GLACIER,
A NAME given to masses of ice which descend from snowy mountains into the adjacent valleys, where they attain a level often far below the upper limit of the surrounding vegetation. The following are the synonyms for a glacier in some different languages and dialects. In French, glacier; German, gletscher; Italian, ghiacciaia; Tyrolean, fern; in Carinthia, hies; in the Valais, biegno; in part of Italy, redretto; in Piedmont, ruize; in the Pyrenees, serneille; in Norway, iisbræ or iisbrede; in Lapland, geikna or jegna; in Iceland, jökull or fall-jökull.
The characteristic appearance of a glacier can be nowhere better studied than in Switzerland and Savoy. The icy mass of the glacier of Bossons at Chamouni—which descends immediately from the highest part of Mont Blanc, but lies, summer and winter, in the valley at a height of no more than 3500 English feet (the height of perpetual snow being about 9000 feet), where it is embosomed amongst luxuriant wood, and is almost in contact with corn-fields—exhibits a spectacle which none who have once seen it can forget, and which attracts more interest and curiosity the more carefully it is considered. The lower glacier of Grindelwald, descending to 3400 feet, is another familiar example of the same phenomenon. In the Arctic regions true glaciers also exist, which, descending the valleys (often of great width and little inclination), enter the sea, and, breaking off, supply the floating ice-islands or icebergs which frequently drift into comparatively low latitudes. These glaciers do not essentially differ from those of alpine countries.
The diminution of temperature as we ascend the slopes
of mountains, as indicated by successive zones of vegetation, and finally by the occurrence of perpetual snow, is stated and explained in other articles (CLIMATE, PHYSICAL GEOGRAPHY, and BOTANY, Part III.), and therefore may be assumed here. Thus in the high mountains of the Andes and Himalaya, between the tropics, the commencement of perpetual snow is found at from 15,000 to 18,000, or even 19,000 feet, according to circumstances; whilst in southern Europe the level is from 8000 to 9000 feet, and in Norway from 5500 to 3000 feet, according to the latitude and the distance from the sea. It was first shown by Baron Humboldt and Von Buch that the limit of perpetual snow depends principally on the temperature of the summer, and not upon that of the whole year.
It has been already explained that an accumulation of snow, even frozen snow, does not constitute properly a glacier. A glacier is a mass of ice, having its origin in the hollows of mountains where perpetual snow accumulates, but which makes its way down towards the lower valleys, where it gradually melts, and it terminates exactly where the melting, due to the contact of the warmer air, earth, and rain of the valley—compensates for the bodily descent of the ice from the snow reservoirs of the higher mountains. From this it is evident, without any formal measurements, that A GLACIER IS ICE IN MOTION.
Geographical Distribution of Glaciers.—Glaciers are not peculiar to any country or region of the earth. It may be that there are extensive snowy mountains wholly devoid of them, as is supposed to be the case in tropical South America; but even this exception requires confirmation.
Glacier. There are peculiarities in the form of mountains, and still more in climate, which, as we shall see, favour the formation of glaciers, or may even totally prevent it.
Although it is only of late years that glaciers have been generally acknowledged to exist in the Himalaya, the descriptions given many years ago by Captain Hodgson of the source of the Ganges could leave no doubt as to the fact on the mind of any one familiar with the glaciers of the Alps. "The Bhagiruttee or Ganges," he writes in 1817, "issues from under a very low arch at the foot of the grand snow-bed. . . . Over the debouche the mass of snow is perfectly perpendicular, and from the bed of the stream to the summit, we estimate the thickness at little less than 300 feet of solid frozen snow, probably the accumulation of ages. It is in layers of some feet thick, each seemingly the remains of a fall of a separate year."1 The level of the source of the Ganges is 12,900 feet, and the chief error of this description is in the interchange of the word snow for ice, and in the absence of a clear perception that the ice could not have always lain there, some thousand feet below the snow-line, but must have travelled progressively down the valley, producing the phenomena of rents and superficial rubbish-heaps which Captain Hodgson describes in another paragraph.
For many years after 1817 the glaciers of the Himalaya, if mentioned at all, are so under the false name of snowbeds, and their relations to physical geography were wholly neglected. This arose from the imperfect education of those clever men who have at different times explored our Indian possessions, who being chiefly bred in that remote land had little acquaintance with the scientific literature of Europe, and still less with its physical features. Scarcely any of our Himalayan travellers had previously visited the Alps.
