The Atlantic Monthly, Volume 13, No. 75, January, 1864

EXTERNAL APPEARANCE OF GLACIERS

Thus far we have examined chiefly the internal structure of the glacier; let us look now at its external appearance, and at the variety of curious phenomena connected with the deposit of foreign materials upon its surface, some of which seem quite inexplicable at first sight. Among the most striking of these are the large boulders elevated on columns of ice, standing sometimes ten feet or more above the level of the glacier, and the sand-pyramids, those conical hills of sand which occur not infrequently on all the large Alpine glaciers. One is at first quite at a loss to explain the presence of these pyramids in the midst of a frozen ice-field, and yet it has a very simple cause.

I have spoken of the many little rills arising on the surface of the ice in consequence of its melting. Indeed, the voice of the waters is rarely still on the glacier during the warm season, except at night. On a summer's day, a thousand streams are born before noontide, and die again at sunset; it is no uncommon thing to see a full cascade come rushing out from the lower end of a glacier during the heat of the day, and vanish again at its decline. Suppose one of these rivulets should fall into a deep, circular hole, such as often occur on the glacier, and the nature of which I shall presently explain, and that this cylindrical opening narrows to a mere crack at a greater or less depth within the ice, the water will find its way through the crack and filter down into the deeper mass; but the dust and sand carried along with it will be caught there, and form a deposit at the bottom of the hole. As day after day, throughout the summer, the rivulet is renewed, it carries with it an additional supply of these light materials, until the opening is gradually filled and the sand is brought to a level with the surface of the ice. We have already seen, that, in consequence of evaporation, melting, and other disintegrating causes, the level of the glacier sinks annually at the rate of from five to ten feet, according to stations. The natural consequence, of course, must be, that the sand is left standing above the surface of the ice, forming a mound which would constantly increase in height in proportion to the sinking of the surrounding ice, had it sufficient solidity to retain its original position. But a heap of sand, if unsupported, must very soon subside and be dispersed; and, indeed, these pyramids, which are often quite lofty, and yet look as if they would crumble at a touch, prove, on nearer examination, to be perfectly solid, and are, in fact, pyramids of ice with a thin sheet of sand spread over them. A word will explain how this transformation is brought about. As soon as the level of the glacier falls below the sand, thus depriving it of support, it sinks down and spreads slightly over the surrounding surface. In this condition it protects the ice immediately beneath it from the action of the sun. In proportion as the glacier wastes, this protected area rises above the general mass and becomes detached from it. The sand, of course, slides down over it, spreading toward its base, so as to cover a wider space below, and an ever-narrowing one above, until it gradually assumes the pyramidal form in which we find it, covered with a thin coating of sand. Every stage of this process may occasionally be seen upon the same glacier, in a number of sand-piles raised to various heights above the surface of the ice, approaching the perfect pyramidal form, or falling to pieces after standing for a short time erect.

The phenomenon of the large boulders, supported on tall pillars of ice, is of a similar character. A mass of rock, having fallen on the surface of the glacier, protects the ice immediately beneath it from the action of the sun; and as the level of the glacier sinks all around it, in consequence of the unceasing waste of the surface, the rock is gradually left standing on an ice-pillar of considerable height. In proportion as the column rises, however, the rays of the sun reach its sides, striking obliquely upon them under the boulder, and wearing them away, until the column becomes at last too slight to sustain its burden, and the rock falls again upon the glacier; or, owing to the unequal action of the sun, striking of course with most power on the southern side, the top of the pillar becomes slanting, and the boulder slides off. These ice-pillars, crowned with masses of rock, form a very picturesque feature in the scenery of the glacier, and are represented in many of the landscapes in which Swiss artists have endeavored to reproduce the grandeur and variety of Alpine views, especially in the masterly Aquarelles of Lory. The English reader will find them admirably well described and illustrated in Dr. Tyndall's work upon the glaciers. They are known throughout the Alps as "glacier-tables"; and many a time my fellow-travellers and I have spread our frugal meal on such a table, erected, as it seemed, especially for our convenience.

Another curious effect is that produced by small stones or pebbles, small enough to become heated through by the sun in summer. Such a heated pebble will of course melt the ice below it, and so wear a hole for itself into which it sinks. This process will continue as long as the sun reaches the pebble with force enough to heat it. Numbers of such deep, round holes, like organ-pipes, varying in size from the diameter of a minute pebble or a grain of coarse sand to that of an ordinary stone, are found on the glacier, and at the bottom of each is the pebble by which it was bored. The ice formed by the freezing of water collecting in such holes and in the fissures of the surface is a pure crystallized ice, very different in color from the ice of the great mass of the glacier produced by snow; and sometimes, after a rain and frost, the surface of a glacier looks like a mosaic-work, in consequence of such veins and cylinders or spots of clear ice with which it is inlaid.

