The Alps

by Sir William Martin Conway

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 time for the biggest mountain mass to be lowered to the level of the plains around it.

Running water is not the only agent that has to be considered. Even more energetic agents act in the higher regions of frost. There the snow that is melted by the sun (whose dissolving power is as operative in the regions so-called of perpetual snow as it is below) percolates into the crevices of the rocks and finds out all their weak places. At night this water freezes, and in freezing expands, thus acting like a wedge and splitting the rock it has penetrated. Next time the sun shines the pieces thus split off may fall. Sooner or later, after repeated operations of the wedge, they must fall, and a new surface of rock will be uncovered to be split and shivered in its turn. The rocks that fall tumble ultimately on to the snow-fields that spread over the high open spaces, where they are taken charge of by the great carrying agents of the heights—the glaciers. The higher a peak is, relatively to its neighbours, the more rapidly will frost attack it, and the more energetic will be the destruction wrought upon it. I have heard it estimated, or perhaps only guessed, that 1000 tons of rock fall daily from the upper portion of the Matterhorn's rock-pyramid. The great peaks of the Himalaya are falling yet more rapidly to pieces.

But what in this relation is the action of the glaciers? At one time they were regarded as a great abrading agency. It was thought that the high valleys were fashioned out by them. Later it was concluded that their hollowing action was a negligible quantity. The general belief now is that it is not considerable. Whatever may be the action of glaciers upon their beds, it is at all events a small matter compared with their action as transporting agents. Glaciers are not hoary accumulations of snow, collected in hollow places since the beginning of the world, as our forefathers supposed, but flowing streams of ice, whose rate of movement varies with the slope, the latitude, the mean temperature, and other factors of their situation. The snow that falls at high elevations lies in great masses where it finds lodgment, or falls to such places from the steep rocks which are unable to give it steady support. By these means it falls and drifts together into those great upper reservoirs we call the snow-fields—resplendent areas of purest white, so toilsome to cross when the sun shines hotly upon them, and so incomparably beautiful to look upon. Here by melting of the surface, percolation into the body of the snow-field, and freezing there, and by the pressure of the ever-increasing accumulation of snow, the substance is gradually changed into granulated ice, and the ice thus formed slowly moves down-hill. The various neighbouring streams of ice flow and unite together, and thus, reaching lower and lower levels and continually melting, they come to a line where the annual increment of snow is equal in amount to the depth of snow annually melted. This is called the snow-line. Still downward flows the mass, and now the amount melted becomes greater than the amount annually received. The thickness of the ice steadily diminishes till at last the total arrival melts and the glacier ends in a so-called snout.

THE FALLS OF TOSA, VAL FORMAZZA

Said to be the grandest in the Alps, 470 feet high. The Tosa falls in three cascades. The first only is shown in the picture.