[Illustration: 16. STORM CLOUDS OVER THE LAKE OF THUN. Looking up the Kander Thal. The Niesen on the right.]
The mountains are rising steadily as the millenniums of years pass on. The rain keeps falling on them, and as they grow higher the snows of winter first, and later of all the year, whiten their summits and gradually descend upon their slopes as the summits reach higher and higher aloft. If the rain always fell uniformly over the whole area, and if the ranges were of rock, homogeneous like a great lump of plaster, equally strong in every direction--if such were the case, each range would remain approximately symmetrical on both sides, and the crest of it would lie evenly between its two flanking troughs. But that is never the case. The rain-bringing winds are sure to come more frequently from one side of the mountain area than from the other. The wet quarter will be the east or the west or the south-west, as the case may be, and more moisture will be precipitated and consequently more denudation effected by it on one side of the ridges than on the other, with important sculpturing results as we shall presently observe.
We may best regard the rising mountain area as a plateau with a wrinkled top, such a plateau as Tibet, for example. As time advances the plateau will present ever loftier walls to the outside world, but the undulations within will not greatly develop by any directly wrinkling process. It is not the wrinkling that splits the plateau up into ranges, but quite other forces. All that the wrinkling does is to give to those forces their first direction. The interior of Tibet shows us what, but for these other forces, a great mountain region would be like. It would be traversed from end to end by low and roughly parallel ridges, separated from one another by shallow valleys raised high aloft on the great plateau-pedestal. In the shallow valleys there would lie many lakes, some having no outlets, others drained by slow streams flowing along the strike of the ranges, and fed by driblets from the slopes of the flanking hills.
But at the ends and around the periphery of the plateau generally a different condition of things will be found. Let us regard the ends first. The slow flowing rivers of the plateau as they reach its extremity will become swift, where they plunge down to the plain. In proportion to their swiftness is the speed with which they cut down their beds into the mass of the plateau-pedestal. If the end of the plateau were a cliff, the rivers would tumble over it in waterfalls, and these would cut their way back and thus dig out cañons in place of the shallow valleys of the original wrinkling. In any case a similar result will be arrived at, and the plateau will be more and more cut down into deep valleys with high ridges between. What were originally small wrinkles above the mean level of the plateau and slight depressions beneath it will be changed by denudation into high mountains and deep valleys, their scale being determined by the amount of general elevation of the plateau above the low-lying country. As the general elevating process goes on, so does the excavation. The deep valleys will be formed first at the edge of the plateau. They will work back into its heart in process of time. The original Tibetan plateau is now greatly reduced, and only the remaining middle part of it preserves any resemblance to its primary surface-form. As you go eastward or westward from that central portion you come into ever deepening valleys and ever relatively higher peaks, measured from the neighbouring valley floor.