Part 25

When we turn to the Alpine Lands, we find that there also the occurrence of former interglacial conditions has been recognised. The interglacial deposits, as described by Heer and others, are well known. These form as definite a geological horizon as the similar fossiliferous zone in the Diluvium of northern Germany. The lignites, as Heer pointed out, represent a long period of time, and this is still further illustrated by the fact that considerable fluviatile erosion supervened between the close of the first and the advent of the later glacial epoch. No mere temporary retreat and re-advance of the ice will account for the phenomena. Let us for a moment consider the conditions under which the accumulations in question were laid down. The glacial deposits underlying the lignite beds contain, amongst other erratics, boulders which have come from the upper valley of the Rhine. This means, of course, that the ancient glacier of the Rhine succeeded in reaching the Lake of Zurich; and it is well known that it extended at the same time to Lake Constance. That glacier, therefore exceeded sixty miles in length. One cannot doubt that the climatic conditions implied by this great extension were excessive, and quite incompatible with the appearance in the low-grounds of Switzerland of such a flora as that of the lignites. The organic remains of the lignite beds indicate a climate certainly not less temperate than that which at present characterises the district round the Lake of Zurich. We may safely infer, therefore, that during interglacial times the glaciers of the Alps were not more extensively developed than at present. Again, as the lignites are overlaid by glacial deposits, it is obvious that the Rhine glacier once more reached Lake Zurich--in other words, there was a return of the excessive climate that induced the first great advance of that and other Swiss glaciers. That these advances were really due to extreme climatic conditions is shown by the fact that it was only under such conditions that the Scandinavian flora could have invaded the low-grounds of Europe, and entered Switzerland. It is impossible, therefore, that the interglacial flora could have flourished in Switzerland while the immigration of these northern plants was taking place.

Lignites of the same age as those of Dürnten and Utznach occur in many places both on the north and south sides of the Alpine chain. At Imberg, near Sonthofen, in Bavaria, for example, they are described by Penck[BJ] as being underlaid and overlaid by thick glacial accumulations. The deposits in question form a terrace along the flanks of the hills, at a height of 700 feet above the Iller. The flora of the lignite has not yet been fully studied, but it is composed chiefly of conifers, which must have grown near where their remains now occur--that is at 3000 feet, or thereabout, above the sea. It is incredible that coniferous forests could have flourished at that elevation during a glacial epoch. A lowering of the mean annual temperature by 3° C. only would render the growth of trees at that height almost impossible, and certainly would be insufficient to cause the glaciers of Algau to descend to the foot of the mountains, as we know they did--a distance of at least twenty-four miles. The Imberg lignites, therefore, are evidence of a climate not less temperate than the present. More than this, there is clear proof that the interglacial stage was long continued, for during that epoch the Iller had time to effect very considerable erosion. The succession of changes shown by the sections near Sonthofen are as follows.

1. The Iller Valley is filled with glacier-ice which flows out upon the low-grounds at the base of the Alps.

2. The glacier retreats, and great sheets of shingle and gravel are spread over the valley.

3. Coniferous forests now grow over the surface of the gravels; and as the lignite formed of their remains attains a thickness of ten feet in all, it obviously points to the lapse of some considerable time.

4. Eventually the forests decay, and their débris is buried under new accumulations of shingle and gravel.

5. The Iller cuts its way down through all the deposits to depths of 680 to 720 feet.

6. A glacier again descends and fills the valley, but does not flow so far as that of the earlier glacial stage.

[BJ] _Die Vergletscherung der deutschen Alpen_, 1882, p. 256.

