Internal Structure and Progression of the Glacier
IT is not my intention, in these articles, to discuss a general theory of the glaciers upon physical and mechanical principles. My special studies, always limited to Natural History, have but indifferently fitted me for such a task, and quite recently the subject has been admirably treated from this point of view by Dr. Tyndall, in his charming volume entitled “ Glaciers of the Alps.” I have worked upon the glaciers as an amateur, devoting my summer vacations, with friends desirous of sharing my leisure, to excursions in the Alps, for the sake of relaxation from the closer application of my professional studies, and have considered them especially in their connection with geological phenomena, with a view of obtaining, by means of a thorough acquaintance with glaciers as they exist now, some insight into the glacial phenomena of past times, the distribution of drift, the transportation of boulders, etc. It was, however, impossible to treat one series of facts without some reference to the other; but such explanations as I have given of the mechanism of the glacier, in connection with its structure, are presented in the language of the unprofessional observer, without any attempt at the technicalities of the physicist. I do not wonder, therefore, that those who have looked upon the glacier chiefly with reference to the physical and mechanical principles involved in its structure and movement should have found my Natural Philosophy defective. I am satisfied with their agreement as to my correct observation of the facts, and am the less inclined to quarrel with the doubts thrown on my theory since I see that the most eminent physicists of the day do not differ from me more sharply than they do from each other. The facts will eventually test all our theories, and they form, after all, the only impartial jury to which we can appeal. In the mean while, I am not sorry that just at this moment, when recent investigations and publications have aroused new interest in the glaciers, the course of these articles brings me naturally to a discussion of the subject in its bearing upon geological questions. I shall, however, address myself especially, as I have done throughout these papers, to my unprofessional readers, who, while they admire the glaciers, may also wish to form a general idea of their structure and mode of action, as well as to know something of the important part they have played in the later geological history of our earth. It would, indeed, be out of place, were I to undertake here a discussion of the different views entertained by the various students who have investigated the glacier itself, among whom Dr. Tyndall is especially distinguished, or those of the more theoretical writers, among whom Mr. Hopkins occupies a prominent position.
Removed, as I am, from all possibility of renewing my ow n observations, begun in 1836 and ended in 1845, I will take this opportunity to call the attention of those particularly interested in the matter to one essential point with reference to which all other observers differ from me. I mean the stratification of the glacier, which I do not believe to be rightly understood, even at this moment. It may seem presumptuous to dissent absolutely from the statements of one who has seen so much and so well as Dr. Tyndall, on a question for the solution of which, from the physicist’s point of view, his special studies have been a far better preparation than mine; and yet I feel confident that I was correct in describing the stratification of the glacier as a fundamental feature of its structure, and the so-called dirt-bands as the margins of the snow-strata successively deposited, and in no way originating in the ice-cascades. I shall endeavor to make this plain to my readers in the course of the present article. I believe, also, that renewed observations will satisfy dissenting observers that there really exists a net-work of capillary fissures extending throughout the whole glacier, constantly closing and reopening, and constituting the channels by means of which water filtrates into its mass. This infiltration, also, has been denied, in consequence of the failure of some experiments in which an attempt was made to introduce colored fluids into the glacier. To this I can only answer, that I succeeded completely, myself, in the self-same experiments which a later investigator found impracticable, and that I see no reason why the failure of the latter attempt should cast a doubt upon the former. The explanation of the difference in the result may, perhaps, be found in the fact, that, as a sponge gorged with water can admit no more fluid than it already contains, so the glacier, under certain circumstances, and especially at noonday in summer, may be so soaked with water that all attempts to pour colored fluids into it would necessarily fail. I have stated, in my work upon glaciers, that my infiltration - experiments were chiefly made at night; and I chose that time, because I knew the glacier would most readily admit an additional supply of liquid from without when the water formed during the day at its surface and rushing over it in myriad rills had ceased to flow.
While we admit a number of causes as affecting the motion of a glacier,— namely, the natural tendency of heavy bodies to slide down a sloping surface, the pressure to which the mass is subjected forcing it onward, the infiltration of moisture, its freezing and consequent expansion,—we must also remember that these various causes, by which the accumulated masses of snow and ice are brought down from higher to lower levels, are not all acting at all times with the same intensity, nor is their action always the same at every point of the moving mass. While the bulk of snow and ice moves from higher to lower levels, the whole mass of the snow, in consequence of its own downward tendency, is also under a strong vertical pressure, arising from its own incumbent weight, and that pressure is, of course, greater at its bottom than at its centre or surface. It is therefore plain, that, inasmuch as the snow can be compressed by its own weight, it will be more compact at the bottom of such an accumulation than at its surface, this cause acting most powerfully at the upper part of a glacier, where the snow has not yet been transformed into a more solid icy mass. To these two agencies, the downward tendency and the vertical pressure, must be added the pressure from behind, which is most effective where the mass is largest and the amount of motion in a given time greatest. In the glacier, the mass is, of course, largest in the centre, where the trough which holds it is deepest, and least on the margins, where the trough slopes upward and becomes more shallow. Consequently, the middle of a glacier always advances more rapidly than the sides.
Were the slope of the ground over which it passes, combined with the pressure to which the mass is subjected, the whole secret of the onward progress of a glacier, it is evident that the rate of advance would be gradually accelerated, reaching its maximum at its lower extremity, and losing its impetus by degrees on the higher levels nearer the point where the descent begins. This, however, is not the case. The glacier of the Aar, for instance, is about ten miles in length ; its rate of annual motion is greatest near the point of junction of the two great branches by which it is formed, diminishing farther down, and reaching a minimum at its lower extremity. But in the upper regions, near their origin, the progress of these branches is again gradually less.
