94

GLACIERS.

[CHAP. rivers, and from every humid surface on the land; and is then precipitated, by cooling, in the several forms of cloud, snow, rain, hail, dew, and hoar frost. We have seen, too, that much of this precipitated water is gradually collected in rivers and so brought back to the sea. We have now to follow it through its various transformations and channels in the interval; noticing the work it does by the way, in modelling the surface, in nourishing the verdure that clothes it, and in rendering it habitable and lovely.

On the higher mountain summits, where the temperature of the air is always at or below the freezing point, all the vapour that is condensed from the atmosphere falls in the form of snow. Since this is a solid, it cannot flow off like rain; it lies where it falls or is drifted by the wind, and accumulates in vast masses in the hollows of the mountain flanks, forming what is termed by Swiss mountaineers, the névé. Under the rays of the sun, the surface is sometimes melted in the day time; and sinking into the mass beneath, is again frozen; so that, in the course of time, the lower parts, pressed upon by the superincumbent weight of the later snow-falls, and cemented by the freezing water, become compacted into ice. Lying, as it does, on the steep slopes of the mountains, the névé gravitates gradually downwards, and, collecting in the loftier parts of the valleys, eventually forms a river of solid ice, termed a glacier. All the higher valleys of the Himálaya and the Alps are occupied by such glaciers, many hundreds of feet in thickness, and sometimes many miles in length. One that descends from the Kárakorám range, separating Tibet from the plains of Yárkand, is not less than thirty-six miles long, and is the longest known glacier in the world. A portion of this gigantic ice river, the glacier of Báltoro, is represented in Fig. 14. The lofty snow-covered peak to the left (only partly represented in the figure) is that of Kárakorám, designated as

I

For this beautiful illustration of a great Himálayan glacier, I am indebted to the kindness of Major H. H. Godwin Austen. The woodcut is copied from a drawing by Mr. R. P. Noble, after Major Godwin Austen's original sketch.

VII.]

THE BALTORO GLACIER.

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K2 by the officers of the Great Trigonometrical Survey, and is the second highest mountain in the world yet measured (28,278 feet). The conical peak to the right, equally snow clad, is Gusherbrúm (the gold-like peak), and rises to 26,378 feet. It may be seen in the drawing how the great trunk glacier is formed by the junction of numerous smaller glaciers, every great mass of névé sending down a separate tributary, just as a great river is formed by the union of innumerable smaller streams. But on a glacier, the several tributaries are for a long time distinguished by lines of

stones and great blocks of rock, termed medial moraines, each of which tails off from the rocky ridge that divides two tributaries. A very excellent illustration of this may be observed in the tributaries on the left that descend from the slopes of K2. Of the origin and destination of this moraine I shall speak presently.

The rate at which a glacier moves varies with the slope; being most rapid when the slope is steepest, and slower on

96

ICE A DESTRUCTIVE AGENT.

[CHAP.

a gentle incline. The central parts of the mass move more rapidly than the sides and bottom, which are retarded by friction against the rocks. In all these respects it moves exactly like a river, adapting itself to the windings and irregularities of its valley, by constant fracturing and refreezing. But the motion is necessarily slow; so slow indeed, that it is only to be detected by observing from time to time the position of objects, such as staves fixed in the ice, or the large masses of rock which are frequently carried down resting on the surface. In this way it has been found by the accurate measurements of Dr. Tyndall, that the rate of glacier motion varies from five to thirty-six inches in a day on the Mer de Glace, one of the largest glaciers of Switzerland.

Though the summits of those lofty mountains which give birth to glaciers are in great part covered with névé, they are not quite concealed. Here and there a mass of splintered rock raises itself bare of the white coverlet, and the greater part of the snow that from time to time falls upon it is soon melted beneath the rays of the sun. Some of the water so formed penetrates the warmed rock; and in the night time or in the winter, when the sun's heat is less powerful, it freezes in the minute crevices which have allowed it entrance. Now ice occupies more space than the water which it yields when melted; and in freezing, water expands with enormous force. In some experiments made by Major Williams in Canada, bomb-shells filled with water and then tightly plugged and exposed to an atmosphere much colder than that at which water freezes, either had the plug blown out to a great distance or were rent in pieces by the expansive force of the freezing water. In like manner, the rocks exposed at great altitudes become cracked and splintered by the frost, giving rise to the sharp needle-like ridges and peaks, so characteristic of the action of frost, and admirably illustrated in Major Godwin Austen's drawing (Fig. 14); and from time to time, masses, varying in size from a mere flake up to boulders many times larger than a house, are detached, and fall on the surface of the moving glacier.