It is since 1840 that we have acquired more correct information as to the glaciers of India. Mr Vigne, in his interesting Travels in Kashmir, has described the perfectly characteristic features of the glaciers of some of the sources of the river Indus occurring in the territory of Little Thibet, about Lat. 35°. Colonel Madden and Captain Richard Strachey directed attention to the glaciers of the central Himalaya (Kumaon) at the source of the rivers Pindur and Kupinee, in Lat. 30° 20', at the level of 11,300 and 12,000 feet respectively, the height of the snow-line being there about 15,000 feet.2 The phenomena and mode of progression of these glaciers, as noted by Captain R. Strachey, appear identical with those which we shall presently describe as characteristic of those of Europe. Farther in the interior of the same chain Dr Thomas Thomson has lately described3 numerous glaciers filling valleys of the central Himalaya (particularly that on the north side of the Bardar or Umasi pass, Lat. 33° 20', Long 76½° E.), which probably exceeds in size any other yet described. Dr Joseph Hooker, in his interesting Himalayan Journals, has described in detail the glaciers of the eastern portion of the same range in the territories of Sikkim and Nepal, where the gigantic mountain Kinchinjunga rears its head to 28,178 feet above the sea, whence the ice descends (he states) in one unbroken mass of 14,000 feet of vertical height to the source of the Thlonok river. Both Dr Thomson and Dr Hooker concur in ascribing to the Himalayan glaciers a formerly much greater extension towards the plains of India, which has left geological evidence of their former sojourn in the lower valleys, in the masses of transported rock and rubbish there accumulated. Thus,
instead of glaciers being rare or unexampled phenomena in the east, as was at one time supposed, we find them developed on a scale commensurate with that of the stupendous mountains with which they are connected, and that from one end to the other of the Himalayan range.
Passing over the less important glaciers of the Caucasus and Altai, we come to the glaciers of Europe, which are principally confined to two great mountainous districts, the Alps and the high lands of Norway.
Referring for minute topographical details to the works which have been published more particularly in connection with the subject, and with those countries,4 we may state generally, that wherever (in Europe) any considerable area of mountainous country rises above the snow-line, there glaciers are found in more or less abundance. In the Alps this level is, on an average, about 7200 feet, including glaciers of all descriptions (Schlagintweit). The great glaciers have of course the lowest mean level. Of these there are, on the same authority, sixty in the whole Alpine chain. Glaciers commence on the south-western prolongation of the chain in the region of Mont Pelvoux and Monte Viso (Lat. 45°), and they extend on the N.E. to the Gross Glockner in Carinthia. The best known and most important glacier-bearing groups in the interval are those of Mont Blanc, Monte Rosa, the Bernese Alps (Finsteraarhorn and Jungfrau), and the Oertler Spitz in the Tyrol. The most considerable individual glaciers are the Mer de Glace of Chamouni, the Gorner glacier near Zermatt (Monte Rosa), the lower glacier of the Aar (Bernese Oberland), the Aletsch glacier and glacier of the Rhone (Vallais), and the Pasterzen glacier (Carinthia). Of these, the first, third, and last have been made the subjects of the most careful surveys and observations.
In Great Britain no mountain fully attains the height of the snow-line, consequently there are no glaciers. But patches of snow, with a more or less icy structure, remain through the summer in the clefts of some of the Scottish hills. Geological appearances, however, strongly indicate the formerly greater extension of glaciers, especially in Scotland and Wales.
In Norway we find two principal groups of glacier-bearing mountains—those in the Bergenstift and those within the arctic circle. The former were well described by M. Durocher.5 Professor J. D. Forbes, in a recent work,6 has detailed his observations on most of them, has given an enumeration of all the known glaciers of Norway, and has compared their conditions and structure with those of the glaciers of the Alps. Of the Bergen group, those of Justedal are the best known, and probably the best worth visiting. Justedal is connected with the inmost ramification of the intricate Sognefjord. On the Fjærlandsfjord, another branch of the same inlet, two important glaciers are found, one of which terminates only 105 feet above the sea level, in Lat. 61°. The Hardanger fjord, somewhat farther south presents one fine glacier, the Bondhusbrae. The more northern group of Norwegian glaciers commences at Fjordal, just within the limits of the arctic circle, where numerous glaciers descend almost to the sea level. About Lat. 70°, on the promontory of Lyngen, are several glaciers, and in the neighbouring Jokulsfjord is one which is stated actually to enter the sea, and to break off in miniature icebergs. About the North Cape the mountains are not sufficiently high to afford any perpetual snow.
Iceland, with a summer temperature far inferior to that
1 Asiatic Researches, vol. xiv., quoted in Captain R. Strachey's paper cited below.
2 Journal of the Asiatic Society of Bengal, vol. xvi., part 2.
4 Gruner's and De Saussure's Travels in the Alps; Prof. J. D. Forbes's Travels in the Alps of Savoy; Agassiz, Etudes sur les Glaciers; Hugl, Alpenreise; Schlagintweit, Untersuchungen; and Johnston's Physical Atlas.
5 Annales des Mines, 4th series, tom. xii.
6 Norway and its Glaciers, visited in 1851.
Glacier. of Norway, abounds in glaciers, which, however, have not been very particularly described. Those of Swina-fels and Holar are stated to be large and characteristic.
The glaciers of Spitzbergen have been minutely described by Dr Scoresby1 and by M. Martins.2 They appear to be essentially of the same nature, and subject to the same laws as those of Switzerland, but modified by the depression of the snow-line and the extreme shortness of the summer. The texture is less icy, the rate of progression probably slower; as the superficial fusion is not great, they descend in vast sheets into the waters of the sea (as at Magdalena Bay), where they form icebergs. The western coasts of Greenland appear to offer the same phenomena, but on a grander scale.
