Thus the first sign of a mountain range will be a series of undulations upon the surface of the supposed plain. These undulations will be roughly parallel to one another. We call the direction of their parallelism the strike of the ranges. From the moment the wrinkling movement begins, a set of forces is put in operation tending to level the wrinkles and fill up the hollows or valleys between them. These are the forces of denudation. People often vaguely speak as though mountains were first elevated to their full height and then only began to be pulled down; but of course the process of mountain sculpture is due to the simultaneous operation of the elevating and destructive forces. Every mountain is being pulled down in the very process of its elevation. It grows only because it is elevated faster than the destructive forces avail to level it. For all we yet know, some of the mountain ranges which seem most rapidly disintegrating may, in fact, still be growing. No one has yet divided the mountain ranges of the world into those which have not yet reached and those which have passed their maturity. When that has been done we shall doubtless find some clearly marked difference in aspect between them which now we do not know enough to recognise. The visible difference once discovered, the two groups will raise different kinds of emotion in the man who sees them. He will note the aspect of growth in one set and of decay in the other, and will be correspondingly affected, as we all now are by the young leaves and buds of spring and the fruits and faded foliage of autumn. Sad folk will love the fading and sanguine folk the growing hills. There will arise a new subject for poets and a new group of similes for preachers and moralists. In this way also science enlarges the material of art.
But we must return to our nascent mountain group, as yet a mere series of parallel wrinklings, higher here, lower there, with lines of depression between them. Rain falling will need to drain away, and in doing so will form pools in hollows, and will run along the furrows till it reaches the open country and can turn away. Thus the first streams of a nascent group of mountains follow and do not flow across the strike. Only the rivulets that actually flow down the slopes will flow in a direction perpendicular to the strike, and will be tributaries to the main lines of drainage that flow along the strike.
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.
VITZNAU AND LAKE OF LUCERNE
Vitznau is the terminus of the Rigi Railway. The two promontories on the right and left of the picture are the Nasen, Ober Nase and Untere Nase.
Thus far we have only spoken of the natural development of the strike rivers, those original lines of flow that follow the direction of the ranges. We must now observe how their course is affected by the development of the tributary streams that flow down the slopes of the ridges approximately at right angles to the strike. In the case of the Himalayas the rains come from southerly quarters. The damp air-current drifts against and over the plateau from that direction. Contact with the elevations against which it drifts causes the rains to fall. As the damp current flows further north it becomes continually dryer, so that less and less rain falls. Thus denudation is most energetic on the southern slopes. As the plateau rises its southern edge (to consider that alone for a moment) is most vigorously cut into by the water pouring down that face and forming gullies, which continuously tend to deepen and to cut back into the mass of the plateau. The process has only to go forward long enough, for the most energetic of these side-streams to eat its way back, right through the outermost wrinkle of the plateau, till it taps the first or southernmost of the strike rivers. From that moment the course of the strike river is changed, and instead of flowing away along its original valley, it turns at right angles and flows out through the gully cut by the side-stream, which thus becomes the main river. The next wrinkle is in turn attacked by the side-streams flowing down its south slope and in turn cut through, so that the second strike river becomes thus tributary to the first. And so the process continues.
Such is the history of the formation of a great river like the Indus. It is filled by the robbed waters of countless smaller rivers, one by one drawn within its drainage area by the action of side-streams cutting through intervening ridges. All these rivers and their tributaries go on cutting their way back with ever-increasing vigour as the trunk outlet is lowered by their united volume. This is the process whereby an original plateau is sculptured into a maze of ridges and valleys. The towering heights we behold were never elevated in isolated magnificence. A different thrust did not send up the Matterhorn, the Weisshorn, and Monte Rosa, but all the neighbourhood was elevated by one great heaving. To begin with, some lines of elevation were a little higher than others, and they determined the position of principal peaks and ridges; but as the mass was elevated the hollows were engraved by the burin of flowing water. The higher the mass was raised the deeper the hollows were impressed and the wider became their opening, for the self-same forces operate on every slope and continually eat it away and open side-valleys and subsidiary side-valleys into them. These forces operating on both sides of every ridge rapidly pull down its crest and ultimately round it off and reduce it lower and lower continually, so that it is only a question of