Geological Evidences of the Antiquity of Man/Chapter 15
EXTINCT GLACIERS OF THE ALPS AND THEIR CHRONOLOGICAL RELATION TO THE HUMAN PERIOD.
EXTINCT GLACIERS OF SWITZERLAND—ALPINE ERRATIC BLOCKS ON THE JURA—NOT TRANSPORTED BY FLOATING ICE—EXTINCT GLACIERS OF THE ITALIAN SIDE OF THE ALPS—THEORY OF THE ORIGIN OF LAKE-BASINS BY THE EROSIVE ACTION OF GLACIERS, CONSIDERED—SUCCESSIVE PHASES IN THE DEVELOPMENT OF GLACIAL ACTION IN THE ALPS—PROBABLE RELATION OF THESE TO THE EARLIEST KNOWN DATE OF MAN—CORRESPONDENCE OF THE SAME WITH SUCCESSIVE CHANGES IN THE GLACIAL CONDITION OF THE SCANDINAVIAN AND BRITISH MOUNTAINS—COLD PERIOD IN SICILY AND SYRIA.
Extinct Glaciers of Switzerland.
WE have seen in the preceding chapters that the mountains of Scandinavia, Scotland, and North Wales have served, during the glacial period, as so many independent centres for the dispersion of erratic blocks, just as at present the ice-covered continent of North Greenland is sending down ice in all directions to the coast, and filling Baffin's Bay with floating bergs, many of them laden with fragments of rocks.
Another great European centre of ice-action during the post-pliocene period was the Alps of Switzerland, and I shall now proceed to consider the chronological relations of the extinct Alpine glaciers to those of more northern countries previously treated of.
The Alps lie far south of the limits of the northern drift described in the foregoing pages, being situated between the 44th and 47th degrees of north latitude. On the flanks of these mountains, and on the subAlpine ranges of hills or plains adjoining them, those appearances which have been so often alluded to, as distinguishing or accompanying the drift, between the 50th and 70th parallels of north latitude, suddenly reappear and assume, in a southern region, a truly arctic development. Where the Alps are highest, the largest erratic blocks have been sent forth; as, for example, from the regions of Mont Blanc and Monte Rosa, into the adjoining parts of Switzerland and Italy; while in districts where the great chain sinks in altitude, as in Carinthia, Carniola, and elsewhere, no such rocky fragments, or a few only and of smaller bulk, have been detached and transported to a distance.
In the year 1821, M. Venetz first announced his opinion that the Alpine glaciers must formerly have extended far beyond their present limits, and the proofs appealed to by him in confirmation of this doctrine were afterwards acknowledged by M. Charpentier, who strengthened them by new observations and arguments, and declared, in 1836, his conviction that the glaciers of the Alps must once have reached as far as the Jura, and have carried thither their moraines across the great valley of Switzerland. M. Agassiz, after several excursions in the Alps with M. Charpentier, and after devoting himself some years to the study of glaciers, published, in 1840, an admirable description of them and of the marks which attest the former action of great masses of ice over the entire surface of the Alps and the surrounding country. He pointed out that the surface of every large glacier is strewed over with gravel and stones detached from the surrounding precipices by frost, rain, lightning, or avalanches. And he described more carefully than preceding writers the long lines of these stones, which settle on the sides of the glacier, and are called the lateral moraines; those found at the lower end of the ice being called terminal moraines. Such heaps of earth and boulders every glacier pushes before it when advancing, and leaves behind it when retreating. When the Alpine glacier reaches a lower and a warmer situation, about 3,000 or 4,000 feet above the sea, it melts so rapidly that, in spite of the downward movement of the mass, it can advance no farther. Its precise limits are variable from year to year, and still more so from century to century; one example being on record of a recession of half a mile in a single year. We also learn from M. Venetz, that whereas, between the eleventh and fifteenth centuries, all the Alpine glaciers were less advanced than now, they began in the seventeenth and eighteenth centuries to push forward, so as to cover roads formerly open, and to overwhelm forests of ancient growth.
These oscillations enable the geologist to note the marks which a glacier leaves behind it as it retrogrades; and among these the most prominent, as before stated, are the terminal moraines, or mounds of unstratified earth and stones, often divided by subsequent floods into hillocks, which cross the valley like ancient earth-works, or embankments made to dam up a river. Some of these transverse barriers were formerly pointed out by Saussure below the glacier of the Rhone, as proving how far it had once transgressed its present boundaries. On these moraines we see many large angular fragments, which, having been carried along the surface of the ice, have not had their edges worn off by friction; but the greater number of the boulders, even those of large size, have been well rounded, not by the power of water, but by the mechanical force of the ice, which has pushed them against each other, or against the rocks flanking the valley. Others have fallen down the numerous fissures which intersect the glacier, where, being subject to the pressure of the whole mass of ice, they have been forced along, and either well rounded or ground down into sand, or even the finest mud, of which the moraine is largely constituted.
As the terminal moraines are the most prominent of all the monuments left by a receding glacier, so are they the most liable to obliteration; for violent floods or debacles are sometimes occasioned in the Alps by the sudden bursting of glacier-lakes, or those temporary sheets of water before alluded to, which are caused by the damming up of a river by a glacier which has increased during a succession of cold seasons, and descending from a tributary into the main valley, has crossed it from side to side. On the failure of this icy barrier, the accumulated waters, being let loose, sweep away and level many a transverse mound of gravel and loose boulders below, and spread their materials in confused and irregular beds over the river-plain.
Another mark of the former action of glaciers, in situations where they exist no longer, is the polished, striated, and grooved surfaces of rocks before described. Stones which lie underneath the glacier and are pushed along by it, sometimes adhere to the ice, and as the mass glides slowly along at the rate of a few inches, or at the utmost two or three feet, per day, abrade, groove, and polish the rock, and the larger blocks are reciprocally grooved and polished by the rock on their lower sides. As the forces both of pressure and propulsion are enormous, the sand, acting like emery, polishes the surface; the pebbles, like coarse gravers, scratch and furrow it; and the large stones scoop out grooves in it. Lastly, projecting eminences of rock, called 'roches moutonnées' (see above, p. 269), are smoothed and worn into the shape of flattened domes where the glaciers have passed over them.