VII.]

MORAINE. GLACIER DENUDATION.

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Thus ice-borne, they are carried down to lower levels; and eventually, are either lodged on some projecting crag by the way, or deposited on the mound of loose rocks that is always formed at the extremity and sides of the melting glacier; such mounds are termed terminal and lateral moraines. They are met with in the Alps and the Himálaya at levels far below those to which glaciers now extend; and, in conjunction with other indications of glacier work, prove that, in former times, the glaciers in both these mountain ranges far exceeded the dimensions of those that still exist. In Sikkim, such accumulations of iceborne blocks are found at about 6,000 feet above the sea; in the Kangra valley, in the North-Western Himalaya, down to 2,000 feet; and, as I am informed by Major Godwin Austen, in the Naga Hills, where no glaciers now exist, down to a level of 4,000 feet above the sea-level. The glaciers of the Himálaya do not now reach much below 11,000 feet.

Glaciers exert a great cutting power. The exposed rocks over which they have passed (however hard,) are found to have been ground down to a polished and flattened surface, frequently scored and grooved by the friction of still harder pebbles, which, firmly frozen in the ice, have been forced onwards with the moving mass. Such polished surfaces are termed roches moutonnées. Many of the Himálayan and Alpine valleys have thus been excavated, in a great measure by the friction of ancient glaciers: the waste of the rocks, in the form of fine sand and mud, is carried away by the stream to which every glacier gives birth, issuing from a cavern in the ice at the lower end of the glacier. The great rivers of the Himálaya, and all their affluents that originate in the snowy range, take their rise from glaciers; the Ganges for instance in that of Gangútri, the Jamna in that of Jamnotri, and the Tísta and its tributaries in the numerous glaciers that seam the flanks of Kanchinjinga, Chomiomo and Donkia.

It is evident that glaciers can be formed only in those regions, where more snow falls than the summer sun is able

B.G.

H

98

THE SNOW-LINE.

[CHAP. to melt; so that much of that which falls during one winter season lasts, till added to and covered up by that of the succeeding winter. On the southern slopes of the Himálaya, this is the case only at elevations above 16,000 feet. At lower elevations, all that falls is melted during the summer time, if not before, and goes to swell the rivers which drain the lower valleys. The line, above which snow lies throughout the year, is called the snow-line. On the northern slopes of the Himálaya it is at a greater height than on the south, viz., about 20,000 feet. This is because, although colder than the latter, the northern slopes and the mountains north of the great snowy range receive much less snow. All the vapour that yields it is brought by southerly winds from the Bay of Bengal, and (in the North-Western Himálaya) from the Arabian Sea; and by far the greater part of this vapour is condensed as rain or snow on the southern slopes of the mountains, especially the towering peaks of the great range. On reaching Tibet, the winds therefore are comparatively dry; and their dryness increases the further they penetrate towards Central Asia. Thus is explained the extreme aridity of Tibet and Chinese Tartary.

In other countries, the snow line descends the lower, the higher the latitude, or the greater the distance from the tropics. In the Alps it is found at 8,500 feet; in the Altai of Central Asia at 7,000 feet; in the mountains of Norway (the Dovre Fjeld) at 4,000 feet; and further north it descends to 2,800 feet on the western face of Scandinavia, which is exposed to the moist winds from the Atlantic. Nearly 4,000 square miles of the mountain area of this country are above the limits of perpetual snow. Lastly, Spitzbergen, within the Arctic circle, is covered with perpetual snow and ice down to the sea level.

In this last-named region, the excess of the accumulated snow is carried off by glaciers just as in the Himálaya, but they form no terminal moraines. In this cold climate, the glaciers flow down to the sea; and are pushed out into a depth of water, such, that the whole mass eventually breaks