My Home in the Alps by Mrs. Aubrey Le Blond - HTML preview

CHAPTER IX.
ON MORAINES.

Now, in order clearly to understand the formation of moraines, I must first say a little more about the movement of glaciers and the débris which they bring down.

I have sometimes heard unthinking persons remark that the snowfall of each winter must tend to increase the height of snow-peaks. This observation shows that such people entirely overlook the four great factors in the maintenance of a uniform height on mountain summits, namely, melting, evaporation (which, in the dry air of the heights, is a very powerful factor in causing the disappearance of snow), glaciers, and avalanches. It is to the two latter of these that we must look for the construction of moraines, in which work they are very largely aided by two other factors, frost and rain. The glacier, starting in its infant purity from some white, unsullied peak, loses, before many years have past, its spotless character. The wintry frosts, gathering into iron bonds the streams which trickle down the mountain-sides, expand the water in freezing, and shatter the rocks with a force that the most solid cliffs cannot resist. Broken and weathered fragments are washed down from the slopes on every fall of rain, and dropping on to the once unspotted bosom of the glacier, swell the burden which is gradually laid upon it with advancing years. Spring after spring, furious avalanches rush down, laden with earth and stones, which they fling recklessly upon the now begrimed edges of the icy stream. The winds and storms, too, contribute their share of dust and sand, and as the glacier still flows on, shrunken in size and laden with heaps of earth and rocks, at length it lays itself to rest, a mass of dirty ice and stones, in the valley towards which it has been ceaselessly progressing.

The glacier of the Alps which comes farthest down into the lower regions is the Grindelwald glacier, which descended to 1080 metres above sea-level in 1870, while that which presents the largest surface is the Aar glacier, and the longest is the Aletsch. Heim gives an estimate, in his valuable work on glaciers, of the number of glaciers existing in Europe, dividing them into those of the first and second order.

The list is as follows:—

Glaciers have regular periods during which they advance or retreat. Many persons who visited the Mer de Glace some twenty or more years ago remember that it then came down nearly to the level of the valley of Chamonix, while the Rhone glacier reached almost to where the lower hotel now stands. In old days, too, the two arms of the Fee glacier united below the Gletscher alp, so that the cows had to pass across the ice in order to reach their summer pastures. A period of advance is always preceded for some years by a noticeable swelling of the upper portions of glaciers; this, of course, is quite what one would expect. A succession of cold, rainy summers and exceptionally snowy winters eventually causes an increase in the glaciers, and the reverse has naturally the contrary effect.

You have learnt that a moraine is a mixture of earth and stones which is borne down by a glacier, and you know how all this débris has accumulated on the ice, chiefly by means of the shattering power of frost on the rocks. Now let us notice the position which moraines assume on a glacier like, say, the Morteratsch. As I have said, persons unaccustomed to the mountain world, and thus unable to estimate the relative sizes of objects seen at a distance, have been known to inquire, when ascending Piz Languard, if the dark streak down the centre of the Morteratsch glacier is a path. They are astonished to learn that it is about fifty feet or more broad, and perhaps twenty feet high in the centre. It is, in fact, neither more nor less than a moraine, and belongs to that class known as medial moraines. Each glacier has a moraine on either side of it, and when two glaciers unite, their lateral moraines join and form a medial moraine. The moraine at the end of a glacier (terminal moraine) is almost entirely formed of the earth and stones which fall off the end of a glacier, and not, as used to be supposed, by any pushing or scooping of the base of the glacier.

In fact, the erosive power of a glacier is infinitesimal as compared with that of water. Dr. Heim cites various examples to show that a glacier leaves undisturbed much of what it finds in its way, and he says that the Forno glacier, which some years ago greatly retreated and left blocks of itself covered with débris behind, rapidly advanced once more in 1884 over the old accumulations at its base, but did not disturb them in any way. Many of our readers will have noticed the many glacier-worn rocks in the Engadine valley; they are especially abundant near Maloja.

Now it will be seen, on near examination, that these rocks have been gently polished by the ice constantly slipping over them, and that they have not those deep smooth hollows which are formed by rushing, eddying water.

The great glaciers which, in the glacier period, flowed down from Mont Blanc to the Jura have left ample proof of their origin in the huge blocks of granite which were transported by the ice, and now lie stranded on the hill-sides at a distance of sixty miles and more from the rocks out of which they were quarried. The size of some of these erratic blocks is very remarkable. The biggest boulder in the Alps is in Val Masino (one of the Italian valleys near the Bernina district). Its dimensions, according to the late Mr. Ball, are—length, 250 feet; breadth, 120 feet; height, 140 feet; in fact, as Mr. Douglas Freshfield remarks, “as tall as an average church tower, and large enough to fill up many a London square.” Many of my readers will remember the great serpentine boulder in front of the little inn at Maltmark, which was no doubt brought down by the glacier which must have originally filled the basin of the lake. The ancient moraines near Aosta are also remarkable evidences of the glacial epoch.

One word here as to the shape of a moraine. It rises, as you know, to a ridge in the centre, and slopes down like the roof of a house at the sides. This is because the heaping together of the earth and stones in the middle has protected the ice from melting as rapidly there as towards the sides; in fact, the same cause brings about the shape of moraines as applies in that of sand cones.

I will close this chapter by a brief explanation of an appearance which many of my readers who have visited Montanvert may have noticed, especially on cloudy, dull days and after sunset. I refer to dirt bands, which are especially noticeable on the Mer de Glace. I observed them under peculiarly favourable circumstances from the summit of the Grandes Jorasses, when a cloudy sky showed them up most distinctly. They are often seen from the Montanvert hotel, however, and take the form of dark bands across the glacier, the convex side of the curve of each being in the direction of the motion of the ice. These dirt bands are very simple in their origin, which is as follows:—At the foot of an ice-fall the tottering blocks reunite and freeze together, presenting a tolerably smooth surface with gentle undulations. The glacier streams sweep dust and small débris into the depressions, which gradually form themselves across the glacier. This dust finally freezes into the ice, and lower down presents the appearance of the famed dirt bands.

Sometimes a photograph will give dirt bands with great distinctness; they are very clearly seen in a view of the Mer de Glace from the Aiguilles Rouge, taken by the late Mr. W. F. Donkin.