Although the surface of almost every kind of rock, when exposed to the open air, wastes away by decomposition, yet some retain for ages their polished and furrowed exterior: and, if they are well protected by a covering of clay or turf, these marks of abrasion seem capable of enduring for ever. They have been traced in the Alps to great heights above the present glaciers, and to great horizontal distances beyond them.
Another effect of a glacier is to lodge a ring of stones round the summit of a conical peak which may happen to project through the ice. If the glacier is lowered greatly by melting, these circles of large angular fragments, which are called 'perched blocks,' are left in a singular situation near the top of a steep hill or pinnacle, the lower parts of which may be destitute of boulders.
Alpine erratic Blocks on the Jura.
Now some or all the marks above enumerated, the moraines, erratics, polished surfaces, domes, striæ, and perched rocks are observed in the Alps at great heights above the present glaciers, and far below their actual extremities; also in the great valley of Switzerland, fifty miles broad; and almost everywhere on the Jura, a chain which lies to the north of this valley. The average height of the Jura is about one-third that of the Alps, and it is now entirely destitute of glaciers; yet it presents almost everywhere moraines, and polished and grooved surfaces of rocks. The erratics, moreover, which cover it present a phenomenon which has astonished and perplexed the geologist for more than half a century. No conclusion can be more incontestable than that these angular blocks of granite, gneiss, and other crystalline formations, came from the Alps, and that they have been brought for a distance of fifty miles and upwards across one of the widest and deepest valleys of the world; so that they are now lodged on the hills and valleys of a chain composed of limestone and other formations, altogether distinct from those of the Alps. Their great size and angularity, after a journey of so many leagues, has justly excited wonder, for hundreds of them are as large as cottages; and one in particular, composed of gneiss, celebrated under the name of Pierre à Bot, rests on the side of a hill about 900 feet above the lake of Neufchatel, and is no less than forty feet in diameter. But there are some far-transported masses of granite and gneiss which are still larger, and which have been found to contain 50,000 and 60,000 cubic feet of stone; and one limestone block at Devens, near Bex, which has travelled thirty miles, contains 161,000 cubic feet, its angles being sharp and unworn.
Von Buch, Escher, and Studer inferred, from an examinationof the mineral composition of the boulders, that those resting on the Jura, opposite the lakes of Geneva and Neufchatel, have come from the region of Mont Blanc and the Valais, as if they had followed the course of the Rhone, to the lake of Geneva, and had then pursued their way uninterruptedly in a northerly direction.
M. Charpentier, who conceived the Alps in the period of greatest cold to have been higher by several thousand feet than they are now, had already suggested that the Alpine glaciers once reached continuously to the Jura, conveying thither the large erratics in question. M. Agassiz, on the other hand, instead of introducing distinct and separate glaciers, imagined that the whole valley of Switzerland might have been filled with ice, and that one great sheet of it extended from the Alps to the Jura, the two chains being of the same height as now relatively to each other. To this idea it was objected that the difference of altitude, when distributed over a space of 50 miles, would give an inclination of two degrees only, or far less than that of any known glacier. In spite of this difficulty, the hypothesis has since received the support of Professor James Forbes, in his very able work on the Alps, published in 1843.
In 1841, I advanced, jointly with Mr. Darwin, the theory that the erratics may have been transferred by floating ice to the Jura, at the time when the greater part of that chain, and the whole of the Swiss valley to the south, was under the sea. We pointed out, that if at that period the Alps had attained only half their present altitude, they would yet have constituted a chain as lofty as the Chilian Andes, which, in a latitude corresponding to Switzerland, now send down glaciers to the head of every sound, from which icebergs, covered with blocks of granite, are floated seaward. Opposite that part of Chili where the glaciers abound, is situated the island of Chiloe, one hundred miles in length, with a breadth of thirty miles, running parallel to the continent. The channel which separates it from the main land is of considerable depth, and twenty-five miles broad. Parts of its surface, like the adjacent coast of Chili, are overspread with recent marine shells, showing an upheaval of the land during a very modern period; and beneath these shells is a boulder deposit, in which Mr. Darwin found large blocks of granite and syenite, which had evidently come from the Andes.
A continuance in future of the elevatory movement, now observed to be going on in this region of the Andes and of Chiloe, might cause the former chain to rival the Alps in altitude, and give to Chiloe a height equal to that of the Jura. The same rise might dry up the channel between Chiloe and the main land, so that it would then represent the great valley of Switzerland.
Sir Roderick I. Murchison, after making several important geological surveys of the Alps, proposed, in 1849, a theory agreeing essentially with that suggested by Mr. Darwin and myself, viz. that the erratics were transported to the Jura, at a time when the great strath of Switzerland, and many valleys receding far into the Alps, were under water. He thought it impossible that the glacial detritus of the Rhone could ever have been carried to the Lake of Geneva, and beyond it by a glacier, or that so vast a body of ice issuing from one narrow valley could have spread its erratics over the low country of the Cantons of Vaud, Friburg, Berne, and Soleure, as well as the slopes of the Jura, comprising a region of about a hundred miles in breadth from south-west to north-east, as laid down in the map of Charpentier. He therefore imagined the granitic blocks to have been translated to the Jura by ice-floats when the intermediate country was submerged. It may be remarked that this theory, provided the water be assumed to have been salt or brackish, demands quite as great an oscillation in the level of the land as that on which Charpentier had speculated, the only difference being that theone hypothesis requires us to begin with a subsidence of 2,500 or 3,000 feet, and the other, with an elevation to the same amount. We should also remember that the crests or watersheds of the Alps and Jura are about eighty miles apart, and if once we suppose them to have been in movement during the glacial period, it is very probable that the movements at such a distance may not have been strictly uniform. If so, the Alps may have been relatively somewhat higher, which would greatly have facilitated the extension of Alpine glaciers to the flanks of the less elevated chain.
Five years before the publication of the memoir last mentioned, M. Guyot had brought forward a great body of new facts in support of the original doctrine of Charpentier, that the Alpine glaciers once reached as far as the Jura, and that they had deposited thereon a portion of their moraines. The scope of his observations and argument was laid with great clearness before the British public in 1852 by Mr. Charles Maclaren, who had himself visited Switzerland for the sake of forming an independent opinion on a theoretical question of so much interest, and on which so many eminent men of science had come to such opposite conclusions.
