The fairy tales of science/Moving Lands
"The ice is here, the ice is there,
The ice is all around;
It cracks and growls, and roars and howls,
Like noises in a swound."
The attention of scientific men has of late been directed to the structure and movement of glaciers, those vast accumulations of ice that fill up the deep valleys of mountains whose summits are covered by perpetual snow. These glaciers form the moving lands which we are about to consider for the edification of our reader. The facts that we have to bring forward relating to these "gigantic icicles" will doubtless be new to the majority of our readers, as they have not yet found their way into elementary scientific treatises. In selecting our fairy tales from the copious budget of science, we have never lost sight of novelty, but have endeavoured to elucidate the most recent discoveries.
As we ascend a lofty mountain the air becomes colder and colder, and at a certain elevation we enter the regions of eternal snow. The vegetation that clothes the slopes undergoes a corresponding change, and at the margin of the snow we find plants resembling those of the arctic circle.
In the upper regions of the ice-world water descends from the clouds in the form of snow but never in the form of rain. The average fall of snow, in the region of the Swiss Alps, from 8000 to 10,000 feet above the level of the sea, has been estimated at sixty feet, that is to say, sufficient snow descends in one year to form a bed of this thickness. What becomes of all this frozen water? How is it that the mountains do not become topheavy? Be patient, gentle reader, we shall be in a position to answer these momentous questions soon, but at present we must confine our attention to the structure of the snow-beds that are formed on the vast tablelands of these elevated regions.
The snow-bed is generally called the névé, and is formed of layers of more or less crystalline snow, which diminish in thickness as their depth increases; in other words, each layer is thinner than that immediately above it. At a certain depth these layers can scarcely be distinguished one from another, and still lower the substance of the névé passes into clear ice. The separate layers represent each considerable fall of snow that has taken place, and their gradual consolidation arises from the percolation of water coming from above, and the pressure of fresh strata of snow which continually accumulate overhead.
The deep valleys that radiate from the central mass of a great mountain are invariably filled with frozen water, and are the outlets of the frozen snowfields, or in the words of a clever writer, "the glacier is a river of ice, and the névé its source." Glaciers sometimes fill up a valley twenty miles long by three or four broad to the depth of six hundred feet. Although apparently solid and stationary, they really move slowly down the valley, and carry with them, either on the surface, frozen into their mass, or grinding and rubbing along the bottom, all the fragments, large and small, from blocks many tons in weight, down to the finest sand and mud, that rain, and ice, and the friction of the moving glacier itself, detach from the adjacent rocks.
The glaciers of the Alps, and probably those of other regions, descend to a vertical depth of nearly 4000 feet below the line of perpetual snow, and into a climate much warmer than that of our own island, before they finally melt away, and leap forth as rivers of running water. The heap of materials of all sorts and sizes which they deposit at their melting extremity is called the moraine, a term which is also applied to the lines of blocks that are being carried along on the surface of the glacier, the floating sticks and straws of the solid river.
Strange to say, the simple fact of the motion of glaciers was not admitted until a comparatively recent date, though it was well known that the lower end of a glacier, in spite of its rapid thawing, remained year after year at about the same point. Were we to attempt to describe the various observations that have been made with a view to determine the rate of glacial movement, we fear we should tax our reader's patience. Let us mention one or two illustrative facts. In the year 1827, M. Hugi built a very solid hut on the glacier of the lower Aar. In 1836 this hut was 4384 feet farther down the valley. Again, Professor Forbes gives an interesting account of a knapsack lost by a guide who fell into a crevass, one of those great chasms which are often observed in glaciers, which was recovered, ten years after, 4300 feet lower down. These facts, were there no others, would suffice to prove that the glaciers move onward at a slow but steady pace.
