A cliff below the snowy regions, if it does not rise out of the sea, is protected at its base by the debris fallen from it. What tumbles from above piles up below, and keeps the foot of the cliff from being eaten away. But a cliff or slope of rock rising out of a glacier or snow-field is deprived of such protection. All the stones that fall from it are carried away by the ice, so that the surface of the whole cliff keeps on peeling off, and that face of the mountain is gradually planed away. Where a great glacier bay reaches into the mountains this action may be very energetic. The whole surrounding cirque is constantly eaten at and continually extends its inner circumference. In some regions this action is more rapid than in others. Where, as in the tropics, the heat is great by day and the frost at high altitudes bitter by night, destruction goes quickly forward, and the mountains are vigorously reduced. Weak points in the rocky structure are soon found out. The range itself will be penetrated. A pass thus formed tends to be continuously lowered. In the neighbourhood of the greatest altitudes the destruction is of course most vigorous. This is the reason why, in so many places, alike in the Himalayas and the Andes, cross-cutting rivers find their way through a range by a gorge that passes quite near a culminating peak. The great Indus gorge below Nanga Parbat is the most notable instance I can recall.
We have thus, in the briefest possible manner, sketched out how some of the chief sculpturing forces operate to form mountains. I have not attempted to go into detail or to explain the various corrections and modifications that have to be applied to make the simple outline correspond with facts. Some valleys are actual depressions formed by the caving in of the earth along a line of weakness. Every mountain region contains examples of such hollows. Now and again by some complication or intersection of the wrinkling process a small area may be forced up considerably higher than the surrounding elevation and thus the mass provided for an exceptionally high peak. Volcanic peaks also remain to be considered, and have been excluded from the foregoing brief survey.
In the main, however, the statement is correct that the mountains of a region are produced by the sculpturing into ridges and subsidiary ridges of a great and slowly elevated mass. What begins as a growing plateau, passes through the stage of rocky and snowy ranges, becomes later on an area of undulating country, and if time sufficed would ultimately flatten out once more into a plain. Between the first stage and the last the sculpturing operations of nature pass through many phases. In the beginning, when the area has only just begun to rise from the level, those forces operate gently. Slopes are slight and streams flow easily down them. When the mountains have been roughly blocked out and the valleys precipitously deepened, the region enters into the dramatic stage of its history. The peaks are at their highest, the valleys at their deepest relatively to the heights. Cliffs are boldest, needles sharpest, torrents most voluminous and rapid. Now is the time when great mountain-falls most frequently occur. The rocks do not merely crumble away stone by stone, but huge masses are undermined and fall with gigantic crash and violence into the valleys, temporarily damming them across and forming lakes, which presently burst, and pour an incredible volume of water in destructive flood down the narrow and winding valley below. The flood transports and grinds up great quantities of rock and carries the material afar, for hundreds of miles perhaps, before the plain is reached and the mud deposited upon it.