« AnteriorContinuar »
glacier is filled up by the winter snow from above. But as may be supposed, it often happens, after mild winters and warm summers, that the supply is not equal to the void, and, vice verså, after severe winters and rainy summers, the glacier is overloaded, as it were; indeed, it is scarcely possible that an exact equilibrium of supply and consumption should be preserved. Yet it seems probable, after all that has been said on the subject, that there is no material variation either in the extent or position of the glaciers among the Alps. Instances have occurred of the sudden advance of a glacier, as in the Gadmenthal (Route 32), where a road has been destroyed by this cause, and even of the formation of new glaciers within the memory of man, as in the Upper Engadine (?), and at the base of the Titlis; but these have been followed by a similar retrocession, and the newly-formed icefields are rarely permanent. It is certain that, at present, both the Mer de Glace, under Mont Blanc, and the Grindelwald Glacier, appear to have shrunk, and sunk considerably below the level they once attained; but this may be merely temporary, or even only their dimensions in summer, when most reduced. Another circumstance has been lost sight of in the consideration of this subject, viz, that the erosive powers of the ice may have, in many instances, considerably enlarged the bed of the glacier.
Professor Hugi has recently made some interesting experiments and observations upon the movement and rate of progress of the glaciers. In 1829 he noted the position of numerous loose blocks lying on the surface of the lower glacier of the Aar, relative to the fixed rocks at its sides. He also measured the glacier and erected signalposts on it. In 1836 he found everything altered;
many of the loose blocks had moved off and entirely disappeared, along with the ice that supported them. A hut, which he had hastily erected, to shelter himself and his companions, had advanced 2184 feet; two blocks of granite, between which it stood, then eight feet apart, had been separated to a distance of 18 feet, the beams and limbers had fallen in between them, and the nails and pieces of iron used in fastening them exhibited not the slightest trace of rust. A mass of granite, containing 26,000 cubic feet, originally buried under the snow of the firn, which was now converted into glacier, had not only been raised to the surface, but was elevated above it, in the air upon two pedestals, or pillars, of ice; so that a large body of men might have found shelter under it. A signal-post, stuck into a mass of granite, had not only made as great an advance as the hut, but the distance between the two had been increased 760 feet by the expansion of the glacier. The mass of the glacier had grown or increased near the point where it begins to descend 206 feet; lower down there was less augmentation perceptible. The advance of the ice-field of the Mer de Glace is calculated at between 400 and 500 feet yearly, and for 8 or 10 years past, the mass of the glacier has been shrinking and retiring gradually.
At the extremity of almost all glaciers a high transverse ridge of rubbish, 'called The Moraine, exists; it consists of fragments of rock which have fallen from the surrounding mountains, the transported debris of the Guffer, and of masses detached by the glacier itself. These are heaped up sometimes to a height of 80 or 100 feet. Not unfrequently there are 3 or 4 such ridges, one behind another, like so many lines of intrenchment. The broken stones, mud, and sand, mixed with shat
tered fragments of ice, of which they are composed, have an unsightly and shabby appearance, being perfectly barren of vegetation; but each heap is, as it were, a geological cabinet, containing specimens of all the neighbouring mountains. The glacier, indeed, seems to have a natural tendency to purge itself from impurities, and whatever happens to fall upon it is gradually discharged in this manner. It likewise exerts great mechanical force, and, like a vast millstone, grinds down, not only the rock which composes its channel, but all the fragments interposed between it and the rock; forming, in the end, a sort of stone-meal. The extent of the moraine depends on the character of the strala of the mountains around the glacier: where they are of granite, or other hard rock, not easily decomposed by the weather, the moraine is of small extent; and it is largest where the boundary rocks are of brittle limestone and fissile slate, Recent researches of Swiss naturalists (Agassiz and Charpentier) have discovered extensive moraires, not only in the lower part of the Vallais, but even on the shores of the Lake Leman, at a height of not more than 200 or 300 feet above it; clearly proving that, during some anterior condition of our planet, the valley of the Rhone was occupied by glaciers, in situations at present 40 or 50 miles distant from the nearest existing ice-field, and 3000 or 4000 feet below it.
It is highly interesting to consider how important a service the glaciers perform in the economy of nature. These dead and chilly fields of ice, which prolong the reign of winter throughout the year, are, in reality, the source of life and the springs of vegetation. They are the locked-up reservoirs, the sealed fountains, from which the vast rivers traversing the great continents of our globe are sustained. The summer heat, which dries up other sources of water, first opens out their bountiful supplies. When the rivers of the plain begin to shrink and dwindle within their parched beds, the torrents of the Alps, fed by melting snow and glaciers, rush down from the tains and supply the deficiency; and, at this season moun (July and August), the rivers and lakes of Switzerland are fullest.
During the whole summer, the traveller who crosses the glaciers hears the torrents rustling and running below him at the bottom of the azure clefts. These plenteous rills gushing forth in their subglacial beds, are generally all collected in one stream, at the foot of the glacier, which, in consequence, is eaten away into a vast dome-shaped arch, sometimes 100 feet high, which gradually increases, until the constant thawing weakens its support, and it gives way and falls in with a crash.. Such caverns of ice are seen in great perfection in some years, at the source of the Arveyron, in the valley of Chamouni, and in the glaciers of Grindelwald. The streams issuing from glaciers are distinguished by their turbid, dirty-white, milky colour.
S 18, AVALANCHES AND SNOW-STORMS.
Avalanches (Germ. Lawinen) are those accumulations of snow which precipitate themselves from the mountains, either by their own weight or by the loosening effects of the sun's heat, into the valleys below, sweeping everything before them, and causing, at times, great destruction of life and property. The fearful crash which accompanies their descent is often heard at a distance of several leagues.
The natives of the Alps distinguish between several different kinds of avalanches. The staublawinen (dust avalanches) are formed of loose fresh-fallen snow, heaped up by the wind early in the winter, before it has begun to melt or combine together. Such a mass, when it reaches the edge of a cliff or declivity, tumbles from point to point, increasing in quantity as well as in impetus every instant, and spreading itself over a wide extent of surface. It descends with the rapidity of lightning, and has been known to rush down a distance of 10 miles from the point whence it was first detached; not only descending one side of a valley, but also ascending the opposite hill, by the velocity acquired in its fall, overwhelming and laying prostrate a whole forest of firs in its descent, and breaking down another forest, up the opposite side, so as to lay the heads of the trees up the hill in its ascent.
Another kind of avalanche, the grund lawinen, occurs in spring, during the months of April and May, when the sun becomes powerful and the snow thaws rapidly under its influence. They fall constantly from different parts of the mountains, at different hours of the day, according as each part is reached by the sun : from the E. side beiweon 10 and 12, from the $. side between 12 and 2, and later in the day from the W. and N. This species is more dangerous in its effects, from the snow being clammy and adhesive, and also hard and compact. Any object buried by it can only be dug out by the most arduous labour. Men or cattle overwhelmed by the staub-lawine can sometimes extricate themselves by their own exertions; or, at any rate, from the snow being less compact, may breathe for some hours through the interstices. In the case of the grund-lawine, the sufferers are