Let us see whether the next cause of displacement, the infiltration of moisture, may not in some measure explain this retardation, at least of the lower part of the glacier. This agency, like that of the compression of the snow by its own weight and the pressure from behind, is most effective where the accumulation is largest. In the centre, where the body of the mass is greatest, it will imbibe the most moisture. But here a modifying influence comes in, not sufficiently considered by the investigators of glacial structure. We have already seen that snow and ice at different degrees of compactness are not equally permeable to moisture. Above the line at which the annual winter snow melts, there is, of course, little moisture; but below that point, as soon as the temperature rises in summer sufficiently to melt the surface, the water easily penetrates the mass, passing through it more readily where the snow is lightest and least compact, — in short, where it has not begun its transformation into ice. A summer's day sends countless rills of water trickling through such a mass of snow. If the snow be loose and porous throughout, the water will pass through its whole thickness, accumulating at the bottom, so that the lower portion of the mass will be damper, more completely soaked with water, than the upper part; if, on the contrary, in consequence of the process previously described, alternate melting and freezing combined with pressure, the mass has assumed the character of icy snow, it does not admit moisture so readily, and still farther down, where the snow is actually transformed into pure compact ice, the amount of surfacewater admitted into its structure will, of course, be greatly diminished. There may, however, be conditions under which even the looser snow is comparatively impervious to water; as, for instance, when rain falls upon a snow-field which has been long under a low temperature, and an ice-crust is formed upon its surface, preventing the water from penetrating below. Admitting, as I believe we must, that the water thus introduced into the snow and ice is one of the most powerful agents to which its motion is due,
we must suppose that it has a twofold influence, since its action when fluid and when frozen would be different. When fluid, it would contribute to the advance of the mass in proportion to its quantity; but when frozen, its expansion would produce a displacement corresponding to the greater volume of ice as compared with water; add to this that while trickling through the mass it will loosen and displace the particles of already consolidated ice. I have already said that I did not intend to trespass on the ground of the physicist, and I will not enter here upon any discussion as to the probable action of the laws of hydrostatic pressure and dilatation in this connection. I will only state, that, so far as my own observation goes, the movement of the glacier is most rapid where the greatest amount of moisture is introduced into the mass, and that I believe there must be a direct relation between these two facts. If I am right in this, then the motion, so far as it is connected with infiltrated moisture or with the dilatation caused by the freezing of that moisture, will, of course, be most rapid where the glacier is most easily penetrated by water, namely, in the region of the neve and iu the upper portion of the glaciertroughs, where the neve begins to be transformed into more or less porous ice. This cause also accounts, in part at least, for another singular fact in the motion of the glacier: that, in its higher levels, where its character is more porous and the water entering at the surface sinks readily to the bottom, there the bottom seems to move more rapidly than the superficial parts of the mass, whereas at the lower end of the glacier, in the region of the compact ice, where the infiltration of the water at the bottom is at its minimum, while the disintegrating influences at the surface admit of infiltration to a certain limited depth, there the motion is greater near the surface than toward the bottom. But, under all circumstances, it is plain that the various causes producing motion, gravitation, pressure, infiltration of water, frost, will combine to propel the mass at a greater rate along its axis than near its margins. For details concerning the facts of the case, I would refer to my work entitled “ Système Glaciaire.”
We will next consider the stratification of the glacier. I have stated in my introductory remarks, that I consider this to be one of its primary and fundamental features, and I confess, that, after a careful examination of the results obtained by my successors in the field of glacial phenomena, I still believe that the original stratification of the mass of snow from which the glacier arises gives us the key to many facts of its internal structure. The ultimate features resulting from this connection are so exceedingly intricate and entangled that their relation is not easily explained. Nevertheless, I trust my readers will follow me in this Alpine excursion, where I shall try to smooth the asperities of the road for them as much as possible.
Imparted to it, at the very beginning of its formation, by the manner in which snow accumulates, and retained through all its transformations, the stratification of a glacier, however distorted, and at times almost obliterated, remains, notwithstanding, as distinct to one who is acquainted with all its phases, as is the stratified character of metamorphic rocks to the skilful geologist, even though they may be readily mistaken for plutonic masses by the common observer. Indeed, even those secondary features, as the dirt-bands, for instance, which we shall see to be intimately connected with snow-strata, and which eventually become so prominent as to be mistaken for the cause of the lines of stratification, do nevertheless tend, when properly understood, to make the evidence of stratification more permanent, and to point out its primitive lines.
On the plains, in our latitude, we rarely have the accumulated layers of several successive snow - storms preserved one above another. We can, therefore, hardly imagine with what distinctness the sequence of such beds is marked in the upper Alpine regions. The first cause of this distinction between the layers is the quality of the snow when it falls, then the immediate changes it undergoes after its deposit, then the falling of mist or rain upon it, and lastly and most efficient of all, the accumulation of dust upon its surface. One who has not felt the violence of a storm in the high mountains, and seen the clouds of dust and sand carried along with the gusts of wind passing over a mountain-ridge and sweeping through the valley beyond, can hardly conceive that not only the superficial aspect of a glacier, but its internal structure also, can be materially affected by such a cause. Not only are dust and sand thus transported in large quantities to the higher mountain-regions, but leaves are frequently found strewn upon the upper glacier, and even pine-cones, and maple-seeds flying upward on their spread wings, are scattered thousands of feet above and many miles beyond the forests where they grew.
This accumulation of sand and dust goes on all the year round, but the amount accumulated over one and the same surface is greatest during the summer, when the largest expanse of rocky wall is bare of snow and its loose soil dried by the heat so as to be easily dislodged. This summer deposit of loose inorganic materials, light enough to be transported by the wind, forms the main line of division between the snow of one year and the next, though only that of the last, year is visible for its whole extent. Those of the preceding years, as we shall see hereafter, exhibit only their edges cropping out lower down one beyond another, being brought successively to lower levels by the onward motion of the glacier.
Other observers of the glacier, Professor Forbes and Dr. Tyndall, have noticed only the edges of these seams, and called them dirt-bands. Looking upon them as merely superficial phenomena, they have given explanations of their appearance which I hold to be quite untenable. Indeed, to consider these successive lines of dirt on the glacier as limited only to its surface, and to explain them from that point of view, is much as if a geologist were to consider the lines presented by the strata on a cut through a sedimentary mass of rock as representing their whole extent, and to explain them as a superficial deposit due to external causes.