M. Guyot had endeavoured to show that the Alpine erratics, instead of being scattered at random over the Jura and the great plain of Switzerland, are arranged in a certain determinate order, strictly analogous to that which ought to prevail if they had once constituted the lateral, medial, and terminal moraines of great glaciers. The rocks chiefly relied on as evidence of this distribution consist of three varieties of granite, besides gneiss, chlorite-slate, euphotide, serpentine, and a peculiar kind of conglomerate, all of them mineral compounds, foreign alike to the great strath between the Alps and Jura, and to the structure of the Jura itself. In these two regions, limestones, sandstones, and clays of the secondary and tertiary formations alone crop out at the surface, so that the travelled fragments of Alpine origin can easily be distinguished, and in some cases the precise localities pointed out from whence they must have come.
The accompanying map or diagram, slightly altered from one given by Mr. Maclaren, will enable the reader more fully to appreciate the line of argument relied on by M. Guyot. The dotted area is that over which the Alpine fragments were spread by the supposed extinct glacier of the Rhone. The site of the present reduced glacier of that name is shown at a. From that point, the boulders may first be traced to b, or Martigny, where the valley takes an abrupt turn at right angles to its former course. Here the blocks belonging to the right side of the river, or derived from c, d, e, have not crossed over to the left side at b, as they should have done had they been transported by floating ice, but continue to keep to the side to which they belonged, assuming
that they once formed part of a right lateral moraine of a great extinct glacier. That glacier, after arriving at the lower end of the long narrow valley of the upper Rhone at f, filled the lake of Geneva, f, i, with ice. From f, as from a great vomitory, it then radiated in all directions, bearing along with it the moraines with which it was loaded, and spreading them out on all sides over the great plain. But the principal icy mass moved straight onwards in a direct line towards the hill of Chasseron, g (precisely opposite f), where the Alpine erratics attain their maximum of height on the Jura, that is to say, 2,015 English feet above the level of the Lake of Neufchatel, or 3,450 feet above the sea. The granite blocks which have ascended to this eminence g, came from the east shoulder of Mont Blanc, h, having travelled in the direction b, f, g.
When these and the accompanying blocks resting on the south-eastern declivity of the Jura are traced from their culminating point g, in opposite directions, whether westward towards Geneva, or eastwards towards Soleure, they are found to decline in height from the middle of the arc g, towards the two extremities i and k, both of which are at a lower level than g, by about 1,500 feet. In other words, the ice of the extinct glacier, having mounted up on the sloping flanks of the Jura in the line of greatest pressure to its highest elevation, began to decline laterally in the manner of a pliant or viscous mass, with a gentle inclination, till it reached two points distant from each other no less than 100 miles.
In further confirmation of this theory, M. Guyot observed that fragments derived from the right bank of the great valley of the Rhone, c, d, e, are found on the right side of the great Swiss basin or strath, as at l and m, while those derived from the left bank, p, h, occur on the left side of the basin, or on the Jura, between g and i; and those again derived from places farthest up on the left bank and nearest the source of the Rhone, as n o, occupy the middle of the great basin, constituting, between m and k, what M. Guyot calls the frontal or terminal moraine of the eastern prolongation of the old glacier.
A huge boulder of talcose granite, now at Steinhoff, ten miles east from k, or Soleure, containing 61,000 cubic French feet, or equal in bulk to a mass measuring 40 feet in every direction, was ascertained by Charpentier, from its composition, to have been derived from n, one of the highest points on the left side of the Rhone valley, far above Martigny. From this spot it must have gone all round by f, which is the only outlet to the deep valley, so as to have performed a journey of no less than 150 miles!
General Transportation of Erratics in Switzerland due to Glaciers and not to floating Ice.
It is evident that the above described restriction of certain fragments of peculiar lithological character to that bank of the Rhone where the parent rocks are alone met with, and the linear arrangement of the blocks in corresponding order on the opposite side of the great plain of Switzerland, are facts, which harmonise singularly well with the theory of glaciers, while they are wholly irreconcilable with that of floating ice. Against the latter hypothesis, all the arguments which Charpentier originally brought forward in opposition to the first popular doctrine of a grand débacle, or sudden flood, rushing down from the Alps to the Jura, might be revived. Had there ever been such a rush of muddy water, said he, the blocks carried down the basins of the principal Swiss rivers, such as the Rhone, Aar, Reuss, and Limmat, would all have been mingled confusedly together instead of having each remained in separate and distinct areas as they do and should do according to the glacial hypothesis.
M. Morlot presented me in 1857 with an unpublished map of Switzerland in which he had embodied the results of his own observations, and those of MM. Guyot, Escher, and others, marking out by distinct colours the limits of the ice-transported detritus proper to each of the great river-basins. The arrangement of the drift and erratics thus depicted accords perfectly well with Charpentier's views, and is quite irreconcilable with the supposition of the scattered blocks having been dispersed by floating ice when Switzerland was submerged.
As opposed to the latter hypothesis, I may also state that nowhere as yet have any marine shells or other fossils than those of a terrestrial character, such as the bones of the mammoth, and a few other mammalia, and some coniferous wood, been detected in those drifts, though they are often many hundreds of feet in thickness.
A glance at M. Morlot's map, above alluded to, will show that the two largest areas, indicated by a single colour, are those over which the Rhone and the Rhine are supposed to have spread out in ancient times their enormous moraines. One of these only, that of the Rhone, has been exhibited in our diagram, fig. 42, p. 299. The distinct character of the drift in the two cases is such as it would be if two colossal glaciers should now come down from the higher Alps through the valleys traversed by those rivers, leaving their moraines in the low country. The space occupied by the glacial drift of the Rhine is equal in dimensions, or rather exceeds, that of the Rhone, and its course is not interfered with in the least degree by the Lake of Constance, forty-five miles long, any more than is the dispersion of the erratics of the Rhone, by the Lake of Geneva, about fifty miles in length. The angular and other blocks have in both instances travelled on precisely as if those lakes had no existence, or as if, which was no doubt the case, they had been filled with solid ice.
During my last visit to Switzerland in 1857, I made excursions, in company with several distinguished geologists, for the sake of testing the relative merits of the two rival theories above referred to, and examined parts of the Jura above Neufchatel in company with M. Desor, the country round Soleure with Mr. Langen, the southern side of the great strath near Lausanne with M. Morlot, the basin of the Aar, around Berne, with M. Escher von der Linth; and having satisfied myself that all the facts which I saw north of the Alps were in accordance with M. Guyot's views, I crossed to the Italian side of the great chain, and became convinced that the same theory was equally applicable to the ancient moraines of the plains of the Po.