The interior of arctic North America has too even a surface, and perhaps too dry and rigorous a climate, to present glaciers in perfection.
In South America, about Lat. 47°, where the climate is one of the worst in the world, numerous glaciers, resembling probably those of Iceland, have been described by Captain King and Mr Darwin. Sir James Clark Ross, in his antarctic voyage, has described and represented by admirable views the stupendous icy barriers which fringe the coast of the inhospitable southern continent.
After reviewing the descriptions of glaciers in all regions of the world, we recur to those of the Alps as presenting all the characteristic features of glaciers in perfect development, and under circumstances the most convenient for study.
General Phenomena of Alpine Glaciers.—The manner in which a glacier protrudes itself into a valley, far below the level of perpetual snow, has been already mentioned. The inference being obvious, that since it is continually melting (during summer) in all its parts, yet retains its general form and place, the waste below must be supplied by the continual advance of the glacier forwards and downwards, we shall consider in the meantime the motion of the glacier as an established fact to which we shall afterwards devote a separate and special discussion. It very frequently happens that the termination of the greater glaciers takes place in an alluvial flat in the bottom of a large alpine valley (as in the glaciers of the Mer de Glace, Brenva, Rhone, Lower Aar, and those of Grindelwald). From a vault in the green-blue ice, more or less perfectly formed each summer, the torrent issues, which represents the natural drainage of the valley, derived partly from land springs, partly from the fusion of the ice. That of the Arveiron, near Chamouni, is perhaps the best known, but almost every glacier possesses such a vault.
Most usually the glacier terminates amidst a wilderness of stones borne down upon its surface and deposited by its fusion. Sometimes these blocks are heaped up in mounds called moraines, which, when in front of the lower end of a glacier, are called its terminal moraines, and mark in a characteristic and certain manner the greatest limit of extension which the glacier has at any one time attained. Sometimes a glacier is seen to have withdrawn very far within its old limits, leaving a prodigious barren waste of stones in advance of it, which, being devoid of soil, nourishes not one blade of grass. At other times the glacier pushes forward its margin beyond the limit which it has ever reached (at least within the memory of man), tears up the ground with its icy ploughshare, and shoves forward the yielding turf in wrinkled folds, uprooting trees, moving vast rocks, and scattering the walls of dwelling houses in fragments before its irresistible onward march.3
The lower end of a glacier is usually steep; sometimes
Glacier. with a dome-shaped unbroken outline, more frequently broken up by intersecting cracks into prismatic masses which the continued action of the sun and rain sharpen into pyramids, often assuming (as in the glacier of Bossons at Chamouni) grotesque or beautiful forms.
The united or crevassed condition of the glacier generally depends almost entirely on the slope of its bed. If it incline rapidly, numerous transverse fissures are formed from the imperfect yielding of the ice during its forced descent along its uneven channel. These cracks often extend for hundreds of yards, and may be hundreds of feet in depth; but their greatest depth is not accurately known, since they are rarely quite vertical. In many cases, however, the crevasses are comparatively few in number, and the glacier may be readily traversed in all directions. This is especially the case if a glacier of considerable dimensions meets with any contraction in its course. The ice is embayed and compressed, and its slope lessens, just as in the case of a river when it nears a similar contraction preceding a fall. Such level and generally traversable spaces may be found about the middle regions of the Mer de Glace, the lower glacier of Grindelwald, the lower glacier of the Aar, and in many other cases. The last named glacier is perhaps the most remarkably even and accessible of any in Switzerland. The slope of its surface is in many places only 3°. The Pasterzen glacier in Carinthia is even less inclined. It is in such portions of a glacier that we commonly find internal cascades, or "moulins." These arise from the superficial water of a glacier being collected into a considerable mass by a long course over its unbroken surface, and then precipitated with violence into the first fissure it meets with. The descending cascade keeps open its channel, which finally loses the form of a fissure, presenting that of an open shaft, often of immense depth.
Nearly connected in their origin with the internal cascades are the gravel cones, occasionally seen on the surface of glaciers, which appear to be formed in this way:—a considerable amount of earthy matter derived by the superficial water-runs from the moraine, accumulates in heaps in the inequalities of the ice, or at the bottom of the "moulins." As the glacier surface wastes by the action of the sun and rain, these heaps are brought to the surface, or rather the general surface is depressed to their level. If the earthy mass be considerable the ice beneath is protected from the radiation of the sun and from the violent washing of the rain; it at length protrudes above the general level of the glacier, and finally forms a cone which appears to be entirely composed of gravel, but is in fact ice at the heart, with merely a protecting cover of earthy matter. These singular cones are very well seen on the glacier of the Aar, but on most others they are comparatively rare.