The surface of the glacier is rough and crumbling, and the traveller can walk upon it without fear of slipping; in some parts it is unbroken and undulating, but in others it is rent by yawning fissures many hundred feet in depth, one set of fissures sometimes crossing another at right angles, and so cutting up the ice in fantastic pinnacles and towers, that occasionally topple over with a terrific crash. The noises that proceed from the glacier cannot be properly described, and we can only vaguely compare the mysterious rumblings, growls, and cracklings that salute the traveller's ear to "noises in a swound."
Various theories have been advanced to account for the motion of glaciers. Saussure, who was the first to observe these wonderful ice-rivers with any attention, asserted that they advance by sliding along their beds, which are constantly lubricated by the melting of the lower strata of ice. But this explanation is far from being satisfactory. Ice is undoubtedly a very slippery substance, but it is scarcely credible that a solid mass of ice some twenty miles in length should glide along by reason of its slipperiness.
To move the Leviathan, our engineers had to make use of the most powerful machines ever constructed before they could overcome the friction between the mighty ship and the surface upon which it rested. But the mass of the Leviathan is immeasurably small compared with that of the glacier; indeed, the river of ice might support a number of such ships, and still move onward at its usual speed. Now, in spite of the lubricating fluid which Saussure imagined to exist between the glacier and its rocky bed, the friction must be immense, and we can scarcely reconcile the steady movement of the frozen mass with the operation of such a powerful retarding force.
Again, it may be asked, how does the huge icicle adapt itself to the irregular form of the valley through which it travels? A solid mass of ice, however large, might possibly slide along a perfectly straight channel, but mere slipperiness would not enable it to pass through a tortuous valley. The diameter of the great basin of the Glacier de Talèfre, on the range of Mont Blanc, is six times as great as the outlet through which the frozen stream eventually squeezes itself. Saussure's explanation throws no light upon this point, and it is quite plain that the philosopher had failed to hit upon the true theory of glacier motion.
We will pass over the theory of M. Agassiz, which was founded on a radical error, and proceed to consider that advanced by Professor James Forbes of Edinburgh. In 1842, this celebrated geologist undertook an extensive series of observations; from which dates the commencement of all sound and accurate knowledge respecting our moving lands. The laws of glacier motion were established by a few simple observations. He showed that the glacier moves onward with such regularity that it is almost possible to tell the hour by the progress of a point placed on the surface; but that the motion is less rapid in summer than in winter, in damp than in dry weather, at night than during the day. The different parts of the same glacier do not advance at a uniform rate, and the centre invariably moves more rapidly than the sides. If a series of points be laid out in a straight line across the glacier, they will be rapidly bent into the form of a regular curve, by the gradual decrease of velocity from the centre to the sides. Further observations in subsequent seasons proved that the upper part of the glacier moves faster than that near to the bottom.
These observations established the strange and unexpected conclusion, that the ice of glaciers, though apparently hard and brittle, can be bent and moulded under the enormous pressure of its own weight, and that instead of moving like an ordinary solid, it flows down the valley just as a viscous substance, such as partially melted pitch, would flow. Professor Forbes actually attributed this manner of motion to a slight degree of plasticity or a demi-semi-fluidity in the ice mass, and announced his new theory of glacier motion in these words:—"A glacier is an imperfect fluid, or a viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts."
Our moving lands are thus robbed of their solidity, and become mere sluggish rivers of a marvellous sticky fluid, which we are unable to define with anything like accuracy,
"For the ice it isn't water, and the water isn't free,
And we cannot say that anything is as it ought to be."
But are we quite sure that the viscous theory is the only possible explanation of glacier motion? It is quite certain "that the manner of movement of the surface of a glacier coincides with the manner of motion of a viscous or semi-fluid body," but we have many reasons for doubting the viscosity of glacier ice. The yawning crevasses, the fantastic towers, and the perpetual crackling noise of a glacier, would seem to prove that it is formed of a very brittle material. But a substance cannot be brittle and viscous at the same time, and we are quite at a loss to explain how it is that the motion of a mass of ice conforms to that of an imperfect fluid.