M. Escher pointed out to me at Trogen in Appenzel, on the left bank of the Rhine, fragments of a rock of a peculiar mineralogical character, commonly called the granite of Pontelyas, the natural position of which is well known near Trons, a hundred miles from Trogen, on the left bank of the Rhine, about thirty miles from the source of that river. All the blocks of this peculiar granite keep to the left bank, even where the valley turns almost at right angles to its former course near Mayenfeld below Chur, making a sharp bend, resembling that of the valley of the Rhone at Martigny. The granite blocks, where they are traced to the low country, still keep to the left side of the Lake of Constance. That they should not have crossed over to the opposite river-bank below Chur is quite inexplicable, if, rejecting the aid of land-ice, we appeal to floating ice as the transporting power.
In M. Morlot's map, already cited, we behold between the areas occupied by the glacial drift of the Rhine and Rhone three smaller yet not inconsiderable spaces, distinguished by distinct colours, indicating the peculiar detritus brought down by the three great rivers, the Aar, Reuss, and Limmat. The ancient glacier of the first of these, the Aar, has traversed the lakes of Brienz and Thun, and has borne angular, polished and striated blocks of limestone and other rocks as far as Berne, and somewhat below that city. The Reuss has also stamped the lithological character of its own mountainous region upon the lower part of its hydrographical basin by covering it with its peculiar Alpine drift. In like manner the old extinct glacier of the Limmat, during its gradual retreat, has left monuments of its course in the Lake of Zurich in the shape of terminal moraines, one of which has almost divided that great sheet of water into two lakes.
The ice-work done by the extinct glaciers, as contrasted with that performed by their dwarfed representatives of the present day, is in due proportion to the relative volume of the supposed glaciers, whether we measure them by the distances to which they have carried erratic blocks, or the areas which they have strewed over with drift, or the hard surfaces of rock and number of boulders which they have polished and striated. Instead of a length of five, ten, or twenty miles and a thickness of 200, 300, or at the utmost 800 feet, those giants of the olden time must have been from 50 to 150 miles long, and between 1,000 and 3,000 feet deep. In like manner the glaciation, although identical in kind, is on so small a scale in the existing Alpine glaciers as at first sight to disappoint a Swedish, Scotch, Welsh or North American geologist. When I visited the terminal moraine of the glacier of the Rhone in 1859, and tried to estimate the number of angular or rounded pebbles and blocks which exhibited glacial polishing or scratches as compared to those bearing no such markings, I found that several thousand had to be reckoned before I arrived at the first which was so striated or polished as to differ from the stones of an ordinary torrent-bed. Even in the moraines of the glaciers of Zermatt, Viesch, and others, in which fragments of limestone and serpentine are abundant (rocks which most readily receive and most faithfully retain the signs of glaciation), I found, for one which displayed such indications, several hundreds entirely free from them. Of the most opposite character were the results obtained by me from a similar scrutiny of the boulders and pebbles of the terminal moraine of one of the old extinct glaciers, namely, that of the Rhone in the suburbs of Soleure. Thus at the point k, in the map, fig. 42, p. 299, I observed a mass of unstratified clay or mud, through which a variety of angular and rubbed stones were scattered, and a marked proportion of the whole were polished and scratched, and the clay rendered so compact, as if by the incumbent pressure of a great mass of ice, that it has been found necessary to blow it up with gun-powder in making railway cuttings through part of it. A marble rock of the age of our Portland stone, on which this old moraine rests, has its surface polished like a looking-glass, displaying beautiful sections of fossil shells of the genera Nerinæa and Pteroceras, while occasionally, besides finer striæ, there are deep rectilinear grooves, agreeing in direction with the course in which the extinct glacier would have moved according to the theory of M. Guyot, before explained.
Extinct Glaciers of the Italian Side of the Alps.
To select another example from the opposite or southern side of the Alps. It will be seen in the elaborate map, recently executed by Signor Gabriel de Mortillet, of the ancient glaciers of the Italian flank of the Alps, that the old moraines descend in narrow strips from the snow-covered ridges, through the principal valleys, to the great basin of the Po, on reaching which they expand and cover large circular or oval areas. Each of these groups of detritus is observed (see map, p. 306) to contain exclusively the wreck of such rocks as occur in situ on the Alpine heights of the hydrographical basins to which the moraines respectively belong.
I had an opportunity of verifying this fact, in company with Signor Gastaldi as my guide, by examining the erratics and boulder formation between Susa and Turin, on the banks of the Dora Riparia, which brings down the waters from Mont Cenis, and from the Alps SW. of it. I there observed striated fragments of dolomite and gypsum, which had come
map of the moraines of extinct glaciers extending from the alps into the plains of the po near turin.
From Map of the ancient Glaciers of the Italian side of the Alps by Signor Gabriel de Mortillet.
a Crest or watershed of the Alps.
b Snow-covered Alpine summits which fed the ancient glaciers.
c Moraines of ancient or extinct glaciers.
gravel, boulders, and large erratics, which extend for fifteen miles from above Ivrea to below Caluso, and which, when seen in profile from Turin, have the aspect of a chain of hills. In many countries, indeed, they might rank as an important range of hills, for where they join the mountains they are more than 1,500 feet high, and retain more than half that height for a great part of their course, rising very abruptly from the plain, often with a slope of from 20° to 30°. This glacial drift reposes near the mountains on ancient metamorphic rocks, and farther from them on marine pliocene strata. Portions of the ridges of till and stratified matter have been cut up into mounds and hillocks by the action of the river, the Dora Baltea, and there are numerous lakes, so that the entire moraine much resembles, except in its greater height and width, the line of glacial drift of Perthshire and Forfarshire, before described, p. 248. Its complicated structure can only be explained by supposing that the ancient glacier advanced and retreated several times, and left large lateral moraines, the more modern mounds within the limits of the older ones, and masses of till thrown down upon the re-arranged and stratified materials of the first set of moraines. Such appearances accord well with the hypothesis of the successive phases of glacial action in Switzerland, to which I shall presently advert.