A few more details may help to make this statement clearer to my readers. Let us imagine that a fresh layer of snow has fallen in these mountain-regions, and that a deposit of dirt has been scattered over its surface, which, if any moisture arises from the melting of the snow or from the falling of rain or mist, will become more closely compacted with it. The next snow-storm deposits a fresh bed of snow, separated from the one below it by the sheet of dust just described, and this bed may, in its turn, receive a like deposit. For greater ease and simplicity of explanation, I speak here as if each successive snow-layer were thus indicated; of course this is not literally true, because snow-storms in the winter may follow each other so fast that there is no time for such a collection of foreign materials upon each newly formed surface. But whenever such a fresh snow-bed, or accumulation of beds, remains with its surface exposed for some time, such a deposit of dirt will inevitably be found upon it. This process may go on till we have a number of successive snow-layers divided from each other by thin sheets of dust. Of course, such seams, marking the stratification of snow, are as permanent and indelible as the seams of coarser materials alternating with the finest mud in a sedimentary rock.
The gradual progress of a glacier, which, though more rapid in summer than in winter, is never intermitted, must, of course, change the relation of these beds to each other. Their lower edge is annually cut off at a certain level, because the snow deposited every winter melts with the coming summer, up to a certain line, determined by the local climate of the place. But although the snow does not melt above this line, we have seen, in the preceding article, that it is prevented from accumulating indefinitely in the higher regions by its own tendency to move down to the lower valleys, and crowding itself between their walls, thus to force its way toward the outlet below. Now, as this movement is very gradual, it is evident that there must be a perceptible difference in the progress of the successive layers, the lower and older ones getting the advance of the upper and more recent ones : that is, when the snow that has covered the face of the country during one winter melts away from the glacier up to the so-called snow-line, there will be seen cropping out below and beyond that line the layers of the preceding years, which are already partially transformed into ice, and have become a part of the frozen mass of the glacier with which they are moving onward and downward. In the autumn, when the dust of a whole season has been accumulated upon the surface of the preceding winter’s snow, the extent of the layer which year after year will henceforth crop out lower down, as a dirt-band, may best be appreciated.
Beside the snow-layers and the sheets of dust alternating with them, there is still another feature of the horizontal and parallel structure of the mass in immediate connection with those above considered. I allude to the layers of pure compact ice occurring at different intervals between the snow-layers. In July, when the snow of the preceding winter melts up to the line of perpetual snow, the masses above, which are to withstand the summer heat and become part of the glacier forever, or at least until they melt away at the lower end, begin to undergo the changes through which all snow passes before it acquires the character of glacial ice. It thaws at the surface, is rained upon, or condenses moisture, thus becoming gradually soaked, and after assuming the granular character of neve - ice, it ends in being transformed into pure compact ice. Toward the end of August, or early in September, when the nights are already very cold in the Alps, but prior to the first permanent autumnal snow-falls, the surface of these masses becomes frozen to a greater or less depth, varying, of course, according to temperature. These layers of ice become numerous and are parallel to each other, like the layers of ice formed from slosh. Such crusts of ice I have myself observed again and again upon the glacier. This stratified snowy ice is now the bottom on which the first autumnal snow-falls accumulate. These sheets of ice may be formed not only annually before the winter snows set in, but may recur at intervals whenever water accumulating upon an extensive snow-surface, either in consequence of melting or of rain, is frozen under a sharp frost before another deposit of snow takes place. Or suppose a fresh layer of light porous snow to have accumulated above one the surface of which has already been slightly glazed with frost; rain or dew, falling upon the upper one, will easily penetrate it; but when it reaches the lower one, it will be stopped by the film of ice already formed, and under a Sufficiently low temperature, it will be frozen between the two. This result may be frequently noticed in winter, on the plains, where sudden changes of temperature take place.
There is still a third cause, to which the same result may possibly be due, and to which I shall refer at greater length hereafter; but as it has not, like the preceding ones, been the subject of direct observation, it must be considered as hypothetical. The admirable experiments of Dr. Tyndall have shown that water may be generated in ice by pressure, and it is therefore possible that at a lower depth in the glacier, where the incumbent weight of the mass above is sufficient to produce water, the water thus accumulated may be frozen into ice-layers. But this depends so much upon the internal temperature of the glacier, about which we know little beyond a comparatively superficial depth, that it cannot at present afford a sound basis even for conjecture.
There are, then, in the upper snowfields three kinds of horizontal deposits: the beds of snow, the sheets of dust, and the layers of ice, alternating with each other. If, now, there were no modifying circumstances to change the outline and surface of the glacier, — if it moved on uninterruptedly through an open valley, the lower layers, forming the mass, getting by degrees the advance of the upper ones, our problem would be simple enough. We should then have a longitudinal mass of snow, inclosed between rocky walls, its surface crossed by straight transverse lines marking the annual additions to the glacier, as in the adjoining figure.
But that mass of snow, before it reaches the outlet of the valley, is to be compressed, contorted, folded, rent in a thousand directions. The beds of snow, which in the upper ranges of the mountain were spread out over broad, open surfaces, are to be crowded into comparatively circumscribed valleys, to force and press themselves through narrow passes, alternately melting and freezing, till they pass from the condition of snow into that of ice, to undergo, in short, constant transformations, by which the primitive stratification will be extensively modified. In the first place, the more rapid motion of the centre of the glacier, as compared with the margins, will draw the lines of stratification downward toward the middle faster than at the sides. Accurate measurements have shown that the axis of a glacier may move tenor twentyfold more rapidly than its margins. This is not the place to introduce a detailed account of the experiments made to ascertain this result; but. I would refer those who are interested in the matter to the measurements given in my “ Systeme Glaciaire,” where it will be seen that the middle may move at a rate of two hundred feet a year, while the margins may not advance more than ten or fifteen or twenty feet. These observations of mine have the advantage over those of other observers, that, while they embrace the whole extent of the glacier, transversely as well as in its length, they cover a period of several successive years, instead of being limited to summer campaigns and a few winter observations. The consequence of this mode of progressing will be that the straight lines drawn transversely across the surface of the glacier above will be gradually changed to curved ones below. After a few years, such a line will appear on the surface of the glacier like a crescent, with the bow turned downward, within which, above, are other crescents, less and less sharply arched up to the last year's line, which may be again straight across the snowfield. (See the subjoined figure, which represents a part of the glacier of the Lauter-Aar.)