Indeed, the aspect of the glacier changes constantly with the different conditions of the temperature. We may see it, when, during a long dry season, it has collected upon its surface all sorts of light floating materials, as dust, sand, and the like, so that it looks dull and soiled,—or when a heavy rain has washed the surface clean from all impurities and left it bright and fresh. We may see it when the heat and other disintegrating influences have acted upon the ice to a certain superficial depth, so that its surface is covered with a decomposed crust of broken, snowy ice, so permeated with air that it has a dead-white color, like pounded ice or glass. Those who see the glacier in this state miss the blue tint so often described as characteristic of its appearance in its lower portion, and as giving such a peculiar beauty to its caverns and vaults. But let them come again after a summer storm has swept away this loose sheet of broken, snowy ice above, and before the same process has had time to renew it, and they will find the compact, solid surface of the glacier of as pure a blue as if it reflected the sky above. We may see it in the early dawn, before the new ice of the preceding night begins to yield to the action of the sun, and the surface of the glacier is veined and inlaid with the water poured into its holes and fissures during the day and transformed into pure, fresh ice during the night,—or when the noonday heat has wakened all its streams, and rivulets sometimes as large as rivers rush along its surface, find their way to the lower extremity of the glacier, or, dashing down some gaping crevasse or open well, are lost beneath the ice.

It would seem from the quantity of water that is sometimes ingulfed within these open breaks in the ice, that the glacier must occasionally be fissured to a very great depth. I remember once, when boring a hole in the glacier in order to let down a self-regulating thermometer into its interior, seeing an immense fissure suddenly rent open, in consequence, no doubt, of the shocks given to the ice by the blows of the instruments. The effect was like that of an earthquake; the mass seemed to rock beneath us, and it was difficult to keep our feet. One of these glacial rivers was flowing past the spot at the time, and it was instantly lost in the newly formed chasm. However deep and wide the fissure might be, such a stream of water, constantly poured into it, and daily renewed throughout the summer, must eventually fill it and overflow, unless it finds its way through the whole mass of the glacier to the bottom on which it rests; it must have an outlet above or below. The fact that considerable rivulets (too broad to leap across, and too deep to wade through safely even with high boots) may entirely vanish in the glacier unquestionably shows one of two things,—that the whole mass must be soaked with water like a wet sponge, or the cavities reach the bottom of the glacier. Probably the two conditions are generally combined.

In direct connection with the narrower fissures are the so-called moulins,—the circular wells on the glacier. We will suppose that a transverse, narrow fissure has been formed across the glacier, and that one of the many rivulets flowing longitudinally along its surface empties into it. As the surface-water of the glacier, producing these rivulets, arises not only from the melting of the ice, but also from the condensation of vapor, or even from rain-falls, and flows over the scattered dust-particles and fragments of rock, it has always a temperature slightly above 32°, so that such a rivulet is necessarily warmer than the icy edge of the fissure over which it precipitates itself. In consequence of its higher temperature it melts the edge, gradually wearing it backward, till the straight margin of the fissure at the spot over which the water falls is changed to a semicircle; and as much of the water dashes in spray and foam against the other side, the same effect takes place there, by which a corresponding semicircle is formed exactly opposite the first. This goes on not only at the upper margin, but through the whole depth of the opening as far down as the water carries its higher temperature. In short, a semicircular groove is excavated on either side of the fissure for its whole depth along the line on which the rivulet holds its downward course. After a time, in consequence of the motion of the glacier, such a fissure may close again, and then the two semicircles thus brought together form at once one continuous circle, and we have one of the round deep openings on the glacier known as moulins, or wells, which may of course become perfectly dry, if any accident turns the rivulet aside or dries up its source. The most common cause of the intermittence of such a waterfall is the formation of a crevasse higher up, across the watercourse which supplied it, and which now begins another excavation.

These wells are often very profound. I have lowered a line for more than seven hundred feet in one of them before striking bottom; and one is by no means sure even then of having sounded the whole depth, for it may often happen that the water meets with some obstacle which prevents its direct descent, and, turning aside, continues its deeper course at a different angle. Such a well may be like a crooked shaft in a mine, changing its direction from time to time. I found this to be the case in one into which I caused myself to be lowered in order to examine the internal structure of the glacier. For some time my descent was straight and direct, but at a depth of about fifty feet there was a landing-place, as it were, from which the opening continued its farther course at quite a different angle. It is within these cylindrical openings in the ice that those accumulations of sand collect which form the pyramids described above.

