAL ROCKS, GRAND CAÑON OF THE COLORADO.] [Illustration: FIG. 9.--MOUNT STEPHEN, SHOWING ITS HORIZONTAL ROCKS.]