Thus the glacier records upon its surface its annual growth and progress, and registers also the inequality in the rate of advance between the axis and the sides.
But these are only surface-phenomena. Let us see what will be the effect upon the internal structure. We must not forget, in considering the changes taking place within glaciers, the shape of the valleys which contain them. A glacier lies in a deep trough, and the tendency of the mass will be to sink toward its deeper part, and to fold inward and downward, if subjected to a strong lateral pressure, — that is, to dip toward the centre and slope upward along the sides, following the scoop of the trough. If, now, we examine the face of a transverse cut in the glacier, we find it traversed by a number of lines, vertical in some places, more or less oblique in others, and frequently these lines are joined together at the lower ends, forming loops, some of which are close and vertical, while others are quite open. These lines are due to the folding of the strata in consequence of the lateral pressure they are subjected to, when crowded into the lower course of the valleys, and the difference in their dip is due to the greater or less force of that pressure. The wood-cut on the next page represents a transverse cut across the Lauter-Aar and the FinsterAar, the two principal tributaries to the great Aar glacier, and includes also a number of small lateral glaciers which join them. The beds on the left, which dip least, and are only folded gently downward, forming very open loops, are those of the Lauter-Aar, where the lateral pressure is comparatively slight. Those which are almost vertical belong in part to the several small tributary glaciers, which have been crowded together and very strongly compressed, and partly to the Finster-Aar. The close uniform vertical lines in this wood-cut represent a different feature in the structure of the glacier, called blue bands, to which I shall refer presently These loops or lines dipping into the internal mass of the glacier have been the subject of much discussion, and various theories have been recently proposed respecting them. I believe them to be caused, as I have said, by the snow-lavers, originally deposited horizontally, but afterward folded into a more or less vertical position, in consequence of the lateral pressure brought to bear upon them. The sheets of dust and of ice alternating with the enow-strata are of course subjected to the same action, and are contorted, bent, and folded by the same lateral pressure.
Dr. Tyndall has advanced the view that the lines of apparent stratification, and especially the dirt-bands across the surface of the glacier, are due to ice-cascades : that is, the glacier, passing over a sharp angle, is cracked across transversely in consequence of the tension, and these rents, where the back of the glacier has been successively broken, when recompacted, cause the transverse lines, the dirt being collected in the furrow formed between the successive ridges. Unfortunately for his theory, the lines of stratification constantly occur in glaciers where no such ice-falls are found. His principal observations upon this subject were made on the Glacier du Geant, where the ice-cascade is very remarkable. The lines may perhaps be rendered more distinct on the Glacier du Geant by the cascade, and necessarily must be so, if the rents coincide with the limit at which the annual snow-line is nearly straight across the glacier. In the region of the Aar glacier, however, where my own investigations were made, all the tributaries entering into the larger glacier are ribbed across in this way, and most of them join the main trunk over uniform slopes, without the slightest cascade.
It must be remembered that these surface-phenomena of the glacier are not to be seen at all times, nor under all conditions. During the first year of my sojourn on the glacier of the Aar, I was not aware that the stratification of its tributaries was so universal as I afterward found it to be ; the primitive lines of the strata are often so far erased that they are not perceptible, except under the most favorable circumstances. But when the glacier has been washed clean by rain, and the light strikes upon it in the right direction, these lines become perfectly distinct, where, under different conditions, they could not be discerned at all. After passing many summers on the same glacier, renewing my observations year after year over the same localities, I can confidently state that not only do the lines of stratification exist throughout the great glacier of the Aar, but in all its tributaries also. Of eourse, they are greatly modified in the lower part of the glacier by the intimate fusion of its tributaries, and by the circumstance that their movement, primarily independent, is merged in the movement of the main glacier embracing them all. We have seen that not only does the centre of a glacier move more rapidly than its sides, but that the deeper mass of the glacier also moves at a different rate from its more superficial portion. My own observations (for the details of which I would again refer the reader to my “ Système Glaciaire”) show that in the higher part of the glacier, especially in the region of the nává,the bottom of the mass seems to move more rapidly than the surface, while lower down, toward the terminus of the glacier, the surface, on the contrary, moves faster than the in which this difference in the motion of the upper and lower portions of the mass is represented, the beds being almost horizontal in the upper snow-fields, while their lower portion slopes more rapidly downward in the neve region, and toward the lower end the upper portion takes the lead, and advances more rapidly than the lower.
I presented these results for the first time in two letters, dated October 9th, 1842, which were published in a German periodical, the Jahrbuch of Leonhard and Bronn. The last three wood-cuts introduced above, the transverse and longitudinal sections of the glacier as well as that representing the concentric lines of stratification on the surface, are the identical ones contained in those communications. These papers seem to have been overlooked by contemporary investigators, and I may be permitted to translate here a passage from one of them, since it sums up the results of the inequality of motion throughout the glacier and its influence on the primitive stratification of the mass in as few words and as correctly as I could give them to-day, twenty years later: — “ Combining these views, it appears that the glacier may be represented as composed of concentric shells which arise from the parallel strata of the upper region by the following process. The primitively regular strata advance into gradually narrower and deeper valleys, in consequence of which the margins are raised, while the middle is bent not only downward, but, from its more rapid motion, forward also, bottom. The annexed wood-cut exhibits a longitudinal section of the glacier,
so that they assume a trough-like form in the interior of the mass. Lower down, the glacier is worn by the surrounding air, and assumes the peculiar form characteristic of its lower course.” The last clause alludes to another series of facts, which we shall examine, in a future article, when we shall see that the heat of the walls in the lower part of its course melts the sides of the glacier, so that, instead of following the trough-like shape of the valley, it becomes convex, arching upward in the centre and sinking at the margins.
I have dwelt thus long, and perhaps my readers may think tediously, upon this part of my subject, because the stratification of the glacier has been constantly questioned by the more recent investigators of glacial phenomena, and has indeed been set aside as an exploded theory. They consider the lines of stratification, the dirt-bands, and the seams of ice alternating with the more porous snow, as disconnected surface-phenomena, while I believe them all to be intimately connected together as primary essential features of the original mass.