Contorted Strata of Glacial Drift south of Ivrea.
At Mazzé near Caluso (see map, p. 306), the southern extremity of this great moraine has recently been cut through in making a tunnel for the railway which runs from Turin to Ivrea. In the fine section thus exposed Signor Gastaldi and I had an opportunity of observing the internal structure of the glacial formation. In close juxtaposition to a great mass of till with striated boulders, we saw stratified beds of alternating gravel, sand, and loam, which were so sharply bent that many of them had been twice pierced through in the same vertical cutting. Whether they had been thus folded by the mechanical power of an advancing glacier, which had pushed before it a heap of stratified matter, as the glacier of Zermatt has been sometimes known to shove forward blocks of stone through the walls of houses, or whether the melting of masses of ice, once interstratified with sand and gravel, had given rise to flexures, in the manner before suggested, pp. 138 and 220; it is at least satisfactory to have detected this new proof of a close connection between ice-action and contorted stratification, such as has been described as so common in the Norfolk cliffs, p. 222, and which is also very often seen in Scotland and North America, where stratified gravel overlies till. I have little doubt that if the marine pliocene strata, which underlie a great part of the moraine below Ivrea, were exposed to view in a vertical section, those fundamental strata would be found not to participate in the least degree in the plications of the sands and gravels of the overlying glacial drift.
To return to the marks of glaciation: in the moraine at Mazzé, there are many large blocks of protogene, and large and small ones of limestone and serpentine, which have been brought down from Monte Rosa, through the gorge of Ivrea, after having travelled for a distance of 100 miles. Confining my attention to a part of the moraine, where pieces of limestone and serpentine were very numerous, I found that no less than one-third of the whole number bore unequivocal signs of glacial action; a state of things which seems to bear some relation to the vast volume and pressure of the ice which once constituted the extinct glacier, and to the distance which the stones had travelled. When I separated the pebbles of quartz, which were never striated, and those of granite, mica schist, and diorite, which do not often exhibit glacial markings, and confined my attention to the serpentine alone, I found no less than nineteen in twenty of the whole number polished and scratched; whereas in the terminal moraines of some modern glaciers, where the materials have travelled not more than ten or fifteen, instead of a hundred miles, scarce one in twenty even of the serpentine pebbles exhibit glacial polish and striation.
Theory of the Origin of Lake-basins by the erosive Action of Glaciers, considered.
Geologists are all agreed, that the last series of movements to which the Alps owe their present form and internal structure, occurred after the deposition of the miocene strata; and it has been usual to refer the origin of the numerous lake-basins of Alpine and sub-Alpine regions, both in Switzerland and Northern Italy, to the same movements; for it seemed not unnatural to suppose, that forces capable of modifying the configuration of the greatest European chain, by uplifting some of its component tertiary strata (those of marine origin of the miocene period) several thousand feet above their former level, after throwing them into vertical and contorted positions, must also have given rise to many superficial inequalities, in some of which large bodies of water would collect. M. Desor, in a memoir on the Swiss and Italian lakes, suggested that they may have escaped being obliterated by sedimentary deposition, by having been filled with ice during the whole of the glacial period.
Subsequently to the retreat of the great glaciers, we know that the lake-basins have been to a certain extent encroached upon and turned into land by river deltas; one of which, that of the Rhone at the head of the lake of Geneva, is no less than twelve miles long and several miles broad, besides which there are many torrents on the borders of the same lake, forming smaller deltas.
M. Gabriel de Mortillet, after a careful study of the glacial formations of the Alps, agreed with his predecessors, that the great lakes had existed before the glacial period, but came to the opinion, in 1859, that they had all been first filled up with alluvial matter, and then re-excavated by the action of ice, which, during the epoch of intense cold, had by its weight and force of propulsion, scooped out the loose and incoherent alluvial strata, even where they had accumulated to a thickness of 2,000 feet. Besides this erosion, the ice had carried the whole mass of mud and stones up the inclined planes, from the central depths to the lower outlets of the lakes, and sometimes far beyond them. As some of these rock-basins are 500, others more than 2,000 feet deep, having their bottoms in some cases 500, in others 1,000 feet below the level of the sea, and having areas from twenty to fifty miles in length and from four to twelve in breadth, we may well be startled at the boldness of this hypothesis.
The following are the facts and train of reasoning which induced M. de Mortillet to embrace these views. At the lower ends of the great Italian lakes, such as Maggiore, Como, Garda and others, there are vast moraines which are proved by their contents to have come from the upper Alpine valleys above the lakes. Such moraines often repose on an older stratified alluvium, made up of rounded and worn pebbles of precisely the same rocks as those forming the moraines, but not derived from them, being small in size, never angular, polished, or striated, and the whole having evidently come from a great distance. These older alluvial strata must, according to M. de Mortillet, be of pre-glacial date, and could not have been carried past the sites of the lakes, unless each basin had previously been filled and levelled up with mud, sand, and gravel, so that the river channel was continuous from the upper to the lower extremity of each basin.
Professor Ramsay, after acquiring an intimate knowledge of the glacial phenomena of the British Isles, had taught, many years before, that small tarns and shallow rock-basins, such as we see in many mountain regions, owe their origin to glaciers which erode the softer rocks, leaving the harder ones standing out in relief and comparatively unabraded. Following up this idea after he had visited Switzerland, and without any communication with M. de Mortillet or cognizance of his views, he suggested in 1859 that the lake-basins were not of pre-glacial date, but had been scooped out by ice during the glacial period, the excavation having for the most part been effected in miocene sandstone, provincially called, on account of its softness, 'molasse.' By this theory he dispensed with the necessity of filling up pre-existing cavities with stratified alluvium, in the manner proposed by M. de Mortillet.
I will now explain to what extent I agree with, and on what points I feel compelled to differ from, the two distinguished geologists above cited. 1st. It is no doubt true, as Professor Ramsay remarks, that heavy masses of ice, creeping for ages over a surface of dry land (whether this comprise hills, plateaus and valleys, as in the case of Greenland, before described (p. 235), or be confined to the bottoms of great valleys, as now in the higher Alps), must often, by their grinding action, produce depressions, in consequence of the different degrees of resistance offered by rocks of unequal hardness. Thus, for example, where quartzose beds of mica schist alternate with clay-slate, or where trap-dykes, often causing waterfalls in the courses of torrents, cut through sandstone or slate—these and innumerable other common associations of dissimilar stony compounds, must give rise to a very unequal amount of erosion, and consequently to lake-basins on a small scale. But the larger the size of any lake, the more certain it will be to contain within it rocks of every degree of hardness, toughness, and softness; and if we find a gradual deepening from the head towards the central parts, and a shallowing again from the middle to the lower end, as in several of the great Swiss and Italian lakes, which are thirty or forty miles in length, we require a power capable of acting with a considerable degree of uniformity on these masses of varying powers of resistance.
