The Alps

by Sir William Martin Conway

The great importance of glaciers in mountain formation is the part they play as carrying agents. There is practically no limit to the weight of rock they will bear down with them in their steady uninterrupted flow. Whatever falls upon the glacier at any part of its course is carried down by it and ultimately dumped off its sides or end. A stone that falls on the highest rim of the snowfield will presently be covered up by newly-fallen snow and will be carried down at, or close to, the floor of the glacier, where it will either be ground to powder or will not emerge till it is melted out at the end of the glacier's snout. A stone that plunges in a crevasse to the bottom of the glacier will have similar experiences. Stones that tumble on to the glacier surface further down will not be so deeply covered by annual accumulations of snow, and will therefore sooner emerge again on to the surface by the melting away of the accumulation above them. Stones that fall on to the glacier below the snow-line will not be covered up at all, but will simply be carried down on the surface.

The visible collections of stone rubbish carried by a glacier are called its moraines. As the surface of a glacier tends to become convex the moraine-stuff tends to be rolled off towards the sides, where it forms the right and left lateral moraines. Where two glaciers flow together and unite, the right lateral moraine of the one and the left of the other will join and be carried down as a medial moraine on the surface of the united glacier. Such medial moraines may be observed in considerable numbers flowing down, side by side, on glaciers formed by the union of a number of higher tributaries. First comers to the Alps, beholding them from a distance, or seeing them in photographs, sometimes have thought they were cart-ruts, thus showing how false a scale of size their unaccustomed vision applies to mountain views.

A given kind of rock subjected to the action of frost and the other disintegrating forces operative at high levels, usually breaks up into debris of a roughly uniform average size. There will, of course, be some large masses and a lot of dust and gravel, but the average lump will be fairly uniform. A climber in a given district comes to know what to expect on a moraine, and he will immediately notice if the average size of the debris is much larger or smaller than usual. Thus, when he sees a debris-slope or a moraine from a distance, he is instinctively conscious that its granulated aspect represents great blocks of rock. That gives him a roughly correct scale for the view. The lowlander, who has never been in contact with a moraine, has no such sense, and can imagine that the brown streak he sees a few miles away is, as it looks to be, a mere line of dust. It was through the aspect of the moraines and debris-slopes that I first obtained an approach to a direct visual understanding of the vaster scale of the Himalayas than that of the Alps.

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.

In the theatrical stage mud avalanches are likewise common. To produce them there must be a great supply of loose debris on steep rocks at a high level and much rapidly melting snow about them, whose water drains into gullies and unites in larger gullies, all with banks of rotten and crumbling rock. On a suitable day in early summer, when the sky is clear and the sun hot, the stones will fall in such numbers that they will plug some gully and dam back the water. It will collect and burst the dam, and a flow of stones, dust, and water will begin. At other neighbouring spots the same thing will happen, and the elements of the avalanche will flow together, block a larger gully, and presently burst that block also. So it will go on till a great mass of mud, water, and rocks collects somewhere and finally bursts loose in an avalanche which sweeps all before it.

Such an avalanche I saw from close at hand on 8th July 1892, in the mountains of Nagar. We were walking up the right bank of a great glacier river, and were forced at intervals to cross its tributaries which came rushing down the hillside on our left. Approaching the mouth of one of these side gullies we heard a noise like thunder and beheld a vast black wave bulging down it. It passed before we arrived and there was silence for a few minutes. Presently the sounds of another were heard aloft, and it soon heaved into view—a terrific sight. The weight of the mud rolled masses of rock down the gully, turning them over and over like so many pebbles. They restrained the muddy torrent and kept it moving slowly with accumulating volume. Each big rock in the vanguard of the avalanche weighed many tons; some were about 10-foot cubes. The stuff behind them filled the gully some 15 feet deep by 40 wide. The thing travelled perhaps at the rate of seven miles an hour. Sometimes a bigger rock than usual barred the way till the mud, piling up behind it, swept it on. The avalanche ate into the sides of the gully and carried away huge undermined masses that fell into it. We saw three enormous avalanches of this sort pass down the same gully in rapid succession, and, after we had gone by, others followed. All the neighbouring similar gullies discharged such groups of mud avalanches during that period of the year. They are one of the chief agents used by nature to pull down mountains during this, the dramatic stage of their existence. The roaring torrential river below carries off the mud and receives the boulders in its bed, where they are rolled along and in time ground to powder.

Mud avalanches are rare now in the Alps, and are only caused by some exceptional event, such as the bursting of a glacier lake. Once they were common. Mountain-falls of any great size are also much rarer in the Alps now than they were formerly or than they are in some Himalayan regions. Alps and Andes have passed beyond the culmination of their dramatic stage. The mountains of Hunza, Nagar, and North Kashmir generally, are in the midst of theirs.

A mountaineer who has acquired a knowledge of how mountains are made, who has seen in action the forces I have briefly described, who has climbed among mountains in sunshine and storm, in heat and frost, who has spent nights on their cold crests, who knows how and where avalanches of snow, ice, and rock are likely to fall and has a realising sense of their force, their frequency, and their mass: a mountaineer who has attained by long experience a knowledge of the ways and action of glaciers, who can as it were feel their weight and momentum, in whose mind, when he looks at them, they are felt to be moving and vigorous agents, who sees the lines of motion upon them, their swing round corners, their energy in mid course, their feebleness at the snout:—such a man can look abroad over a mountain panorama with an understanding, a sense of the significance of what he beholds, which, far from detracting from its aspect of beauty, adds greatly to it.