There is another feature of glacial structure, intimately connected, by similarity of position and aspect, with the stratification, which has greatly perplexed the students of glacial phenomena. I allude to the so-called blue bands, or bands of infiltration, also designated as veined structure, ribboned or laminated structure, marginal structure, and longitudinal structure. The difficulty lies, 1 believe, in the fact that two very distinct structures, that of the stratification and the blue bands, are frequently blended together in certain parts of the glacier in such a manner as to seem identical, while elsewhere the one is prominent and the other subordinate, and vice versa. According to their various opportunities of investigation, observers have either confounded the two, believing them to be the same, or some have overlooked the one and insisted upon the other as the prevailing feature, while that very feature has been absolutely denied again by others who have seen its fellow only, and taken that to be the only prominent and important fact in this peculiar structural character of the ice.
We have already seen how the stratification of the glacier arises, accompanied by layers of dust and other material foreign to the glacier, and how blue bands of compact ice may be formed parallel to the surface of these strata. We have also seen how the horizontally of these strata may be modified by pressure till they assume a position within the mass of the glacier, varying from a slightly oblique inclination to a vertical one. Now, while the position of the strata becomes thus altered under pressure, other changes take place in the constitution of the ice itself.
Before attempting to explain how these changes take place, let us consider the facts themselves. The mass of the glacial ice is traversed by thin bands of compact blue ice, these bands being very numerous along the margins of the glacier, where they constitute what Dr. Tyndall calls marginal structure, and still more crowded along the line upon which two glaciers unite, where he has called it longitudinal structure. In the latter case, where the extreme pressure resulting from the junction of two glaciers has rendered the strata nearly vertical, these blue bands follow their trend so closely that it is difficult to distinguish one from the other. It will be seen, on referring to the wood-cut on page 758, where the close, uniform, vertical lines represent the true veined structure, that at several points of that section the lines of stratification run so nearly parallel with them, that, were the former not drawn more strongly, they could not be easily distinguished from the latter. Along the margins, also, in consequence of the retarded motion, the blue bands and the lines of stratification run nearly parallel with each other, both following the sides of the trough in which they move.
Undoubtedly, in both these instances, we have twfinds of blu bands, namely : those formed primitively in a horizontal position, indicating seams of stratification, and those which have arisen subsequently in connection with the movement of the whole mass, which I have occasionally called bands of infiltration, as they appeared to me to be formed by the infiltration and freezing of water. The fact that these blue bands are most numerous where two glaciers are crowded together into a common bed naturally suggests pressure as their cause. And since the beautiful experiments of Dr. Tyndall have illustrated the iwnal liquefaction of ice by pressure, it ...ecomes highly probable that his theory ’ the origin of these secondary blue bat s is the true one. He suggests that layers of water may be formed in the glacier at right angles with the pressure, and pass into a state of solid ice upon the removal of that pressure, the pressure being of course relieved in proportion to the diminution in the body of the ice by compression. The number of blue bands diminishes as we recede from the source of the pressure,—few only being formed, usually at right angles with the surfaces of stratification, in the middle of a glacier, half-way between its sides. If they are caused by pressure, this diminution of their number toward the middle of the glacier would be inevitable, since the intensity of the pressure naturally fades as we recede from the motive power.
Dr. Tyndall also alludes to another structure of the same kind, which he calls transverse structure, where the blue bands extend in crescent-shaped curves, more or less arched, across the surface of the glacier. Where these do not coincide with the stratification, they are probably formed by vertical pressure in connection with the unequal movement of the mass.
With these facts before us, it seems to me plain that the primitive blue bands arise with the stratification of the snow in the very first formation of the glacier, while the secondary blue bands are formed subsequently, in consequence of the onward progres of the glaand the pressure to which it is subjected. The secondary blue bands intersect the planes of stratification at every possible angle, and may therefore seem identical with the stratification in some places, while in others they cut it at right angles. It has been objected to my theory of glacial structure, that I have considered the socalled blue bands as a superficial feature when compared with the stratification. And in a certain sense this is true ; since, if my views are correct, the glacier exists and is in full life and activity before the secondary blue bands ari in it, whereas the stratification is a feare of its embryo condition, already established in the accumulated snow befor begins its transformation into glacmr-ice. In other words, the veined structure of the glacier is not a primary structural feature of its whole mass, but the result of various local influences acting upon the constitution of the ice: the marginal structure resulting from the resistance of the sides of the valley to the onward movement of the glacier, the longitudinal structure arising from the pressure caused by two glaciers uniting in one common bed, the transverse structure being produced by vertical pressure in consequence of the weight of the mass itself and the increased rate of motion at the centre.
In the nává fields, where the strata are still horizontal, the few blue bands observed are perpendicular to the strata of snow, and therefore also perpendicular to the blue seams of ice and the sheets of dust alternating with them. Upon the sides of the glacier they are more or less parallel to the slopes of the valley ; along the line of junction of two glaciers they follow the vertical trend of the axis of the mass; while at intermediate positions they are more or less oblique. Along the outcropping edges of the strata, on the surface of the glacier, they follow more or less the dip of the strata themselves ; that is to say, they are more or less parallel with the dirt-bands. In conclusion, I would recommend future investigators to examine the glaciers, with reference to the distribution of the blue bands, after heavy rains and during foggy days, when the surface is freed from the loose materials and decomposed fragments of ice resulting from the prolonged action of the sun.