In this section, as in those at Dürnten and Utznach, we have conclusive evidence of two glacial epochs, sharply marked off the one from the other. Nor does that evidence stand alone, for at various points between Lake Geneva and the lower valley of the Inn similar interglacial deposits occur. Sometimes these appear at the foot of the mountains, as at Mörschweil on Lake Constance; sometimes just within the mountain area, as at Imberg; sometimes far in the heart of the Alpine Lands, as at Innsbruck. Professor Penck has further shown, and his observations have been confirmed by Brückner, Blaas, and Böhm, that massive sheets of fluviatile gravel are frequently met with throughout the valleys of the Alps, occupying interglacial positions. These gravels are exactly comparable to the interglacial gravels of the Sonthofen sections. And it has been demonstrated that they occur on two horizons, separated the one from the other by characteristic ground-moraine, or boulder-clay. The lower gravels rest on ground-moraine, and the upper gravels are overlaid by sheets of the same kind of glacial detritus. In short, three separate and distinct ground-moraines are recognised. The gravels, one cannot doubt, are simply the torrential and fluviatile deposits laid down before advancing and retreating glaciers; and it is especially to be noted that each sheet of gravel, after its accumulation, was much denuded and cut through by river-action. In a word, as Penck and others have shown, the valleys of Upper Bavaria have been occupied by glaciers at three successive epochs--each separated from the other by a period during which much river-gravel was deposited and great erosion of the valley-bottoms was effected.

On the Italian side of the Alps, similar evidence of climatic changes is forthcoming. The lignites and lacustrine strata of Val Gandino, and of Val Borlezza, as I have elsewhere shown,[BK] are clearly of interglacial age. From these deposits many organic remains have been obtained--amongst the animals being _Rhinoceros hemitoechus_ and _R. leptorhinus_. According to Sordelli, the plants indicate a climate as genial as that of the plains of Lombardy and Venetia, and warmer therefore than that of the upland valleys in which the interglacial beds occur. Professor Penck informs me that some time ago he detected evidence in the district of Lake Garda of three successive glacial epochs--the evidence being of the same character as that recognised in the valleys of the Bavarian Alps.

[BK] _Prehistoric Europe_, p. 303.

In the glaciated districts of France similar phenomena are met with. Thus in Cantal, according to M. Rames,[BL] the glacial deposits belong to two separate epochs. The older morainic accumulations are scattered over the surface of the plateau of Archæan schistose rocks, and extend up the slopes of the great volcanic cone of that region to heights of 2300 to 3300 feet. One of the features of these accumulations are the innumerable gigantic erratics, known to the country folk as _cimetière des enragés_. Sheets of fluvio-glacial gravel are also associated with the moraines, and it is worthy of note that both have the aspect of considerable age--they have evidently been subjected to much denudation. In the valleys of the same region occurs a younger series of glacial deposits, consisting of conspicuous lateral and terminal moraines, which, unlike the older accumulations, have a very fresh and well-preserved appearance. With them, as with the older moraines, fluvio-glacial gravels are associated. M. Rames shows that the interval that supervened between the formation of the two series of glacial deposits must have been prolonged, for the valleys during that interval were in some places eroded to a depth of 900 feet. Not only was the volcanic _massif_ deeply incised, but even the old plateau of crystalline rocks on which the volcanic cone reposes suffered extensive denudation in interglacial times. M. Rames further recognises that the second glacial epoch was marked by two advances of the valley-glaciers, separated by a marked episode of fusion, the evidence for which is conspicuous in the valley of the Cère.

[BL] _Bull. Soc. Géol. de France_, 1884.

The glacial and interglacial phenomena of Auvergne are quite analogous to those of Cantal. Dr. Julien has described the morainic accumulations of a large glacier that flowed from Mont Dore. After that glacier had retreated a prolonged period of erosion followed, when the morainic deposits were deeply trenched, and the underlying rocks cut into. In the valleys and hollows thus excavated freshwater beds occur, containing the relics of an abundant flora, together with the remains of elephant (_E. meridionalis_), rhinoceros (_R. leptorhinus_), hippopotamus, horse, cave-bear, hyæna, etc.--a fauna comparable to that of the Italian interglacial deposits. After the deposition of the freshwater beds, glaciers again descended the Auvergne valleys and covered the beds in question with their moraines.[BM]

[BM] _Des Phénomènes glaciaires dans le Plateau central de France, etc._

According to the researches of Martins, Collomb, Garrigou, Piette, and Penck, there is clear evidence in the Pyrenees of two periods of glaciation, separated by an interval of much erosion and valley-excavation. Penck, indeed, has shown that the valleys of the Pyrenees have been occupied at three successive epochs by glaciers--each epoch being represented by its series of moraines and by terraces of fluvio-glacial detritus, which occur at successively lower levels.