2ndly. Several of the great lakes are by no means in the line of direction which they ought to have taken had they been scooped out by the pressure and onward movement of the extinct glaciers. The Lake of Geneva, for instance, had it been the work of ice, would have been prolonged from the termination of the upper valley of the Rhone towards the Jura, in the direction from f to g of the map, fig. 42, p. 299, instead of running from f to i.
3rdly. It has been ascertained experimentally, that in a glacier, as in a river, the rate of motion is accelerated or lessened, according to the greater or less slope of the ground; also, that the lower strata of ice, like those of water, move more slowly than those above them. In the Lago Maggiore, which is more than 2,600 feet deep (797 metres), the ice, says Professor Ramsay, had to descend a slope of about 3° for the first twenty-five miles, and then to ascend for the last twelve miles (from the deepest part towards the outlet), at an angle of 5°. It is for those who are conversant with the dynamics of glacier motion to divine whether, in such a case, the discharge of ice would not be entirely effected by the superior and faster moving strata, and whether the lowest would not be motionless or nearly so, and would therefore exert very little, if any, friction on the bottom.
4thly. But the gravest objection to the hypothesis of glacial erosion on so stupendous a scale is afforded by the entire absence of lakes of the first magnitude in several areas where they ought to exist if the enormous glaciers which once occupied those spaces had possessed the deep excavating power ascribed to them. Thus in the area laid down on the map, p. 306, or that covered by the ancient moraine of the Dora Baltea, we see the monuments of a colossal glacier derived from Mont Blanc and Monte Rosa, which descended from points nearly a hundred miles distant, and then emerging from the narrow gorge above Ivrea, deployed upon the plains of the Po, advancing over a floor of marine pliocene strata of no greater solidity than the miocene sandstone and conglomerate in which the lake-basins of Geneva, Zurich, and some others are situated. Why did this glacier fail to scoop out a deep and wide basin rivalling in size the lakes of Maggiore or Como, instead of merely giving rise to a few ponds above Ivrea, which may have been due to ice action? There is one lake, it is true—that of Candia, near the southern extremity of the moraine, which is larger; but even this, as will be seen by the map, p. 306, is quite of subordinate importance, and whether it is situated in a rock basin or is simply caused by a dam of moraine matter, has not yet been fully made out.
There ought also to have been another great lake, according to the theory under consideration, in the space now occupied by the moraine of the Dora Riparia, between Susa and Turin (see map, p. 306). Signor Gastaldi has shown that all the ponds in that area consist exclusively of what M. de Mortillet has denominated morainic lakes, i.e. caused by barriers of glacier-mud and stones.
5thly. In proof of the great lakes having had no existence before the glacial period, Professor Ramsay observes that we do not find in the Alps any freshwater strata of an age intermediate between 'the close of the miocenic and the commencement of the glacial epoch.' But although such formations are scarce, they are by no means wholly wanting; and if it can be shown that any one of the principal lakes, that of Zurich for example, existed prior to the glacial era, it will follow that in the Alps the erosive power of ice was not required to produce lake-basins on a large scale. The deposits alluded to on the borders of the lake of Zurich are those of Utznach and Dürnten, situated each about 350 feet above the present level of the lake, and containing valuable beds of lignite.
The first of them, that of Utznach, is a delta formed at the head of the ancient and once more extensive lake. The argillaceous and lignite-bearing strata, more than 100 feet in thickness, rest unconformably on highly inclined and sometimes vertical miocene molasse. These clays are covered conformably by stratified sand and gravel sixty feet thick, partly consolidated, in which the pebbles are of rocks belonging to the upper valleys of the Limmat and its tributaries, all of them small and not glacially striated, and wholly without admixture of large angular stones. On the top of all repose very large erratic blocks, affording clear evidence that the colossal glacier which once filled the valley of the Limmat covered the old littoral deposit. The great age of the lignite is partly indicated by the bones of Elephas antiquus found in it.
I visited Utznach in company with M. Escher von der Linth in 1857, and during the same year examined the lignite of Dürnten, many miles further down on the right bank of the lake, in company with Professor Heer and Mr. Marcou. The beds there are of the same age and within a few feet of the same height above the level of the lake. They might easily have been overlooked or confounded with the general glacial drift of the neighbourhood, had not the bed of lignite, which is from five to twelve feet thick, been worked for fuel, during which operation many organic remains came to light. Among these are the teeth of Elephas antiquus, determined by Dr. Falconer, and Rhinoceros leptorhinus? (R. megarhinus Christol), the wild bull and red deer (Bos primigenius Boj., and Cervus Elaphus L.), the last two determined by Professor Rütemeyer. In the same beds I found many freshwater shells of the genera Paludina, Limnea, &c., all of living species. The plants named by Professor Heer are also recent, and agree singularly with those of the Cromer buried forest, before described (p. 214).
Among them are the Scotch and spruce firs, Pinus sylvestris and Pinus Abies, and the buckbean, or Menyanthes trifoliata, &c., besides the common birch and other European plants.
Overlying this lignite are, first, as at Utznach, stratified gravel, not of glacial origin, about thirty feet thick; and, secondly, highest of all, huge angular erratic blocks, clearly indicating the presence of a great glacier, posterior in date to all the organic remains above enumerated.
If any one of the existing Swiss lakes were now lowered by deepening its outlet, or by raising the higher portion of it relatively to the lower, we should see similar deltas of comparatively modern date exposed to view, some of them with embedded trunks of pines of the same species drifted down during freshets. Such deposits would be most frequent at the upper ends of the lakes, but a few would occur on either bank not far from the shore, where torrents once entered, agreeing in geographical position with the lignite formations of Utznach and Dürnten.