The most important facts, then, to be considered with reference to the motion of the glacier are as follows. First, that the rate of advance between the axis and the margins of a glacier differs in the ratio of about ten to one and even less; that is to say, when the centre is advancing at a rate of two hundred and fifty feet a year, the motion toward the sides may be gradually diminished to two hundred, one hundred and fifty, one hundred, fifty feet, and so on, till nearest the margin it becomes almost inappreciable. Secondly, the rate of motion is not the same throughout the length of the glacier, the advance being greatest about half-way down in the region of the neve, and diminishing in rapidity both above and below; thus the onward motion in the higher portion of a glacier may not exceed twenty to fifty feet a year, while it reaches its maximum of some two hundred and fifty feet annually in the neve region, and is retarded again toward the lower extremity, where it is reduced to about onefourth of its maximum rate. Thirdly, the glacier moves at different rates throughout the thickness of its mass; toward the lower extremity of the glacier the bottom is retarded, and the surface portion moves faster, while in the upper region the bottom seems to advance more rapidly. 1 say seems, because upon this latter point there are no positive measurements, and it is only inferred from general appearances, while the former statement has been demonstrated by accurate experiments. Remembering the form of the troughs in which the glaciers arise, that they have their source in expansive, open fields of snow and nává, and that these immense accumulations move gradually down into ever narrowing channels, though at times widening again to contract anew, their surface wasting so little from external influences that they advance far below the line of perpetual snow without any sensible diminution in size, it is evident that an enormous pressure must have been brought to bear upon them before they could have been packed into the lower valleys through which they descend.
Physicists seem now to agree that pressure is the chief agency in the motion of glaciers. No doubt, all the facts point that way ; but it now becomes a matter of philosophical interest to determine in what direction it acts most powerfully, and upon this point glacialists are by no means agreed. Thc latest conclusion seems to be, that the weight of the advancing mass is itself the efficient cause of the motion. But while this is probably true in the main, other elements tending to the same result, and generally overlooked by investigators, ought to be taken into consideration ; and before leaving the subject, I would add a few words upon infiltration in this connection.
The weight of the glacier, as a whole, is about the same all the year round. If, therefore, pressure, resulting from that weight, be the all-controlling agency, its progress should be uniform during the whole year, or even greatest in winter, which is by no means the case. By a series of experiments, I have ascertained that the onward movement, whatever be its annual average,is accelerated in spring and early summer. The average annual advance of the glacier being, at a given point, at the rate of about two hundred feet, its average summer advance, at the same point, will be at a rate of two hundred and fifty feet, while its average rate of movement in winter will be about one hundred and fifty feet. This can be accounted for only by the increased pressure due to the large accession of water trickling in spring and early summer into the interior through the net-work of capillary fissures pervading the whole mass. The unusually large infiltration of water at that season is owing to the melting of the winter snow. Careful experiments made on the glacier of the Aar, respecting the water thus accumulating on the surface, penetrating its mass, and finally discharged in part at its lower extremity, fully confirm this view. Here, then, is a powerful cause of pressure and consequent motion, quite distinct from the permanent weight of the mass itself, since it operates only at certain seasons of the year. In midwinter, when the infiltration is reduced to a minimum, the motion is least. The water thus introduced into the glacier acts, as we have seen above, in various ways : by its weight, by loosening the particles of snow through which it trickles, and by freezing and consequent expansion, at least within the limits and during the season at which the temperature of the glacier sinks below 32° Fahrenheit. The simple fact, that in the spring the glacier swells on an average to about five feet more than its usual level, shows how important this infiltration must be. I can therefore only wonder that other glacialists have given so little weight to this fact. It is admitted by all, that the waste of a glacier at its surface, in consequence of evaporation and melting, amounts to about nine or ten feet in a year. At this rate of diminution, a glacier, even one thousand feet in thickness, could not advance during a single century without being exhausted. The water supplied by infiltration no doubt repairs the loss to a great degree. Indeed, the lower part of the glacier must be chiefly maintained from this source, since the annual increase from the fresh accumulations of snow is felt only above the snow-line, below which the yearly snow melts away and disappears. In a complete theory of the glaciers, the effect of so great an accession of plastic material cannot he overlooked.
I now come to some points in the structure of the glacier, the consideration of which is likely to have a decided influence in settling the conflicting views respecting their motion. The experiments of Faraday concerning regelation, and the application of the facts made known by the great English physicist to the theory of the glaciers, as first presented by Dr. Tyndall in his admirable work, show that fragments of ice with moist surfaces are readily reunited under pressure into a solid mass. It follows from these experiments, that glacier-ice, at a temperature of 32° Fahrenheit, may change its form and preserve its continuity during its motion, in virtue of the pressure to which it is subjected. The statement is, that, when two pieces of ice with moistened surfaces are placed in contact, they become cemented together by the freezing of a film of water between them, while, when the ice is below 32° Fahrenheit, and therefore dry, no effect of the kind can be produced. The freezing was also found to take place under water; and the result was the same, even when the water into which the ice was plunged was as hot as the hand can bear.
The fact that ice becomes cemented under these circumstances is fully established, and my own experiments have confirmed it to the fullest extent. I question, however, the statement, that regelation takes place by the freezing of a film of water between the fragments. I never have been able to detect any indication of the presence of such a film, and am, therefore, inclined to consider this result as akin to what takes place when fragments of moist clay or marl are pressed together and thus reunited. When examining beds of clay and marl, or even of compact limestone, especially in large mountain - masses, I have frequently observed that the rock presents a net-work of minute fissures pervading the whole, without producing a distinct solution of continuity, though generally determining the lines according to which it breaks under sudden shocks. The network of capillary fissures pervading the glacier may fairly be compared to these rents in hard rocks,—with this difference, however, that in ice they are more permeable to water than in stone.
How this net-work of capillary fissures is formed has not been ascertained by direct observation. Following, however, the transformation of the snow and neve' into compact ice, it is easily conceived that the porous mass of snow, as it falls in the upper regions of the Alps, and in the broad caldrons in which the glaciers properly originate, cannot pass into solid ice, by the process described in a former article, without retaining within itself larger or smaller quantities of air. This air is finally Surrounded from all sides by the cementation of the granules of neve, through the freezing of the water that penetrates it. So inclosed, the bubbles of air are subject to the same compression as the ice itself, and become more flattened in proportion as the snow has been more fully transformed into compact ice. As long as the transformation of snow into ice is not complete, a rise of its temperature to 32° Fahrenheit, accompanied with thawing, reduces it at once again to the condition of loose grains of neve'; but when more compact, it always presents the aspect of a mass composed of angular fragments, wedged and dovetailed together, and separated by capillary fissures, the flattened air-bubbles trending in the same direction in each fragment, but varying in their trend from one fragment to another. There is, moreover, this important point to notice, —that, the older the neve, the larger are its composing granules; and where neve passes into porous ice, small angular fragments are mixed with rounded návágranules, the angular fragments appearing larger and more numerous, and the nává-granules fewer, in proportion as the neve-ice has undergone most completely its transformation into compact glacierice. These facts show conclusively that the dimensions and form of the návágranules, the size and shape of the angular fragments, the porosity of the ice, the arrangement of its capillary fissures, and the distribution and compression of the air-bubbles it contains, are all connected features, mutually dependent. Whether the transformation of snow into ice be the result of pressure only, or, as I believe, quite as much the result of successive thawings and freezings, these structural features can equally be produced, and exhibit these relations to one another. It may be, moreover, that, when the glacier is at a temperature below 32°, its motion produces extensive fissuration throughout the mass.