I have referred in some detail to these discoveries of interglacial phenomena because they so strongly corroborate the conclusions arrived at a number of years ago by glacialists in our own country. Many additional examples might be cited from other parts of Europe, but those already given may serve to show that at least one epoch of interglacial conditions supervened during the Pleistocene period. Before leaving this part of my subject, however, I may point out the significant circumstance that long before much was known of glaciation, and certainly before the periodicity of ice-epochs had been recognised, Collomb had detected in the Vosges conspicuous evidence of two successive glaciations.[BN]

[BN] _Preuves de l'existence d'anciens glaciers dans les vallées des Vosges_, 1847, p. 141.

Having shown that alike in the regions formerly occupied by the great northern ice-sheet, and in the Alpine Lands of central and southern Europe, alternations of cold and genial conditions characterised the so-called Glacial period, we may now glance at the evidence supplied by those Pleistocene deposits that lie outside of the glaciated areas. Of these we have a typical example in the river-accumulations of the Rhine Valley between Bâle and Bingen. Here and there these deposits have yielded remains of extinct and no longer indigenous mammals and relics of Palæolithic man--one of the most interesting deposits from which mammalian remains have been obtained being the Sands of Mosbach, between Wiesbaden and Mayence. The fauna in question is characteristically Pleistocene, nor can it be doubted that the Mosbach Sands belong to the same geological horizon as the similar fluviatile deposits of the Seine, the Thames, and other river-valleys in western Europe. Dr. Kinkelin has shown,[BO] and with him Dr. Schumacher agrees,[BP] that the Mosbach deposits are of interglacial age; while Dr. Pohlig has no hesitation in assigning them to the same horizon.[BQ] It is true there are no glacial accumulations in the region where they occur, but they rest upon a series of unfossiliferous gravels which are recognised as the equivalents of the fluvio-glacial and glacial deposits of the Vosges, the Black Forest, the Alps, etc. These gravels are traced at intervals up to considerable heights above the Rhine, and contain numerous erratics, some of which are several feet in diameter, while a large proportion are not at all water-worn, but roughly and sharply angular. The blocks have unquestionably been transported by river-ice, and imply therefore cold climatic conditions. The overlying Mosbach Sands have yielded not only _Elephas antiquus_ and _Hippopotamus major_, but the reindeer, the mammoth, and the marmot--two strongly contrasted faunas, betokening climatic changes similar to those that marked the accumulation of the river-deposits of the Thames, the Seine, etc. Of younger date than the Mosbach Sands is another series of unfossiliferous gravels, which, like the older series, are charged with ice-floated erratics. The beds at Mosbach are thus shown to be of interglacial age: they occupy the same geological horizon as the interglacial beds of Switzerland and other glaciated tracts in central and northern Europe.

[BO] Kinkelin: _Bericht über die Senckenberg. naturf. Ges. in Frankfurt a. M._, 1889.

[BP] Schumacher: _Mittheilungen d. Commission für d. geolog. Landes-Untersuch. v. Elsass-Lothringen_, Bd. ii., 1890, p. 184.

[BQ] _Zeitschr. d. deutsch. geolog. Ges._, 1887, p. 806.