The similar protective action of large stones detached from the moraines and lying on the surface of the ice often produces the striking phenomenon of glacier tables. Stones of any considerable size almost invariably stand upon a slightly elevated pillar of ice; but when they are broad and flat they occasionally attain a height of 6 and even of 12 feet above the general level. A striking instance has been described and drawn by Professor Forbes, in his Travels in the Alps of Savoy.
To this peculiar tendency of glaciers apparently to elevate heavy and opaque bodies above their surface—in reality to have their surface depressed beneath them—is no doubt mainly owing to the striking, and at first sight perplexing, fact, that stones or dirt are scarcely ever seen imbedded in the massive ice of glaciers. The Swiss peasants
1 Arctic Regions, vol. i.
2 Bibliothèque Universelle, 1840.
3 Such a sudden and disastrous increase took place in many of the glaciers of Switzerland and Savoy in 1818 (occasioning the catastrophe of the Val de Bagnes), and in those of the Bergenstift in Norway about 1740. The retreat of a glacier far within its old moraines is well exemplified in most of the glaciers of the latter country, and especially in that of Nygaard.
Glacier. attribute to them an intrinsic power of rejecting impurities. The fact is, that year by year, and month by month, fresh thicknesses of virgin ice become revealed by the fusion of the surface. That ice, formed in the highest mountain hollows, never was or could be impure. The rocks and earth have fallen upon the surface since; and, by the conditions which we have mentioned, once there, there they remain. Even those blocks which fall into the crevasses are usually arrested at no great depth, and by the general lowering of the glacier-surface, soon attain its level.
The superficial waste of a glacier is thus a very important phenomenon. Owing to it the body of the ice has its vertical thickness rapidly diminished during the heats of summer, and, as we have already intimated, the lower end of a glacier has its position determined by the amount of this waste. Suppose a glacier to move along its bed at the rate of 300 feet per annum, and imagine (merely for the sake of illustration) its yearly superficial waste to be 20 feet: then the thickness of the glacier will diminish by 20 feet for every 300 feet of its length, or at the rate of 360 feet per mile; so that the longitudinal section of a glacier has the form of a wedge; and however enormous its original thickness, after a certain course we must at length come to the thin end of the wedge, and that the more rapidly as the causes of melting increase towards the lower extremity. These causes are indeed so various that it is difficult to estimate them with accuracy. We have (1) the direct solar heat; (2) the contact of warm air; (3) the washing of rain. All these causes act on the surface, and produce the ablation of the surface. Besides these, the ice of the glacier wastes somewhat beneath by the contact of the soil and the washing of the inferior streams. This may be called its subsidence. Further, the natural slope of the rocky bed of the glacier causes any point of the surface to stand absolutely lower each day in consequence of the progressive motion. These three causes united produce the geometrical depression of the surface. Professor Forbes has shown how the several effects may usually be distinguished by observation.1 During the height of summer, near the Montanvert, he found the daily average ablation to be 3.62 inches, the daily subsidence to be 1.63 inches. Seven-tenths of the geometrical depression are due therefore to the former cause, and three-tenths to the latter. This is a very large amount, and it is certain that during the colder period of the year, and whilst the glacier is covered with snow, the subsidence is not only suspended, but that the glacier recruits in thickness a portion of its waste during the season of summer and autumn. To this subject we shall again return.
One point about moraines we have not yet mentioned. As we ascend any considerable glacier we almost invariably observe several parallel trails of debris extending throughout its length, and not mixing with one another. These medial moraines may in all cases be traced to a rocky promontory where two tributary glaciers have united. The rocky masses detached by frost and rain which have rolled upon the margin of the confluent glaciers are borne along by the progress of each to the point of union. But where the icy streams unite the trails of rock do so also; and being continually retained on the surface by the causes we have mentioned, float, as it were, down the middle of the common glacier, preserving throughout the distinctive character of their origin. Four such medial moraines may readily be traced to their sources on the great glacier of Chamouni; but the grandest specimen of a medial moraine is that on the glacier of the Lower Aar, effectively represented in one of the plates in M. Agassiz' work (Études sur les Glaciers).
The middle region of the great glaciers of the Alps ex-
tends from the level of about 6000 to 8000 feet above the sea. The inclination is usually there most moderate—say from to . But this is not invariably the case. Beyond 8000 feet we reach the snow line. The snow line is a fact as definite on the surface of a glacier as on that of a mountain, only in the former case it occurs at a somewhat lower level. It cannot be too distinctly understood that the fresh snow annually disappears from the glacier proper. Where it ceases entirely to melt, it of course becomes incorporated with the glacier. We have therefore arrived at the region where the glacier forms; everywhere below it only wastes. This snowy region of the glacier is called in French névé; in German, firn. As we ascend the glacier it passes gradually from the state of ice to the state of snow. The superficial layers are more snowy and white, in fact nearly pure snow; the deeper ones have more colour and consistence, and break on the large scale into vast fragments, which at Chamouni are called seracs. The névé moves, as the glacier proper does, and it is fissured by the inequalities of the ground over which it passes. These fissures are less regular than those of the lower glacier. They are often much wider, in fact of stupendous dimensions, and being often covered with treacherous snowy roofs, constitute one of the chief dangers of glacier traveling. The constitution of the névé may be well studied on the Glacier du Géant, a tributary of the Mer de Glace.
