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.
CHAPTER X. ON AVALANCHES.
Those who during the summer of 1888 visited Switzerland had very unusual opportunities for studying both the appearance and the effect of avalanches. It is, indeed, rare to see a huge mass of winter snow lying in the Rosegthal in mid-summer, and the remains of numerous other avalanches were that year still unmelted in many high-lying Alpine valleys. I wonder if the crowds who, out of curiosity, visited the snowy débris, knew anything of the various causes which formed the avalanche and launched it down the hill-side, or if they could tell to what class of avalanche it belonged, and at what time of year it is likely to have fallen.
Avalanches vary immensely in their characteristics, and can be classed under three headings according to their peculiarities. The different kinds of avalanches are as follows:—Staublawinen, or dust avalanches; Grundlawinen, or compact avalanches; Eislawinen, or ice avalanches. Dust avalanches are the most to be feared of any, for while the others fall according to certain well-known rules and at particular times of the year, the dust avalanches are erratic in their movements, uncertain in the periods at which they come down, and most terrible in their results. Dust avalanches consist of cold, dry, powdery snow, which falling on a slope of ice or hard snow, or even on a steep slope of grass, slides off on the slightest provocation.