One may often trace the gradual formation of these wells, because, as they require certain similar conditions, they are very apt to be found in various stages of completion along the same track where these conditions occur. Fissures, for instance, will often be produced along the same line, because, as the mass of the glacier moves on, its upper portions, as they advance, come successively in contact with inequalities of the bottom, in consequence of which the ice is strained beyond its power of resistance and cracks across. Rivulets are also likely to be renewed summer after summer over the same track, because certain conditions of the surface of the glacier, to which I have not yet alluded, and which favor the more rapid melting of the ice, remain unchanged year after year. Of course, the wells do not remain stationary any more than any other feature of the glacier. They move on with the advancing mass of ice, and we consequently find the older ones considerably lower down than the more recent ones. In ascending such a track as I have described, along which fissures and rivulets are likely to occur, we may meet first with a sand-pyramid; at a certain distance above that there may be a circular opening filled to its brim with the sand which has just reached the surface of the ice; a little above may be an open well with the rivulet still pouring into it; or higher up, we may meet an open fissure with the two semicircles opposite each other on the margins, but not yet united, as they will be presently by the closing of the fissure; or we may find near by another fissure, the edges of which are just beginning to wear in consequence of the action of the water. Thus, though we cannot trace the formation of such a cylindrical shaft in the glacier from the beginning to the end, we may by combining the separate facts observed in a number decipher their whole history.

In describing the surface of the glacier, I should not omit the shallow troughs which I have called "meridian holes," from the accuracy with which they register the position of the sun. Here and there on the glacier there are patches of loose materials, dust, sand, pebbles, or gravel, accumulated by diminutive water-rills, and small enough to become heated during the day. They will, of course, be warmed first on their eastern side, then, still more powerfully, on their southern side, and in the afternoon with less force again on their western side, while the northern side will remain comparatively cool. Thus around more than half of their circumference they melt the ice in a semicircle, and the glacier is covered with little crescent-shaped troughs of this description, with a steep wall on one side and a shallow one on the other, and a little heap of loose materials in the bottom. They are the sundials of the glacier, recording the hour by the advance of the sun's rays upon them.

In recapitulating the results of my glacial experience, even in so condensed a form as that in which I intend to present them here, I shall be obliged to enter somewhat into personal narration, though at the risk of repeating what has been already told by the companions of my excursions, some of whom wrote out in a more popular form the incidents of our daily life which could not be fitly introduced into my own record of scientific research. When I first began my investigations upon the glaciers, now more than twenty-five years ago, scarcely any measurements of their size or their motion had been made. One of my principal objects, therefore, was to ascertain the thickness of the mass of ice, generally supposed to be from eighty to a hundred feet, and even less. The first year I took with me a hundred feet of iron rods, (no easy matter, where it had to be transported to the upper part of a glacier on men's backs,) thinking to bore the glacier through and through. As well might I have tried to sound the ocean with a ten-fathom line. The following year I took two hundred feet of rods with me, and again I was foiled. Eventually I succeeded in carrying up a thousand feet of line, and satisfied myself, after many attempts, that this was about the average thickness of the glacier of the Aar, on which I was working. I mention these failures, because they give some idea of the discouragements and difficulties which meet the investigator in any new field of research; and the student must remember, for his consolation under such disappointments, that his failures are almost as important to the cause of science and to those who follow him in the same road as his successes. It is much to know what we cannot do in any given direction,—the first step, indeed, toward the accomplishment of what we can do.

A like disappointment awaited me in my first attempt to ascertain by direct measurement the rate of motion in the glacier. Early observers had asserted that the glacier moved, but there had been no accurate demonstration of the fact, and so uniform is its general appearance from year to year that even the fact of its motion was denied by many. It is true that the progress of boulders had been watched; a mass of rock which had stood at a certain point on the glacier was found many feet below that point the following year; but the opponents of the theory insisted that it did not follow, because the mass of rock had moved, that therefore the mass of ice had moved with it. They believed that the boulder might have slid down for that distance. Neither did the occasional encroachment of the glaciers upon the valleys prove anything; it might he solely the effect of an unusual accumulation of snow in cold seasons. Here, then, was another question to be tested; and one of my first experiments was to plant stakes in the ice to ascertain whether they would change their position with reference to the sides of the valley or not. If the glacier moved, my stakes must of course move with it; if it was stationary, my stakes would remain standing where I had placed them, and any advance of other objects upon the surface of the glacier would be proved to be due to their sliding, or to some motion of their own, and not to that of the mass of ice on which they rested. I found neither the one nor the other of my anticipated results; after a short time, all the stakes lay flat on the ice, and I learned nothing from my first series of experiments, except that the surface of the glacier is wasted annually for a depth of at least five feet, in consequence of which my rods had lost their support, and fallen down. Similar disappointment was experienced by my friend Escher upon the great glacier of Aletsch.