To this position must likewise be assigned the Pleistocene river-alluvia of other districts. There is no other horizon, indeed, on which these can be placed. That they are not of post-glacial age is shown by the fact that in many places the angular gravels and flood-loams of the Glacial period overlie them. And that they cannot all belong to pre-glacial times is proved by the frequent occurrence underneath them of glacial or fluvio-glacial accumulations. It is quite possible, of course, that here and there in the valleys of western and southern Europe some of the Pleistocene alluvia may be of pre-glacial age. But in the main these alluvia must be regarded as the equivalents of the glacial and interglacial deposits of northern and Alpine districts. This will appear a reasonable conclusion when we bear in mind that long before the Pliocene period came to a close the climate of Europe had begun to deteriorate. In England, as we know, glacial conditions supervened almost at the advent of the Pleistocene period. And the same was the case in the Alpine Lands of the south. Again, in the glaciated areas of north and south alike, the closing stage of the Pleistocene was characterised by cold climatic conditions. And thus in those regions the glacial and interglacial epochs were co-extensive with that period. It follows, therefore, that the Pleistocene deposits of extra-glacial areas must be the equivalents of the glacial and interglacial accumulations elsewhere. If we refused to admit this we should be puzzled indeed to tell what the rivers of western and southern Europe were doing throughout the long-continued Glacial period. There is no escape from the conclusion that the Pleistocene river-alluvia and cave-accumulations must be assigned to the same general horizon as the glacial and interglacial deposits. This is now admitted by Continental palæontologists who find in the character of Pleistocene organic remains abundant proof that the old river-alluvia and cave-accumulations were laid down under changing climatic conditions. Did neither glacial nor interglacial deposits exist, the relics of the Pleistocene flora and fauna met with in extra-glacial regions would yet lead us to the conclusion that after the close of the Pliocene period, extremely cold and very genial climates alternated up to the dawn of the present. Thus during one stage of the Pleistocene "clement winters and cool summers permitted the wide diffusion and intimate association of plants which have now a very different range. Temperate and southern species like the ash, the poplar, the sycamore, the fig-tree, the judas-tree, etc., overspread all the low-grounds of France as far north at least as Paris. It was under such conditions that the elephants, rhinoceroses, hippopotamuses, and the vast herds of temperate cervine and bovine species ranged over Europe, from the shores of the Mediterranean up to the latitude of Yorkshire, and probably even further north still, and from the borders of Asia to the western ocean. Despite the presence of numerous fierce carnivora--lions, hyænas, tigers, and others--Europe at that time, with its shady forests, its laurel-margined streams, its broad and deep-flowing rivers--a country in every way suited to the needs of a race of hunters and fishers--must have been no unpleasant habitation for Palæolithic man." But during another stage of the Pleistocene period, the climate of our continent presented the strongest contrast to those genial conditions. At that time "the dwarf birch of the Scottish Highlands, and the Arctic willow, with their northern congeners, grew upon the low-grounds of middle Europe. Arctic animals, such as the musk-sheep and the reindeer, lived then, all the year round, in the south of France; the mammoth ranged into Spain and Italy; the glutton descended to the shores of the Mediterranean; the marmot came down to the low-grounds at the foot of the Apennines; and the lagomys inhabitated the low-lying maritime districts of Corsica and Sardinia. The land and freshwater molluscs of many Pleistocene deposits tell a similar tale: high alpine, boreal, and hyperborean forms are characteristic of those deposits in central Europe; even in the southern regions of our continent the shells testify to a former colder and wetter climate. It was during the climax of these conditions that the caves of Aquitaine were occupied by those artistic men who appear to have delighted in carving and engraving."[BR] Such, in brief, is the testimony of the Pleistocene flora and fauna of extra-glacial regions. It is from the deposits in these regions, therefore, that we derive our fullest knowledge of the life of the period. But a comparison of their organic remains with those that occur in the glacial and interglacial deposits of alpine and northern lands shows us that the Pleistocene accumulations of glacial and extra-glacial countries are contemporaneous--for there is not a single life-form obtained from interglacial beds which does not also occur in the deposits of extra-glacial regions. The converse is not true--nor is that to be wondered at, for interglacial deposits have only been sparingly preserved. In regions liable to glaciation such superficial accumulations must frequently have been ploughed up and incorporated with ground-moraine. It was only in the extra-glacial tracts that alluvia of interglacial age were at all likely to be preserved in any abundance. To appreciate fully the climatic conditions of the Pleistocene period, therefore, it is necessary to combine the evidence derived from the glaciated areas with that obtained from the lands that lay beyond the reach of the ice-plough. The one is the complement of the other, and this being so, it is obvious that any attempted explanation of the origin of the Glacial period which does not fully realise the importance of the interglacial phase of that period cannot be accepted.