Now that water pervades this net-work of fissures in the glacier to a depth not yet ascertained, my experiments upon the glacier of the Aar have abundantly proved; and that the fissures themselves exist at a depth of two hundred and fifty feet I also know, from actual observation. All this can, of course, take place, even if the internal temperature of the glacier never should fall below 32° Fahrenheit; and it has actually been assumed that the temperature within the glacier does not fall below this point, and that, therefore, no phenomena, dependent upon a greater degree of cold, can take place beyond a very superficial depth, to which the cold outside may be supposed to penetrate. I have, however, observed facts which seem to me irreconcilable with this assumption. In the first place, a thermometrograph Indicating —2° Centigrade, (about 28° Fahrenheit,) at a depth of a little over two metres, that is, about six feet and a half, has been recovered from the interior of the glacier of the Aar, while all my attempts to thaw out other instruments placed in the ice at a greater depth utterly failed, owing to the circumstance, that, after being left for some time in the glacier, they were invariably frozen up in newly formed water-ice, entirely different in its structure from the surrounding glacier-ice. This freezing could not have taken place, did the mass of the glacier never fall below 32° Fahrenheit. And this is not the only evidence of hard frost in the interior of the glaciers. The innumerable large walls of water-ice, which may be seen intersecting their mass in every direction and to any depth thus far reached, show that water freezes in their interior. It cannot be objected, that this is merely the result of pressure: since the thin fluid seams, exhibited under pressure in the interesting experiments of Dr. Tyndall, and described in his work under the head of Crystallization and Internal Liquefaction, cannot be compared to the large, irregular masses of water-ice found in the interior of the glacier, to which I here allude.
In the absence of direct thermometric observations, from which the lowest internal temperature of the glacier could be determined with precision in all its parts, we are certainly justified in assuming that every particle of water-ice found in the glacier, the formation of which cannot be ascribed to the mere fact of pressure, is due to the influence of a temperature inferior to 32° Fahrenheit at the time of its consolidation. The fact that the temperature in winter has been proved by actual experiment to fall as low is 28 Fahrenheit, that is, four degrees below the freezing-point, at a depth of six feet below a thick covering of snow, though not absolutely conclusive as to the temperature at a greater depth, is certainly very significant.
Under these circumstances, it is not out of place to consider through what channels the low temperature of the air surrounding the glacier may penetrate into the interior. The heavy cold air may of course sink from the surface into every large open space, such as the crevasses, large fissures, and moulins or milllike holes to be described in a future article ; it may also penetrate with the currents which ingulf themselves under the glacier, or it may enter through its terminal vault, or through the lateral openings between the walls of the valley and the ice. Indeed, if all the spaces in the mass of the glacier, not occupied by continuous ice, could be graphically represented, I believe it would be seen that cold air surrounds the glacier-ice itself in every direction, so that probably no masses of a greater thickness than that already known to be permeable to cold at the surface would escape this contact with the external temperature. If this be the case, it is evident that water may freeze in any part of the glacier.
To substantiate this position, which, if sustained, would prove that the dilatation of the mass of the glacier is an essential element of its motion, I may allude to several other well-known facts. The loose snow of the upper regions is gradually transformed into compact ice. The experiments of Dr. Tyndall prove that this may be the result, of pressure; but in the region of the neve it is evidently owing to the transformation of the snow-flakes into ice by repeated melting and freezing, for it takes place in the uppermost layers of the snow, where pressure can have no such effect, as well as in its deeper beds. I take it for granted, also, that no one, familiar with the presence of the numerous ice-seams parallel to the layers of snow in these upper regions of the glacier, can doubt that they, as well as the neve, are the result of frost. But be this as it may, the difference between the porous ice of the upper region of the glacier and the compact blue ice of its lower track seems to me evidence direct that at times the whole mass must assume the rigidity imparted to it by a temperature inferior to the freezing-point. We know that at 32° Fahrenheit, regelation renders the mass continuous, and that it becomes brittle only at a temperature below this. In other words, the ice can break up into a mass of disconnected fragments, such as the capillary fissures and the infiltration-experiments described in my “ Systeme Glaciaire,” show to exist, only when it is below 320 Fahrenheit. If it be contended that ice at 3 2° does break, and that therefore the whole mass of the glacier may break at that temperature, setting aside the contradiction to the facts of regelation which such an assumption involves, I would refer to Dr. Tyndall’s experiments concerning the vacuous spots in the ice.