My failure, however, taught me to sink the next set of stakes ten or fifteen feet below the surface of the ice, instead of five; and the experiment was attended with happier results. A stake planted eighteen feet deep in the ice, and cut on a level with the surface of the glacier, in the summer of 1840, was found, on my return in the summer of 1841, to project seven feet, and in the beginning of September it showed ten feet above the surface. Before leaving the glacier, in September, 1841, I planted six stakes at a certain distance from each other in a straight line across the upper part of the glacier, taking care to have the position of all the stakes determined with reference to certain fixed points on the rocky walls of the valley. When I returned, the following year, all the stakes had advanced considerably, and the straight line had changed to a crescent, the central rods having moved forward much faster than those nearer the sides, so that not only was the advance of the glacier clearly demonstrated, but also the fact that its middle portion moved faster than its margins. This furnished the first accurate data on record concerning the average movement of the glacier during the greater part of one year. In 1842 I caused a trigonometric survey of the whole glacier of the Aar to be made, and several lines across its whole width were staked and determined with reference to the sides of the valley;² for a number of successive years the survey was repeated, and furnished the numerous data concerning the motion of the glacier which I have published. I shall probably never have an opportunity of repeating these experiments, and examining anew the condition of the glacier of the Aar; but as all the measurements were taken with reference to certain fixed points recorded upon the map mentioned in the note, it would be easy to renew them over the same locality, and to make a direct comparison with my first results after an interval of a quarter of a century. Such a comparison would be very valuable to science, as showing any change in the condition of the glacier, its rate of motion, etc., since the time my survey was made.

These observations not only determined the fact of the motion of the glacier itself, as well as the inequality of its motion in different parts, but explained also a variety of phenomena indirectly connected with it. Among these were the position and direction of the crevasses, those gaping fissures of unknown depths, sometimes a mile or more in length, and often measuring several hundred feet in width, the terror, not only of the ordinary traveller, but of the most experienced mountaineers. There is a variety of such crevasses upon the glacier, but the most numerous and dangerous are the transverse and lateral ones. The transverse ones were readily accounted for after the motion of the glacier was admitted; they must take place, whenever, the glacier advancing over inequalities or steeper parts of its bed, the tension of the mass was so great that the cohesion of the particles was overcome, and the ice consequently rent apart. This would be especially the case wherever some steep angle in the bottom over which it moved presented an obstacle to the even advance of the mass. But the position of the lateral ones was not so easily understood. They are especially apt to occur wherever a promontory of rock juts out into the glacier; and when fresh, they usually slant obliquely upward, trending from the prominent wall toward the head of the glacier, while, when old, on the contrary, they turn downward, so that the crevasses around such a promontory are often arranged in the shape of a spread fan, diverging from it in different directions. When the movement of the glacier was fully understood, however, it became evident, that, in its effort to force itself around the promontory, the ice was violently torn apart, and that the rent must take place in a direction at right angles with that in which the mass was moving. If the mass be moving inward and downward, the direction of the rent must be obliquely upward. As now the mass continues to advance, the crevasses must advance with it; and as it moves more rapidly toward the middle than on the margins, that end of the crevasse which is farthest removed from the projecting rock must move more rapidly also; the consequence is, that all the older lateral crevasses, after a certain time, point downward, while the fresh ones point upward.

Not only does the glacier collect a variety of foreign materials on its upper surface, but its sides as well as its lower surface are studded with boulders, stones, pebbles, sand, coarse and fine gravel, so that it forms in reality a gigantic rasp, with sides hundreds of feet deep, and a surface thousands of feet wide and many miles in length, grinding over the bottom and along the walls between which it moves, polishing, grooving, and scratching them as it passes onward. One who is familiar with the track of this mighty engine will recognize at once where the large boulders have hollowed out their deeper furrows, where small pebbles have drawn their finer marks, where the stones with angular edges have left their sharp scratches, where sand and gravel have rubbed and smoothed the rocky surface, and left it bright and polished as if it came from the hand of the marble-worker. These marks are not to be mistaken by any one who has carefully observed them; the scratches, furrows, grooves, are always rectilinear, trending in the direction in which the glacier is moving, and most distinct on that side of the surface-inequalities facing the direction of the moving mass, while the lee-side remains mostly untouched.