There are other freshwater formations with lignite, besides those on the lake of Zurich, as those of Wetzikon, near the Pfaffikon Lake, of Kaltbrunnen, of Buchberg, and that of Morschweil between St. Gall and Rorschach, but none probably older than the Dürnten beds. Like the buried forest of Cromer (p. 214), they are all pre-glacial, yet they by no means represent the older nor even the newer pliocene period, but rather the beginning of the post-pliocene. It is therefore true, as Professor Ramsay remarks, that, as yet, no strata 'of the age of the English Crag' have been detected in any Alpine valley. In other words, there are no freshwater formations yet known corresponding in date to the pliocene beds of the upper Val d'Arno, above Florence—a fact from which we may infer (though with diffidence, as the inference is based on negative evidence), that, although the great Alpine valleys were eroded in pliocene times, the lake-basins were, nevertheless, of post-pliocene date—some of them formed before, others during, the glacial epoch.
6thly. In what manner, then, did the great lake basins originate if they were not hollowed out by ice? My answer is, they are all due to unequal movements of upheaval and subsidence. We have already seen that the buried forest of Cromer, which, by its organic contents, seems clearly to be of the same age as the lignite of Dürnten, was pre-glacial, and that it has undergone a great oscillation of level (about 500 feet in both directions, see p. 227) since its origin, having first sunk to that extent below the sea, and then been raised up again to the sea-level. In the countless post-miocene ages which preceded the glacial period there was ample time for the slow erosion by water of all the principal hydrographical basins of the Alps, and the sites of all the great lakes coincide, as Professor Ramsay truly says, with these great lines of drainage. The lake-cavities do not lie in synclinal troughs, following the strike and foldings of the strata, but often, as the same geologist remarks, cross them at high angles; nor are they due to rents or gaping fissures, although these, with other accidents connected with the disturbing movements of the Alps, may sometimes have determined originally the direction of the valleys. The conformity of the lake-basins to the principal watercourses is explicable if we assume them to have resulted from inequalities in the upward and downward movements of the whole country in post-pliocene times, after the valleys were eroded.
We know that in Sweden the rate of the rise of the land is far from uniform, being only a few inches in a century near Stockholm, while north of it, and beyond Gefle, it amounts to as many feet in the same number of years. Let us suppose, with Charpentier, that the Alps gained in height several thousand feet at the time when the intense cold of the glacial period was coming on. This gradual rise would be an era of aqueous erosion, and of the deepening, widening, and lengthening of the valleys. It is very improbable that the elevation would be everywhere identical in quantity, but if it was never in excess in the outskirts as compared to the central region or crest of the chain, it would not give rise to lakes. When, however, the period of upheaval was followed by one of gradual subsidence, the movement not being everywhere strictly uniform, lake-basins would be formed where-ever the rate of depression was in excess in the upper country. Let the region, for example, near the head waters of the great rivers sink at the rate of from four to six feet per century, while only half as much subsidence occurs towards the circumference of the mountains—the rate diminishing about an inch per mile, in a distance, say of forty miles—this might convert many of the largest and deepest valleys at their lower ends into lakes.
We have no certainty that such movements may not now be in progress in the Alps; for if they are as slow as we have assumed, they would be as insensible to the inhabitants, as is the upheaval of Scandinavia or the subsidence of Greenland to the Swedes and Danes who dwell there. They only know of the progress of such geographical revolutions, because a slight change of level becomes manifest on the margin of the sea. The lines of elevation or depression above supposed might leave no clear geological traces of their action on the high ridges and table-lands separating the valleys of the principal rivers; it is only when they cross such valleys, that the disturbance caused in the course of thousands of years in the drainage becomes apparent. If there were no ice, the sinking of the land might not give rise to lakes. To accomplish this in the absence of ice, it is necessary that the rate of depression should be sufficiently fast to make it impossible for the depositing power of the river to keep pace with it, or, in other words, to fill up the incipient cavity, as fast as it begins to form. Such levelling operations once complete, the running water, aided by sand and pebbles, will gradually cut a gorge through the newly raised rock, so as to prevent it from forming a barrier. But if a great glacier fill the lower part of the valley, all the conditions of the problem are altered. Instead of the mud, sand, and stones drifted down from the higher regions being left behind in the incipient basin, they all travel onwards in the shape of moraines on the top of the ice, passing over and beyond the new depression, so that when, at the end of fifty or a thousand centuries, the glacier melts, a large and deep basin representing the difference in the movement of two adjoining mountain areas—namely, the central and the circumferential—is for the first time rendered visible.
By adopting this hypothesis, we concede that there is an intimate connection between the glacial period and a predominance of lakes, in producing which the action of ice is threefold; first, by its direct power in scooping out shallow basins where the rocks are of unequal hardness; an operation which can by no means be confined to the land, for it must extend to below the level of high water a thousand feet and more, in such friths as have been described as filled with ice in Greenland (see above, p. 236).
2ndly. The ice will act indirectly by preventing cavities caused by inequalities of subsidence or elevation from becoming the receptacles first of water, and then of sediment, by which the cavities would be levelled up and the lakes obliterated.
3rdly. The ice is also an indirect cause of lakes, by heaping up mounds of moraine matter, and thus giving rise to ponds and even to sheets of water several miles in diameter.
The comparative scarcity, therefore, of lakes of post-pliocene date in tropical countries, and very generally south of the fortieth and fiftieth parallels of latitude, may be accounted for by the absence of glacial action in such regions.
Post-glacial Lake-dwelling in the North of Italy.
We learn from M. de Mortillet that in the peat which has filled up one of the 'morainic lakes' formed by the ancient glacier of the Ticino, M. Moro has discovered at Mercurago the piles of a lake-dwelling like those of Switzerland, together with various utensils, and a canoe hollowed out of the trunk of a tree. From this fact we learn that south of the Alps, as well as north of them, a primitive people having similar habits flourished after the retreat of the great glaciers.
Successive Phases of Glacial Action in the Alps, and their Relation to the Human Period.
According to the geological observations of M. Morlot, the following successive phases in the development of ice-action in the Alps are plainly recognisable:—
1st. There was a period when the ice was in its greatest excess, as described at p. 300 et seq., when the glacier of the Rhone not only reached the Jura, but climbed to the height of 2,015 feet above the lake of Neufchatel, and 3,450 above the sea, at which time the Alpine ice actually entered the French territory at some points, penetrating by certain gorges, as through the defile of the Fort de l'Ecluse, among others.