[BR] _Prehistoric Europe_, p. 67.

But if the climatic changes of Pleistocene times are the most important phenomena which the geologist who essays to trace the history of that period is called upon to consider, he cannot ignore the evidence of contemporaneous geographical mutations. These are so generally admitted, however, that it is only necessary here to state the well-known fact that everywhere throughout the maritime tracts of the glaciated lands of Europe and North America frequent changes in the relative level of land and sea took place during Pleistocene and post-glacial times.

I must now very briefly review the evidence bearing on the climatic conditions of post-glacial times. And first, let it be noted that the closing stage of the Pleistocene period was one of cold conditions, accompanied in north-western Europe by partial depression of the land below its present level. This is shown by the late-glacial marine deposits of central Scotland and the coast-lands of Scandinavia. The historical records of the succeeding post-glacial period are furnished chiefly by raised beaches, river- and lake-alluvia, calcareous tufas, and peat-bogs. An examination of these has shown that the climate, at first cold, gradually became less ungenial, so that the Arctic-alpine flora and northern fauna were eventually supplanted in our latitude by those temperate forms which, as a group, still occupy this region. The amelioration of the climate was accompanied by striking geographical changes, the British Islands becoming united with themselves and the opposite coasts of the continent. The genial character of the climate at this time is shown by the great development of forests, the remains of which occur under our oldest peat-bogs. Not only did trees then grow at greater altitudes in these regions than is at present the case, but forests ranged much further north, and flourished in lands where they cannot now exist. In Orkney and Shetland, in the far north of Norway, and even in the Faröe Islands and in Iceland relics of this old forest-epoch are met with. In connection with these facts reference may be made to the evidence obtained from certain raised beaches on both sides of the N. Atlantic, and from recent dredgings in the intervening sea. The occurrence of isolated colonies of southern molluscs in our northern seas, and the appearance in raised beaches of many forms which are now confined to the waters of more southern latitudes, seem to show that in early post-glacial times the seas of these northern latitudes were warmer than now. And it is quite certain that the southern forms referred to are not the relics of any pre-glacial or interglacial immigration. They could only have entered our northern seas after the close of the Glacial period, and their evidence taken in connection with that furnished by the buried trees of our peat-bogs, leads to the conclusion that a genial climate supervened after the cold of the last glacial epoch and of earliest post-glacial times had passed away.

To this genial stage succeeded an epoch of cold humid conditions, accompanied by geographical changes which resulted in the insulation of Britain and Ireland--the sea encroaching to some extent on what are now our maritime regions. The climate was less favourable to the growth of forests, which began to decay and to become buried under widespread accumulations of growing peat. At this time glaciers reappeared in the glens of the Scottish Highlands, and here and there descended to the sea. The evidence for these is quite conspicuous, for the moraines are found resting on the surface of post-glacial beaches. Thus my friend Mr. L. Hinxman, of the Geological Survey, tells us that at the foot of Glen Thraill well-formed moraines are seen in section reposing on beach-deposits at the distance of about three-quarters of a mile above the head of Loch Torridon.[BS] The evidence of this recrudescence of glacial conditions in post-glacial times is not confined to Scotland. I believe it will yet be recognised in many other mountain-regions; but already Prof. Penck has detected it in the valleys of the Pyrenees.[BT] Dr. Kerner von Marilaun has also described similar phenomena in the higher valleys of Tyrol, while Professor Brückner has obtained like evidence in the Salzach region.[BU]