The mountain-clefts in which large glaciers lie, usually expand in their higher portions (in conformity with the ordinary structure of valleys) into extensive basins in which snow is perpetual, and which therefore contain the névé, the true origin and material of the glacier, which is literally the overflow of these snowy reservoirs. The amount of overflow, or the discharge of the glacier—upon which depends the extent of its prolongation into the lower valleys—depends in its turn on the extent of the névé or collecting reservoir. Glaciers with small reservoirs, of necessity perish soon. Their thickness is small, and consequently the wedge of the glacier soon thins out. Such glaciers are common in confined clefts of the higher mountains. Being destitute of reservoirs, they soon terminate abruptly. Such are the glaciers of the second order described by De Saussure. They are exceedingly numerous in all glacier-bearing chains of mountains, but from their comparative smallness and inaccessibility, they usually attract but little attention. Their slope is often very great—from to .
Structure of Glacier Ice, and Dirt Bands.—The ice of the glacier proper has a very peculiar structure, quite distinct from the stratification of the snow on the névé (the relics of its mode of deposit), and one which requires special notice. When we examine the appearance of the ice in the wall of an ordinary crevasse (especially if it be tolerably near the side of the glacier) we are struck with the beautiful vertically laminated structure which commonly it presents, resembling delicately-veined marble, in shades varying from bluish-green, through green, to white. It sometimes resembles the marble called in Italy cioppino. When we trace the direction of the planes constituting the laminated structure, by observing them on the surface of the glacier (where they are usually well seen after rain, or in the channels of superficial water-runs), we find that where best developed (or not very far from the sides of the glacier) these laminae are nearly parallel to the sides, but rather incline from the shore to the centre of the ice stream as we follow the declivity of the glacier.
The general out-crop of the veined structure may best be seized at a glance by means of a correlative phenomenon thus described by Professor Forbes, who first observed it:—“On the evening of the 24th July (1842), the day follow-
1 Eleventh Letter on Glaciers. Ed. Phil. Journal, 1846. Observations of the ablation of the ice have also been made by MM. Martins and Agassiz. The amount, of course, depends materially on the elevation and exposure of the glacier as well as on the weather and season.
Glacier. ing my descent from the Col du Géant, I walked up the hill of Charmoz to a height of 600 or 700 feet above the Montanvert, or 1000 feet above the level of the glacier. The tints of sunset were cast in a glorious manner over the distant mountains, whilst the glacier was thrown into comparative shadow. This condition of half-illumination is far more proper for distinguishing feeble shades of colour on a very white surface like that of a glacier than the broad day. Accordingly, whilst revolving in my mind, during this evening's stroll, the singular problems of the ice world, my eye was caught by a very peculiar appearance of the surface of the ice, which I was certain that I now saw for the first time. It consisted in a series of nearly hyperbolic brownish bands on the glacier, the curves pointing downwards, and the two branches mingling indiscriminately with the (lateral) moraines, presenting an appearance of waves some hundred feet apart, and having opposite to the Montanvert the form which I have attempted to show upon the map,1 where they are represented in the exact figure and number in which they occur. . . . . I was satisfied, from the general knowledge which I then had of the 'veined structure' of the ice, that these coloured bands probably followed that direction.2 Farther examination confirmed this conjecture, and showed that these superficial discolorations in the form of excessively elongated hyperbolas are due to the recurrence (at intervals of some hundred feet along the course of the glacier) of portions of ice in which the veined structure is more energetically developed than elsewhere, and where, by the decomposition of the softer laminae, portions of sand and dirt become entangled in the superficial ice, and give rise to the phenomena of dirt bands, which thus at a distance display (though in a manner requiring some attention to discover) the exact course of this singular structure on the surface of the glacier. The annexed figure, No. 1, displays the superficial form of the dirt bands, and the course of the structural
marked by the snow lying in them during the period of its partial disappearance.3 Glacier.
The Motion of Glaciers, and its causes.—The most characteristic and remarkable feature of glaciers is their motion downwards from the née towards the lower valley. The explanation of it is by far the most important application of mechanical physics connected with the subject.
Obvious as the fact itself must appear by what has been already stated, manifest confusion has obtained in the minds of intelligent persons regarding it. Thus Ebel, in his well-known Swiss guide-book, affirms the motion of the glaciers of Chamouni to be 14 feet, and those of Grindelwald 25 feet in a year; quantities which, if they have any meaning, must refer to the apparent advance of the lower termination of those glaciers into the valley, which therefore only indicate the difference of the real motion, and of the waste in any particular season, and which may become null, or even negative, if the summer be more than usually warm. The peasants, however
—who are inevitably made aware of the progressive motion of the ice by observing the progressive advance of conspicuous blocks on its surface—commonly ascribe to the glaciers the more correct measure of several hundred feet per annum.