2nd. To this succeeded a prolonged retreat of the great glaciers, when they evacuated not only the Jura and the low country between that chain and the Alps, but retired some way back into the Alpine valleys. M. Morlot supposes their diminution in volume to have accompanied a general subsidence of the country, to the extent of at least 1,000 feet. The geological formations of the 2nd period consist of stratified masses of sand and gravel, called the 'ancient alluvium' by MM. Necker and Favre, corresponding to the 'older or lower diluvium' of some writers. Their origin is evidently due to the action of rivers, swollen by the melting of ice, by which the materials of parts of the old moraines were rearranged and stratified, and left usually at considerable heights above the level of the present valley plains.
3rd. The glaciers again advanced and became of gigantic dimensions, though they fell far short of those of the first period. That of the Rhone, for example, did not again reach the Jura, though it filled the lake of Geneva, and formed enormous moraines on its borders, and in many parts of the valley between the Alps and Jura.
4th. A second retreat of the glaciers took place when they gradually shrank nearly into their present limits, accompanied by another accumulation of stratified gravels, which form in many places a series of terraces above the level of the alluvial plains of the existing rivers.
In the gorge of the Dranse, near Thonon, M. Morlot discovered no less than three of these glacial formations in direct superposition, namely, at the bottom of the section, a mass of compact till or boulder-clay (No. 1) twelve feet thick, including striated boulders of Alpine limestone, and covered by regularly stratified ancient alluvium (No. 2) 150 feet thick, made up of rounded pebbles in horizontal beds. This mass is in its turn overlaid by a second formation (No. 3) of unstratified boulder clay, with erratic blocks and striated pebbles, which constituted the left lateral moraine of the great glacier of the Rhone, when it advanced for the second time to the lake of Geneva. At a short distance from the above section, terraces (No. 4) composed of stratified alluvium are seen at the heights of 20, 50, 100, and 150 feet above the lake of Geneva, which, by their position, can be shown to be posterior in date to the upper boulder-clay, and therefore belong to the fourth period, or that of the last retreat of the great glaciers. In the deposits of this fourth period, the remains of the mammoth have been discovered, as at Morges, for example, on the lake of Geneva. The conical delta of the Tinière, mentioned at p. 27 as containing at different depths monuments of the Roman as well as of the antecedent bronze and stone ages, is the work of alluvial deposition going on when the terrace of 50 feet was in progress. This modern delta is supposed by M. Morlot to have required 10,000 years for its accumulation. At the height of 150 feet above the lake, following up the course of the same torrent, we came to a more ancient delta, about ten times as large, which is therefore supposed to be the monument of about ten times as many centuries, or 100,000 years, all referable to the fourth period mentioned in the preceding page, or that which followed the last retreat of the great glaciers.
If the lower flattened cone of Tinière be referred in great part to the age of the oldest lake-dwellings, the higher one might, perhaps, correspond with the post-pliocene period of St. Acheul, or the era when man and the Elephas primigenius flourished together; but no human remains or works of art have as yet been found in deposits of this age, or in any alluvium containing the bones of extinct mammalia in Switzerland.
Upon the whole, it is impossible not to be struck with an apparent correspondence in the succession of events of the glacial period of Switzerland, and that of the British Isles before described. The time of the first Alpine glaciers of colossal dimensions, when that chain perhaps was several thousand feet higher than now, may have agreed with the first continental period alluded to at pp. 241 and 282, when Scotland was invested with a universal crust of ice. The retreat of the first Alpine glaciers, caused partly by a lowering of that chain, may have been synchronous with the period of great submergence and floating ice in England. The second advance of the glaciers may have coincided in date with the re-elevation of the Alps, as well as of the Scotch and Welsh mountains; and lastly, the final retreat of the Swiss and Italian glaciers may have taken place when man and the extinct mammalia were colonising the north-west of Europe, and beginning to inhabit areas which had formed the bed of the glacial sea during the era of chief submergence.
But it must be confessed, that in the present state of our knowledge, these attempts to compare the chronological relations of the periods of upheaval and subsidence of areas so widely separated as are the mountains of Scandinavia, the British Isles, and the Alps, or the times of the advance and retreat of glaciers in those several regions, and the greater or less intensity of cold, must be looked upon as very conjectural.
We may presume with more confidence that when the Alps were highest and the Alpine glaciers most developed, filling all the great lakes of northern Italy, and loading the plains of Piedmont and Lombardy with ice, the waters of the Mediterranean were chilled and of a lower average temperature than now. Such a period of refrigeration is required by the conchologist to account for the prevalence of northern shells in the Sicilian seas about the close of the newer pliocene or commencement of the post-pliocene period. For such shells as Cyprina islandica, Natica clausa, and some others, enumerated among the fossils of the latest tertiary formations of Sicily by Philippi and Edward Forbes, point unequivocally to a former more severe climate. Dr. Hooker also, in his late journey to Syria (in the autumn of 1860), found the moraines of extinct glaciers, on which the whole of the ancient cedars of Lebanon grow, to descend 4,000 feet below the summit of that chain. The temperature of Syria is now so much milder, that there is no longer perpetual snow even on the summit of Lebanon, the height of which was ascertained to be 10,200 feet above the Mediterranean.
Such monuments of a cold climate in latitudes so far south as Syria and the north of Sicily, between 33° to 38° north, may be confidently referred to an early part of the glacier period, or to times long anterior to those of man and the extinct mammalia of Abbeville and Amiens.
- Agassiz, Études sur les Glaciers et Système Glaciaire.
- D'Archiac, Histoire des Progrès, &c. tom. ii. p. 249.
- See Elements of Geology, 2nd ed. 1841.
- Quarterly Geological Journal, 1850, vol. vi. p. 65.
- Bulletin de la Société des Sciences Naturelles de Neufchâtel, 1845.
- Edinburgh New Philosophical Magazine, October 1852.
- See map, Geological Quarterly Journal, vol. xviii. pl. 18, p. 185.
- Geol. Quart. Journ. vol. xviii.
- Morlot, Terrain quaternaire du Bassin de Léman, Bulletin de Société Vaudoise des Sciences Naturelles, No. 44.
- Hooker, Natural History Review, No. 5, January 1862, p. 11.