Professor Tyndall has recently cleared up the mystery, and has shown that ice may be plastic without being viscous. Some time ago, Professor Faraday discovered that two pieces of ice when placed in contact, would freeze together, even under hot water, and that any number of fragments would unite into a solid mass, provided sufficient pressure were applied to bring their surfaces together. The plasticity of ice has since been established beyond all question by the beautiful experiments of the younger philosopher. Spheres of ice have been flattened into cakes, cakes have been formed into transparent lenses, a block of ice has been moulded into a crystal cup, and a straight bar six inches long has been bent into a semi-ring. Ice can be forced into a mould and made to take what shape we please, not because it is an imperfect fluid like plaster of Paris, but because it possesses the peculiar property of re-uniting by the contact of adjoining surfaces, after having been broken into fragments. In forcing a cube of ice into a cup-shaped mould, we crush it to a powder, but the particles composing this powder immediately freeze together again into a solid and transparent cup. The plasticity of ice may therefore be explained as the effect of breakage and re-freezing, or in scientific language, fracture and regelation.
This strange property of ice fully accounts for its obedience to the law of glacier motion discovered by Professor Forbes. “All the phenomena of motion,” says Tyndall, “on which the idea of viscosity has been based are brought by such experiments as the above into harmony with the demonstrable property of ice. In virtue of this property the glacier accommodates itself to its bed, while preserving its general continuity; crevasses are closed up; and the broken ice of a cascade, such as that of the Talèfre or the Rhone, is re-compacted into a solid continuous mass.
“But if the glacier accomplishes its movements in virtue of the incessant fracture and regelation of its parts, such a process will be accompanied by a crackling noise, corresponding in intensity to the nature of the motion, and which would be absent if the motion were that of a viscous body. It is well known that such noises are heard, from the rudest crashing and quaking down to the lowest decrepitation, and they thus receive a satisfactory explanation.” The reader will now be able to comprehend the wonderful phenomena presented by our moving lands; a glacier does not slide along its bed like a launching ship along her ways, nor does it flow, in virtue of any viscous quality, like thick mud or melted pitch; but its motion is the result of the minute, almost molecular, fracture and regelation of the ice particles, which move as if they were sand, continually thawing and re-freezing.
We have said that glaciers generally carry large fragments of rock, which they deposit in confused heaps at their lower extremities. It sometimes happens, however, that a glacier descends into a lake, or into the sea, before it melts, and large masses of it, or icebergs, are floated off with their freight of rock fragments. These loaded icebergs are sometimes carried great distances before they entirely dissolve, and in this manner large unworn angular blocks of rock may be dropped on the bed of the sea hundreds of miles from their original site.
In many parts of Great Britain the geologist finds heaps of gravel and sand containing large fragments of rock which exactly resemble the terminal heaps or moraines of modern glaciers. He also finds huge blocks of rock or boulders resting upon the bare surface of rocks of quite a different character. One of the largest of the boulders is situated at the head of the Devil's Glen, in the county of Wicklow, its dimensions being twenty-seven feet long, by eighteen wide, and fifteen high. It consists of granite, and rests upon a bed of slate six or eight miles from the granite district, a wide shallow valley intervening. Another large boulder of granite has recently been discovered in the chalk near Croydon, and geologists have come to the conclusion that this mass of rock must have wandered hither from the North of Europe.
These curious heaps and boulders prove that "once upon a time" the glens of our present mountains were encumbered with glaciers, and that our low lands were entirely submerged. By the action of these glaciers the rocks were scored and rounded, polished and grooved, and masses of rock carried down and heaped into moraines; while great blocks were transported on fragments of those glaciers which dipped into the sea and formed icebergs, being often carried far over the shallow seas and dropped many miles from their parent sites, generally on the banks and shallows (now the hill-tops) which arrested the laden icebergs in their course.
We have said that our moving lands advance with great regularity. Let the reader glance at the illustration which precedes this chapter, and he will find that our artist has represented this motion by the figure of Time using his scythe as an alpen-stock, and sliding along with the glacier upon which he stands.