M. Hugi, of Soleure, measured, with some accuracy, year by year, the progress of a conspicuous block on the glacier of the Aar, which he found to be 2200 feet in nine years, or about 240 feet per annum.4 M. Agassiz continued some of these annual measures, but only in a rough way by causing his guides to reckon the distance of a block on the moraine by lengths of a pole or rod from a fixed rock some thousand feet off. These measures appear not to have been altogether trustworthy.
The principal theories to account for the progressive motion of glaciers which were prevalent previous to 1842, may be briefly characterized as De Saussure's and De Charpentier's, though each had been maintained in times long antecedent by the earlier Swiss writers. The first may for brevity be called the gravitation theory, the latter the dilatation theory. Both suppose that the motion of the ice takes place by its sliding bodily over its rocky bed, but they differ as to the force which urges it over the obstacles opposed by friction and the irregularities of the surface on which it moves.
laminae projected horizontally. No. 2 shows an ideal transverse section of the glacier; and No. 3 another vertical section parallel to its length. These three sections in rectangular planes will serve to give a correct idea of the course of this remarkable structure within the ice, but a more popular conception will be formed of it from the imaginary sections of a canal-shaped glacier in the annexed woodcut, No. 4. The structure of the compound glacier, originally double, becomes gradually single; and the frontal dip of the laminae at the loop of the horizontal curves which in the upper region of the glacier is nearly vertical, gradually slopes forwards until at the lower termination it has a very slight dip inwards, or indeed may be reversed and fall outwards and forwards. The general form of a structural lamina of a glacier rudely resembles that of a spoon.
This structure and the accompanying dirt bands have been recognized by different observers in almost all glaciers, including those of Norway and of India. The interval between the dirt bands has been shown in the case of the Mer de Glace (and therefore probably in other cases) to coincide with annual rate of progression, and in the higher parts of the glacier (towards the née) to be accompanied by wrinkles or inequalities of the surface which are well
1 Map of the Mer de Glace of Chamouni, &c., in Professor Forbes's Travels in the Alps.
2 Travels in the Alps of Savoy, &c., 2d edit., p. 162.
3 Fifth Letter on Glaciers. Edin. Phil. Journal, 1844; and Travels in the Alps, 2d edit.
4 Agassiz, Etudes sur les Glaciers, p. 150.
The following quotation from De Saussure explains his views with his usual precision:—"These frozen masses, carried along by the slope of the bed on which they rest, disengaged by the water (arising from their fusion owing to the natural heat of the earth) from the adhesion which they might otherwise contract to the bottom—sometimes even elevated by the water—must gradually slide and descend along the declivity of the valleys or mountain slopes (croupes) which they cover. It is this slow but continual sliding of the icy masses (des glaces) on their inclined bases which carries them down into the lower valleys, and which replenishes continually the stock of ice in valleys warm enough to produce large trees and rich harvests."1 Very sufficient objections have been urged against this theory. It is evident that De Saussure considered a glacier as an accumulation of icy fragments, instead of a great and continuous mass, throughout which the fissures and crevasses bear a small proportion to the solid portion; and that he has attributed to the subglacial water a kind and amount of action for which there exists no sufficient or even probable evidence. The main objection, however, is this, that a sliding motion of the kind supposed, if it commence, must be accelerated by gravity, and the glacier must slide from its bed in an avalanche. The small slope of most glacier-valleys, and the extreme irregularity of their bounding walls, are also great objections to this hypothesis.
The Dilatation theory ingeniously meets the difficulty of the want of a sufficient moving power to drag or shove a glacier over its bed, by calling in the well-known force with which water expands on its conversion into ice. The glacier being traversed by innumerable capillary fissures, and being in summer saturated with water in all its parts, it was natural to invoke the freezing action of the night to convert this water into ice, and by the amount of its expansion to urge the glacier onwards in the direction of its greatest slope. In answer to this it is sufficient to observe, in the first place, that during the height of summer the portions of those glaciers which move fastest are never reduced below the freezing point, and that even in the most favourable cases of nocturnal radiation producing congelation at the surface, it cannot (by well-known laws of conduction) penetrate above a few inches into the interior of the glacier. Again, the ascertained laws of glacier motion are (as will be immediately seen) entirely adverse to this theory, as it is always accelerated by hot weather and retarded by cold, yet does not cease even in the depths of winter.2
It is singular how slow observers were to perceive the importance to the solution of the problem of glacier motion of ascertaining with geometrical precision the amount of motion of the ice, not only from year to year, but from day to day, whether constant or variable at the same point, whether continuous or by starts; if variable, on what circumstances it depended, and in what manner it was affected at different points of the length and breadth of a glacier.
