“Ye ice-falls! ye that from the mountain’s brow
Adown enormous ravines slope amain—
Torrents, methinks, that heard a mighty voice,
And stopped at once amid their maddest plunge!
Motionless torrents! silent cataracts!
Who made you glorious as the gates of Heaven
Beneath the keen full moon? Who bade the sun
Clothe you with rainbows? Who with living flowers
Of loveliest blue, spread garlands at your feet? God! let the torrents, like a shout of nations,
Answer! and let the ice-plains echo, God!
God! sing ye meadow-streams with gladsome voice!
Ye pine-groves, with your soft and soul-like sounds!
And they too have a voice, yon piles of snow,
And in their perilous fall shall thunder, God!”[29]
A Glacier is an enormous mass of solid ice filling up a valley, and stretching from the eternal snows which crown the summits of the mountains, down to the smiling cornfields and rich pastures of the plains. It is constantly fed by the accumulated snows of winter, which, slipping and rolling down the slopes of the mountains, lodge in the valleys below, and are there converted into ice. For it must be remembered that the Glacier properly so called does not commonly extend much higher than 9000 feet above the level of the sea. Beyond that elevation the compact and massive ice gradually passes into frozen snow, called by the French Nevé, and by the Germans Firn. The change which takes place in the condition of the snow as it descends into the valley is chiefly owing to these two circumstances: first, it is closely compacted together by the weight of the snowy masses pressing down upon it from above; and secondly, in the summer months it is thawed upon the surface during the day by the heat of the sun, and frozen again at night. On a small scale this process is practically familiar to every school-boy. When he makes a snow-ball he is practically converting a mass of snow into ice, and that by a series of operations very closely resembling those which Nature employs in the manufacture of a Glacier.
In Switzerland the Glacier is often two or three miles in breadth, from twenty to thirty miles in length, and five or six hundred feet in depth. Though so vast in its bulk and so solid in its character, it is not, as might be supposed, a fixed, immovable mass. On the contrary, it is moving incessantly, but slowly, down the valley which it occupies, at the rate of several inches—sometimes one or two feet, and even more—in the day. In Greenland a Glacier explored by Doctor Hayes, in his expedition to the North Pole, was found to move for a whole year at the average rate of a hundred feet a day. It may be thought, perhaps, that this fact requires further confirmation; but at all events it is certain that the language of the poet, when he addresses the Glaciers as “motionless torrents,” though it conveys an accurate and beautiful idea of the appearance they present to the eye, is not rigorously true in a scientific sense. Indeed, it is just because the Glaciers are not motionless that they serve as instruments of Denudation.
Their agency in this respect “consists partly in their power of transporting gravel, sand, and huge stones, to great distances, and partly in the smoothing, polishing, and scoring of their rocky channels, and the boundary walls of the valleys through which they pass. At the foot of every steep cliff or precipice in high Alpine regions, a sloping heap is seen of rocky fragments detached by the alternate action of frost and thaw. If these loose masses, instead of accumulating on a stationary base, happen to fall upon a Glacier, they will move along with it, and, in place of a single heap, they will form in the course of years a long stream of blocks. If a Glacier be twenty miles long, and its annual progression about five hundred feet, it will require about two centuries for a block thus lodged upon its surface to travel down from the higher to the lower regions, or to the extremity of the icy mass. This terminal point usually remains unchanged from year to year, although every part of the ice is in motion, because the liquefaction by heat is just sufficient to balance the onward movement of the Glacier, which may be compared to an endless file of soldiers, pouring into a breach, and shot down as fast as they advance.
“The stones carried along on the ice are called in Switzerland the moraines of the Glacier. There is always one line of blocks on each side or edge of the icy stream, and often several in the middle, where they are arranged in long ridges or mounds of snow and ice, often several yards high. The reason of their projecting above the general level, is the non-liquefaction of the ice in those parts of the surface of the Glacier which are protected from the rays of the sun, or the action of the wind, by the covering of the earth, sand, and stones. The cause of medial moraines was first explained by Agassiz, who referred them to the confluence of tributary Glaciers. Upon the union of two streams of ice, the right lateral moraine of one of the streams comes in contact with the left lateral moraine of the other, and they afterward move on together, in the centre, if the confluent Glaciers are equal in size, or nearer to one side if unequal.
“Fragments of stone and sand which fall through crevasses in the ice, and get interposed between the moving Glacier and the fundamental rock, are pushed along so as to have their angles more or less worn off, and many of them are entirely ground down into mud. Some blocks are pushed along between the ice and the steep boundary rocks of the valley, and these, like the rocky channel at the bottom of the valley, often become smoothed and polished, and scored with parallel furrows, or with lines and scratches produced by hard minerals, such as crystals of quartz, which act like the diamond upon glass. The effect is perfectly different from that caused by the action of water, or a muddy torrent forcing along heavy stones; for these not being held like fragments of rock in ice, and not being pushed along under great pressure, cannot scoop out long rectilinear furrows or grooves parallel to each other. The discovery of such markings at various heights far above the surface of existing Glaciers, and for miles beyond their present terminations, affords geological evidence of the former extension of the ice beyond its present limits in Switzerland and other countries.”[30]
Fig. 2.—Iceberg seen in mid-ocean 1400 miles from any known land.
Sometimes, however, it happens, especially in extreme northern and southern latitudes, that the glacier valley leads down to the sea. In such cases, huge masses of ice are floated off, and, with their ponderous burden of gravel, mud, and rocks, are carried away by currents toward the equator. Immense numbers of these floating islands of ice, or Icebergs, as they are called, are seen by mariners drifting along in the Northern and Southern oceans. In 1822 Scoresby counted five hundred between the latitudes 69° and 70° N., many of which measured a mile in circumference, and rose two hundred feet above the surface of the sea.[31] The annexed drawing, copied by kind permission of the author from Sir Charles Lyell’s Principles of Geology, affords a good idea of the appearance that such Icebergs present to the eye. The one represented in the fore-ground was supposed to reach a height of nearly three hundred feet, and was observed with many others floating about in the Southern Ocean at a distance of 1400 miles from any known land. An angular mass of rock was visible on the surface. The part exposed was twelve feet high and from five to six broad: but it was conjectured, from the color of the surrounding ice, that the greater part of the stone was concealed from view.
How enormous must be the magnitude of those ponderous masses may be learned from the fact that the bulk of ice below the level of the water is about eight times as great as that above: and in point of fact, Captain Sir John Ross saw several of them aground in Baffin’s Bay, where the water was 1500 feet deep. It has been calculated that the beds of earth and stones which they carry along cannot be less than from 50,000 to 100,000 tons in weight. Sir Charles Lyell, writing in 1865 from the results of the latest investigations on this subject, says: “Many had supposed that the magnitude commonly attributed to icebergs by unscientific navigators was exaggerated; but now it appears that the popular estimate of their dimensions has rather fallen within than beyond the truth. Many of them, carefully measured by the officers of the French exploring expedition of the Astrolabe, were between 100 and 225 feet high above water, and from two to five miles in length. Captain d’Urville ascertained one of them, which he saw floating, to be thirteen miles long, and a hundred feet high, with walls perfectly vertical.”[32]