It may be asked, how it is known that the glacier carries this powerful apparatus on its sides and bottom, when they are hidden from sight. I answer, that we might determine the fact theoretically from certain known conditions respecting the conformation of the glacier; to which I shall allude presently; but we need not resort to this kind of evidence, since we have ocular demonstration of the truth. Here and there on the sides of the glacier it is possible to penetrate between the walls and the ice to a great depth, and even to follow such a gap to the very bottom of the valley, and everywhere do we find the surface of the ice fretted as I have described it, with stones of every size, from the pebble to the boulder, and also with sand and gravel of all sorts, from the coarsest grain to the finest, and these materials, more or less firmly set in the ice, form the grating surface with which, in its onward movement down the Alpine valleys, it leaves everywhere unmistakable, traces of its passage.

We come now to the moraines, those walls of loose materials built by the glaciers themselves along their road. They have been divided into three classes, namely, lateral, medial, and terminal moraines. Let us look first at the lateral ones; and to understand them we must examine the conformation of the glacier below the névé, where it assumes the character of pure compact ice. We have seen that the fields of snow, where the glaciers have their origin, are level, and that lower down, where these masses of snow begin to descend toward the narrower valley, they follow its trough-like shape, sinking toward the centre and sloping upward against the sides, so that the surface of the glacier, about the region of the névé, is slightly concave. But lower down in the glacier proper, where it is completely transformed into ice, its surface becomes convex, for the following reason: The rocky walls of the valley, as they approach the plain, partake of its higher temperature. They become heated by the sun during the day in summer, so that the margins of the glacier melt rapidly in contact with them. In consequence of this, there is always in the lower part of the glacier a broad depression between the ice and the rocky walls, while, as this effect is not felt in the centre of the glacier, it there retains a higher level. The natural result of this is a convex surface, arching upward toward the middle, sinking toward the sides. It is in these broad, marginal depressions that the lateral moraines accumulate; masses of rock, stones, pebbles, dust, all the fragments, in short, which become loosened from the rocky walls above, fall into them, and it is a part of the materials so accumulated which gradually work their way downward between the ice and the walls, till the whole side of the glacier becomes studded with them. It is evident, that, when the glacier runs in a northerly or southerly direction, both the walls will be affected by the sun, one in the morning, the other in the afternoon, and in such a case the sides will be uniform, or nearly so. But when the trend of the valley is from east to west, or from west to east, the northern side only will feel the full force of the sun; and in such a case, only one side of the glacier will be convex in outline, while the other will remain nearly on a level with the middle. The large masses of loose materials which accumulate between the glacier and its rocky walls and upon its margins form the lateral moraines. These move most slowly, as the marginal portions of the glacier advance at a much slower rate than its centre.

The medial moraines arise in a different way, though they are directly connected with the lateral moraines. It often happens that two smaller glaciers unite, running into each other to form a larger one. Suppose two glaciers to be moving along two adjoining valleys, converging toward each other, and running in an easterly or westerly direction; at a certain point these two valleys open into a single valley, and here, of course, the two glaciers must meet, like two rivers rushing into a common bed. But as glaciers consist of a solid, and not a fluid, there will be no indiscriminate mingling of the two, and they will hold their course side by side. This being the case, the lateral moraine on the southern side of the northernmost glacier and that on the northern side of the southernmost one must meet in the centre of the combined glaciers. Such are the so-called medial moraines formed by the junction of two lateral ones. Sometimes a glacier may have a great number of tributaries, and in that case we may see several such moraines running in straight lines along its surface, all of which are called medial moraines in consequence of their origin midway between two combining glaciers. The glacier of the Aar represented in the wood-cut below affords a striking example of a large medial moraine. It is formed by the junction of the glaciers of the Lauter-Aar, on the right-hand side of the wood-cut, and the Finster-Aar, on the left; and the union of their inner lateral moraines, in the centre of the diagram, forms the stony wall down the centre of the larger glacier, called its medial moraine. This moraine at some points is not less than sixty feet high. We have here an effect similar to that of the glacier-tables and the sand-pyramids. The wall protects the ice beneath it, and prevents it from sinking at the same rate as the surrounding surface, while its heated surface increases the melting of the adjacent surfaces of ice, thus forming longitudinal depressions along the medial moraines, in which the largest rivulets and the most conspicuous sand-pyramids, the deepest wells and the finest waterfalls, are usually met with. As the medial moraines rest upon that part of the glacier which moves fastest, they of course advance much more rapidly than the lateral moraines.