This method of studying the question was taken up by Professor Forbes of Edinburgh. His observations were commenced on the Mer de Glace of Chamouni, in June 1842. Between the 26th and 27th of that month the motion of the ice opposite a point called the "Angle" was found, by means of a theodolite, to be 16.5 inches in 26 hours; between the 27th and 28th, 17.4 inches in 25½ hours; and from about 6 A.M. to 6 P.M. on the 28th the motion was 9.5 inches, or 17.5 inches in 24 hours; whilst even the proportional motion during an hour and a half was observed. No doubt could therefore remain that
the motion of the ice is continuous and tolerably uniform—in short, that it does not move by jerks. He also ascertained about the same time that the motion of the ice is greatest towards the centre of a glacier and slower at the sides, contrary to an opinion then maintained on high authority. He next found that the rate of motion varied at different points of the length of the same glacier, being on the whole greatest where the inclination of its surface is greatest. As the season advanced, he observed notable changes in the rate of motion of the same part of the ice, and connected it by a very striking direct relation with the temperature of the air. These facts were established during the summer of 1842, and promptly published. By means of occasional observations during the following winter and spring by his guide, Auguste Balmat of Chamouni, and by a more full comparison of the entire motion of a glacier for 12 months with its motion during the hot season of the year, another generally received error was rectified: the motion of the glacier continues even in winter, and it has a very perceptible ratio to the summer motion. Last of all, it was found that the surface of a glacier moves faster than the ice nearer the bottom or bed.3
These and some minor laws of motion are undoubted expressions of the way in which glaciers move; and having been successively confirmed by succeeding observers, seem to admit of but one expression in the form of an approximate theory, and it is that given to them by Professor Forbes: "A glacier is an imperfect fluid or a viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts."4 The analogy subsisting between the motion of a glacier and that of a river (which is a viscous fluid;—were it not so, its motion would be widely different) will be best perceived by stating more precisely its laws of motion in order.
1. Each portion of a glacier moves, not indeed with a constant velocity, but in a continuous manner, and not by sudden subsidences with intervals of repose. This, of course, is characteristic also of a river.
2. The ice in the middle part of the glacier moves much faster than that near the sides or banks; also the surface moves faster than the bottom. Both these facts obtain in the motion of a river in consequence of the friction of the fluid on its banks, and in consequence also of that internal friction of the fluid which constitutes its viscosity.
Thus, at four stations of the Mer de Glace, distant respectively from the west shore of the glacier
100 230 305 365 yds.,
the relative velocities were... 1.000 1.332 1.356 1.367.
3. The variation of velocity (as in a river) is most rapid near the sides, whilst the middle parts move nearly uniformly. This and the preceding laws are also fully brought out by the subsequent experiments of M. Agassiz on the glacier of the Aar, and of MM. Schlagintweit on the Pasterzen glacier.5
4. The variation of velocity of a glacier from the sides to the middle varies nearly in proportion to the absolute velocity of the glacier; whether that absolute velocity change in the same place in consequence of change of season, or between one point and another of the length of the same glacier, depending on its declivity. (See (5.) and (6.) below.) These facts, clearly brought out in Professor Forbes's observations of 1842, present a striking analogy to the phenomena of rivers, as observed by Dubuat.6
5. The glacier, like a stream, has its pools and its rapids. Where it is embayed by rocks it accumulates, its declivity increases, and its velocity at the same time. When it passes
1 Voyages dans les Alpes, § 535.
2 The fullest exposition of the Dilatation Theory is to be found in De Charpentier, Essai sur les Glaciers (1841), and Agassiz, Etudes sur les Glaciers (1840).
3 Eleventh Letter. Edin. Phil. Journal, 1846.
4 Travels in the Alps, p. 365.
5 See Mousson, Gletscher der Jetzt-zeit, p. 126.
6 Traité d'Hydraulique, arts, 37, 49, 65.
Glacier. down a steep, issuing by a narrow outlet, its velocity increases. Thus the approximate declivities of the inferior, middle, and superior region of the Mer de Glace (taken in the direction of its length) are ..... and the relative velocities are as the { 1:398 :574 :925 } numbers.....
6. A fact not less important than any of the preceding, and equally well-established, not only by the observations of Professor Forbes, but by those of succeeding experimenters, is this: that increased temperature of the air favours the motion of the ice; and generally whatever tends to increase the proportion of the watery to the solid constituents of a glacier, as mild rains, and especially the thawing of the superficial snow in spring. The velocity does not, however, descend to nothing even in the depth of winter. Indeed, in the lower and most accessible portions of the Mer de Glace (or Glacier des Bois) and the Glacier des Bossons, the ratio of the winter to the summer motion is almost exactly 1:2. On endeavouring to establish a relation between the velocity of the glacier and the temperature of the ambient air, we find that these two quantities diminish together in an almost regular manner down to the freezing-point; below which the velocity seems to remain constant.2
The circumstances of motion detailed in the six preceding propositions appear to be reconcilable with the assumption of what may be called the Viscous or Plastic Theory of glacier motion, and with that alone.
