Mountains as we see them are never in the shape which they would present were the forces of compression and upheaval alone concerned in their formation. Every mountain range has been profoundly affected by the agencies of denudation, and their ridges and peaks, their cliffs and valleys, have been carved out of swelling folds and domes, or angular, tilted fault-blocks. As upheaval is a slow process, denudation must have begun its work as soon as the crests of the folds made their appearance above the sea, or above the level of the ground, so that probably no range ever had the full height which the strata, if free from denudation, would have given to it. Upheaval, though sometimes slow enough to allow rivers to keep their channels open, is yet too rapid to be kept in check by the processes of general atmospheric weathering, and so the ranges grew into great uplifts. But as soon as the movement of elevation ceased, denudation began to get the upper hand, for as we have learned, mountains are the scene of rapid erosion.

Lofty, or alpine, ranges are subject to peculiarly rapid and effective denudation, quite different in character from that which operates in the lowlands. Above the limit of the growth of trees (tree- or timber-line) rock destruction goes on with great rapidity, as is indicated by the wild and chaotic confusion of rock-pieces. In German this is called Felsenmeer (sea of rock), but there is no English term for it, and it is preeminently characteristic of lofty mountain slopes. These masses of shattered rock are not only evidences of rapid disintegration by frost, but afford an immensely increased surface to destructive weathering. The wind, which blows with great violence, is an important agent of destruction; avalanches carry down great quantities of rock, and the combined agencies of frost and gravity produce the vast talus slopes of all high mountains.

Another very effective agent among alpine summits is the glacier, which, by widening and cutting back the cirque at its head, eats rapidly into the mountain mass. When many glaciers rise on the different sides of a mountain, the recession of the cirques will develop sharp crests and knife-edges. To this cause has been chiefly attributed the extreme ruggedness of the high Alps, Sierra Nevada, and other ranges.

For a long period the effect of denudation is greatly to increase the ruggedness of the mountains, carving folds into ridges and cliffs, and ridges into bold and inaccessible peaks, but sooner or later the mountains are worn down lower and lower, and are eventually levelled with the plains from which they spring. In the process of degradation, the synclines often resist wear better than the anticlines, and standing up above the level, form the synclinal mountains of many ancient ranges.

From the geological point of view mountains must be regarded as short-lived and ephemeral; low-lying plains persist for a longer time than do lofty ranges, as rivers may outlast many generations of lakes. Consequently, among the mountain chains of the globe, we everywhere find that the lofty ranges are those of comparatively recent date, while ancient mountains have been worn down into mere stumps. The Appalachians have been reduced nearly to base-level, and their present condition is that of a reelevated and dissected peneplain, the ridges and valleys of which are determined by the position, attitude, and alternation of harder and softer strata. In its pristine state this very ancient range may have been as lofty as the Alps or Andes. Of course, there is no mathematical ratio between the youth of a range and its height, for moderately folded strata of moderate thickness never could have formed very high mountains, but in a general way it is true, that very high ranges are youthful, and that very old ranges are low. The process of degradation may go so far as to sweep away a mountain range to its very roots, leaving only the intensely plicated strata of the plain as evidence that mountains ever existed there.

Of such a nature is the upland of southern New England and the great metamorphic area of Canada, both of which probably once carried ranges of high mountains.