Those who have read his startling investigations will remember that by sending a beam of sunlight through ice he brought to view the primitive crystalline forms to which it owes its solidity, and that he insisted that these star-shaped figures are always in the plane of crystallization. Without knowing what might be their origin, I had myself noticed these figures, and represented them in a diagram, part of which is reproduced in the annexed wood-cut. I had considered them to be compressed air-bubbles; and though I cannot, under my present circumstances, repeat the experiment of Dr. Tyndall upon glacier-ice, I conceive that the star - shaped figures represented upon PI. VII. figs. 8 and 9, in my “ Systeme Glaciaire,” may refer to the same phenomenon as that observed by him in pond - ice. Yet while I make this concession, I still maintain, that besides these crystalline figures there exist compressed air-bubbles in the angular fragments of the glacier-ice, as shown in the above wood-cut; and that these bubbles are grouped in sets, trending in the same direction in one and the same fragment, and diverging under various angles in the different fragments. I have explained this fact concerning the position of the compressed air-bubbles, by assuming that ice, under various pressure, may take the appearance it presents in each fragment with every compressed airbubble trending in the same direction, while their divergence in the different fragments is owing to a change in the respective position of the fragments resulting from the movement of the whole glacier. I have further assumed, that throughout the glacier the change of the snow and porous ice into compact ice is the result of successive freezing, alternating with melting, or at least with the resumption of a temperature of 32° Fahrenheit in consequence of the infiltration of liquid water, to which the effects of pressure must be added, the importance of which in this connection no one could have anticipated prior to the experiments of Dr. Tyndall. Of course, if the interior temperature of the glacier never falls below 32’, the changes here alluded to could not take place. But if the vacuous spaces observed by Dr. Tyndall are really identical with the spaces I have described as extremely flattened air-bubbles, I think the arrangement of these spaces as above described proves that it freezes in the interior of the glacier to the depth at which these crosswise fragments have been observed : that is, at a depth of two hundred feet. For, since the experiments of Dr. Tyndall show that the vacuous spaces are parallel to the surface of crystallization, and as no crystallization of water can take place unless the surrounding temperature fall below 32°, it follows that these vacuous spaces could not exist in such large continuous fragments, presenting throughout the fragments the same trend, if there had been no frost within the mass, affecting the whole of such a fragment while it remained in the same position.
The most striking evidence, in my opinion, that at times the whole mass of the glacier actually freezes, is drawn from the fact, already alluded to, that, while the surface of the glacier loses annually from nine to ten feet of its thickness by evaporation and melting, it swells, on the other hand, in the spring, to the amount of about five feet. Such a dilatation can hardly be the result of pressure and the packing of the snow and ice, since the difference in the bulk of the ice brought down, during one year, from a point above to that under observation, would not account for the swelling. It is more readily explained by the freezing of the water of infiltration during spring and early summer, when the infiltration is most copious and the winter cold has been accumulating for the longest time. This view of the case is sustained by Élie de Beaumont, who states his opinion upon this point as follows: —
“ Pendant I'hiver, la temperature de la surface du glacier s’abaisse a un grand nombre de degres au-dessous de zero, et cette basso temperature penetre, quoique avec un affaiblissement graduel, dans l’interieur de la masse. Le glacier se fendille par l’effet de la contraction resultant de ce refroidissement, Los fentes restent d’abord vides, et conconrent au refroidissement des glaciers en favorisant I'introduction de fair froid extárieur ; mais au printemps, lorsque les rayons du soleil áchauffent la surface de la neige qui couvre le glacier, ils la remenent d’abord it zero, et ils produisent ensuite de L'eau à zero qui tombe dans le glacier refroidi et fendill e. Cette eau s’y congàle l’instant, en laissant degager de la chaleur qui tend à ramener le glacier it zero ; et la phenomàne se continue jusqu’à ce que la masse entiere du glacier refroidi soit ramene à la temperature de zero.” 1 But where direct observations are still so scanty, and the interpretations of the facts so conflicting, it is the part of wisdom to be circumspect in forming opinions. This much, however, I believe to be already settled: that any theory which ascribes the very complicated phenomena of the glacier to one cause must be defective and one-sided. It seems to me most probable, that, while pressure has the larger share in producing the onward movement of the glacier, as well as in the transformation of the snow into ice, a careful analysis of all the facts will show that this pressure is owing partly to the weight of the mass itself, partly to the pushing on of the accumulated snow from behind, partly to its sliding along the surface upon which it rests, partly to the weight of water pervading the whole, partly to the softening of the rigid ice by the infiltration of water, and partly, also, to the dilatation of the mass, resulting from the freezing of this water. These causes, of course, modify the ice itself, while they contribute to the motion. Further investigations are required to ascertain in what proportion these different influences contribute to the general result, and at what time and under what circumstances they modify most directly the motion of the glacier.
That a glacier cannot be altogether compared to a river, although there is an unmistakable analogy between the flow of the one and the onward movement of the other, seems to me plain, — since the river, by the combination of its tributaries, goes on increasing in bulk in consequence of the incompressibility of water, while a glacier gradually thins out in consequence of the packing of its mass, however large and numerous may be its accessions. The analogy fails also in one important point, that of the acceleration of speed with the steepness of the slope. The motion of the glacier bears no such direct relation to the inclination of its bed. And though in a glacier, as in a river, the axis of swiftest motion is thrown alternately on one or the other side of the valley, according to its shape and slope, the very nature of ice makes it impossible that eddies should be formed in the glacier, and the impressive feature of whirlpools is altogether wanting in them. What have been called glacier-cascades bear only a remote resemblance to river-cascades, as in the former the surface only is thrown into confusion by breaking, without affecting the primitive structure;2 and I reiterate my formerly expressed opinion, that even the stratification of the upper regions is still recognizable at the lower end of the glacier of the Rhone.
The internal structure of the glacier has already led me beyond the limits I had proposed to myself in the present article. But I trust my readers will not be discouraged by this dry discussion of various theories concerning it, and will meet me again on the glacier, when we will examine together some of its more picturesque features, its crevasses, its rivulets and cascades, its moraines, its boulders, etc., and endeavor also to track its ancient course and boundaries in earlier geological times.
- “During the winter, the temperature at the surface of the glacier sinks a great many degrees below 32° Fahrenheit, and this low temperature penetrates, though at a gradually decreasing rate, into the interior of the mass. The glacier becomes fissured in consequence of the contraction resulting from this cooling process. The cracks remain open at first, and contribute to lower the temperature of the glacier by favoring the introduction of the cold air from without; but in the spring, when the rays of the sun raise the temperature of the snow covering the glacier, they first bring it back to ,320 Fahrenheit, and presently produce water at 32’, which falls into the chilled and fissured mass of the glacier. There this water is instantly frozen, releasing heat which tends to bring back the glacier to the temperature of 32’; and this process continues till the entire mass of the cooled glacier returns to the temperature of 320.”↩
- For the evidence of this statement I must, however, refer to my work on Glaciers, already so often quoted in this article, where it may be found with all the necessary details.↩