Plastic Nature of Glacier Ice.—Notwithstanding the apparent paradox of calling a vast mass of coherent ice a semi-fluid body, there is something about a glacier which almost inevitably conveys to the mind the idea of a stream. This may be traced in the descriptions of unscientific tourists, of poets, and of some of those who have addressed themselves more seriously to the question of the real nature of these bodies. To the latter class of observers belong Captain Basil Hall and Monseigneur Rendu, bishop of Anecy, who had much more than hinted at the possibility of a true mechanical connection between the descent of a glacier and that of a mountain torrent, or of a stream of lava. But until the actual conditions of motion were reduced to rule, it was impossible to know how far the analogy was real or apparent.
The viscous theory of glaciers, as deduced from observation by Professor Forbes, though now very generally accepted, had to struggle with numerous and strongly-urged objections; of which the oftener repeated was, that ice is by its nature a brittle solid, and not sensibly possessed of any viscous or plastic quality. In answer to this, it may be urged that the qualities of solid bodies of vast size, and acted on by stupendous and long-continued forces, cannot be estimated from experiments on a small scale, especially if short and violent; that sealing-wax, pitch, and other similar bodies mould themselves, with time, to the surfaces on which they lie, even at atmospheric temperatures, and whilst they maintain, at the same time, the quality of excessive brittleness under a blow or a rapid change of form; that even ice does not pass at once, and per saltum, from the solid to the liquid state, but absorbs its latent heat throughout a certain small range of temperature (between and of Fahrenheit), which is precisely that to which the ice of glaciers is actually exposed; that, after all,
Glacier. a glacier is not a crystalline solid, like ice, tranquilly frozen in a mould, but possesses a peculiar fissured and laminated structure, through which water enters (at least for a great part of the year) into its intrinsic composition. But, waiving the inferences from all these facts, the main argument in favour of the view now maintained is this, that admitting the preceding propositions as to the velocity of its parts (which no one now contests), the quasi-fluid or viscous motion of the ice of glaciers is not a theory but a FACT. A substance which is seen to pour itself out of a large basin through a narrow outlet without losing its continuity,—the different parts of which, from top to bottom, and from side to centre, possess distinct, though related velocities, which moves over slopes inconsistent with the friction between its surface and the ground on which it rests—which surmounts obstacles, and, even if cleft into two streams by a projecting rock, instead of being thereby anchored as a solid would necessarily be, reunites its streams below, and retains no trace of the fissure, leaving the rock an islet in the icy flood—a substance which moves in such a fashion cannot, in any true sense of the word, be termed a rigid solid, but must be granted to be ductile, viscous, plastic, or semifluid, or to possess qualities represented by any of these terms which we may choose to adopt as least shocking to our ordinary conception of the brittleness of ice.3
Origin of the Veined Structure of Ice.—Any mechanical theory of glaciers must be more or less imperfect which does not explain the remarkable veined or ribbed structure of the ice, which, with its peculiar course through the interior of the glacier, has been described at page 637. In applying the theory of quasi-fluid motion to explain this structure, two great difficulties are experienced; first, our confessed ignorance of the modus operandi of those molecular forces which induce pervading structures of this kind; secondly, our imperfect acquaintance with the laws of motion of semifluids under the action of gravity, even in simple cases. Nevertheless it seems possible to prove, partly from admitted mechanical principles, partly from direct experiments, that the tendency of the motion is to produce such a structure.
The fundamental idea is this, that the veined or ribbed structure of the ice is the result of internal forces, by which one portion of ice is dragged past another in a manner so gradual as not necessarily to produce large fissures in the ice, and the consequent sliding of one detached part over another, but rather the effect of a general bruise over a considerable space of the yielding body. According to this view, the delicate veins seen in the glacier, often less than a quarter of an inch wide, have their course parallel to the direction of the sliding effort of one portion of the ice over another. Amongst other proofs of this fundamental conception that the veined structure is the external symbol of this forced internal motion of a body comparatively solid, we may mention one very striking instance recorded by Professor Forbes, as observed on the glacier of La Brenva, on the south side of Mont Blanc, subsequently to the publication of his principal work on the subject. In this case the ice of the glacier, forcibly pressed against the naked rocky face of an opposing hill is turned into a new direction; and in thus shoving and squeezing past a prominence of rock, he observed developed in the ice a "veined
1 The absolute velocity of a glacier depends upon so many circumstances besides its declivity that this law must not be sought to be verified, except under like conditions. The breadth and depth of a glacier (as of a river) no doubt materially affect its rate of motion, and its elevation has a not less important influence. Small lofty glaciers of the second order move slowly over steep inclinations. See Phil. Trans., 1846, p. 177.
2 Phil. Trans., 1846, p. 191; and Edin. Phil. Journal, 1847.
3 For a fuller reply to the objections which have been urged against the theory of the plasticity of glaciers, see Phil. Trans., 1846, particularly pp. 162, &c. The confirmatory observations of MM. Agassiz and Schlagintweit on other glaciers, and their adoption of views virtually the same with those of Professor Forbes, have proved convincing to a majority of those who at first rejected a theory apparently opposed to commonly received notions. See Mousson, Die Gletscher der Jetztzeit, who says, p. 162, speaking of the plastic theory, "Es steht noch heute unangefochtene da."