Chapter XIV: The Geographical Development of Coast-Lines 393 (11)

When we compare the conditions of the Ice Age with those of the present we are struck with the fact that the former were only an exaggeration of the latter. The development of glaciation was in strict accordance with existing conditions. Thus in Pleistocene times North America was more extensively glaciated than northern Europe, just as to-day Greenland shows more snow and ice than Scandinavia. No traces of glaciation have been observed as yet in northern Asia or in northern Alaska, and to-day the only glaciers and ice-sheets that exist in northern regions are confined to the formerly glaciated areas. Again, in Pleistocene Europe glacial phenomena were more strongly developed in the west than in the east. Large glaciers, for example, existed in central France, and a considerable ice-flow poured into the basin of the Douro. But in the same latitudes of eastern Europe we meet with few or no traces of ice-action. Again, the Vosges appear to have been more severely glaciated than the mountains of middle Germany; and so likewise the old glaciers of the western Alps were on a much more extensive scale than those towards the east end of the chain. Similar contrasts may be noted at the present day. Thus we find glaciers in Norway under lat. 60°, while in the Ural Mountains in the same latitude there is none. The glaciers of the western Alps, again, are larger than those in the eastern part of the chain. The Caucasus region, it is true, has considerable glaciers, but then the mountains are higher.

Now turn for a moment to North America. The eastern area was covered by one immense ice-sheet, while in the mountainous region of the west gigantic glaciers existed. In our own day we see a similar contrast. In the north-east lies Greenland well-nigh drowned in ice, while the north-west region on the other hand, although considerably higher and occurring in the same latitude, holds only local glaciers. We may further note that at the present day very dry regions, even when these are relatively lofty and in high latitudes, such as the uplands of Siberia, contain no glaciers. And the same was the case in the Glacial period. These facts are sufficient to show that the conditions of glacial times bore an intimate relation to those that now obtain. Could the requisite increase of precipitation and lowering of temperature take place, we cannot doubt that ice-sheets and glaciers would reappear in precisely the same regions where they were formerly so extensively developed. No change in the relative elevation of the land would be required--increased precipitation accompanied by a general lowering of the snow-line for 3000 or 3500 feet would suffice to reintroduce the Ice Age.

From the foregoing considerations we may conclude:--(1) That the cold of the Glacial period was a general phenomenon, due to some widely-acting cause--a cause sufficient to influence contemporaneously the climate of Europe and North America; (2) that glaciation in our continent increased in intensity from east to west, and from south to north; (3) that where now we have the greatest rainfall, in glacial times the greatest snow-fall took place, and the snow tended most to accumulate; (4) that in the extreme south of Europe, and in north Africa and west Asia, increased rain precipitation accompanied lowering of temperature, from which it may be inferred that precipitation in glacial times was greater generally than it is now.

Having considered the climatic conditions that obtained at the climax of the Glacial period, I have next to recapitulate what is known as to the climatic changes of Pleistocene times. It is generally admitted that the glacial conditions of which I have been speaking were repeated twice, some say three times, during the Pleistocene period; while others maintain that even a larger number of glacial episodes may have occurred. Two glacial epochs, at all events, have been recognised generally both in Europe and North America. These were separated by an interglacial stage of more genial conditions, the evidence for which is steadily increasing. No one now calls in question the existence of interglacial deposits, but, as their occurrence is rather a stumbling-block in the way of certain recently resuscitated hypotheses, some attempt has been made to minimise their importance--to explain them away, in fact. It has been suggested, for example--(and the suggestion is by no means new)--that the deposits in question only show that there were local oscillations during the advance and retreat of the old ice-sheets and glaciers. This, however, is not the view of those who have observed and described interglacial beds--who know the nature of the organic remains which they have yielded, and the conditions under which the beds must have been accumulated. I need not refer to the interglacial deposits of our own country further than to remark that they certainly cannot be explained away in that summary fashion. The peat and freshwater beds that lie between the lower and upper tills in the neighbourhood of Edinburgh, for example, are of themselves sufficient to prove a marked and decided change of climate. No mere temporary retreat and re-advance of the ice-sheet will account for their occurrence. The lower till is unquestionably the bottom-moraine of an ice-sheet which, in that region, flowed towards the east. When the geographical position of the deposits in question is considered it becomes clear that an easterly flow of ice in Mid-Lothian proves beyond gainsaying that during the accumulation of the lower till all Scotland was drowned in ice. But when water once more flowed over the land-surface--when a temperate flora, composed of hazels and other plants, again appeared, it is obvious that the ice-sheet had already vanished from central Scotland. This is not the case of a mere temporary recession of the ice-front. It is impossible to believe that a temperate or even cold-temperate flora could have flourished in central Scotland at a period when thick glacier-ice mantled any portion of our Lowlands. Again, in the upper till we read the evidence of a recurrence of extreme glacial conditions--when central Scotland was once more overwhelmed by confluent ice-streams coming from the Highlands and the southern Uplands. Similar evidence of recurrent glacial conditions, I need hardly remind you, has been detected in other parts of the country. We are justified, then, in maintaining that our interglacial beds point to distinct oscillations of climate--oscillations which imply a long lapse of time. Continental observers are equally convinced that the interglacial epoch, of which so many interesting relics have been preserved over a wide region, was marked at its climax by a temperate climate and endured for a long period. The interglacial beds of northern and central Europe form everywhere marked horizons in the glacial series.

Geologists sometimes forget that in every region where glacial accumulations are well developed, good observers had recognised an upper and lower series of "drift-deposits" long before the idea of two separate glacial epochs had presented itself. Thus, in north Germany, so clearly is the Upper differentiated from the Lower Diluvium that the two series had been noted and mapped as separate accumulations for years before geologists had formulated the theory of successive ice-epochs.[BD] The division of the German Diluvium into an upper and a lower series is as firmly established as any other well-marked division in historical geology. The stratigraphical evidence has been much strengthened, however, by the discovery between upper and lower boulder-clays of true interglacial beds, containing lignite, peat, diatomaceous earth, and marine, brackish, and freshwater molluscs, fish, etc., and now and again bones of Pleistocene mammals.[BE] A similar strongly-marked division characterises the glacial accumulations of Sweden, as has been clearly shown by De Geer,[BF] who thinks that the older and younger epochs of glaciation were separated by a protracted period of interglacial conditions. In short, evidence of a break in the glacial succession has been traced at intervals across the whole width of the Continent, from the borders of the North Sea to central Russia. M. Krischtafowitsch has recently detected in the neighbourhood of Moscow[BG] certain fossiliferous interglacial beds, the flora and fauna of which indicate a warmer and moister climate than the present. The interglacial stage, he says, must have been of long duration, and separated in Russia as in western Europe two distinct epochs of glaciation.

[BD] Wahnschaffe: _Forschungen zur deutschen Landes- und Volkskunde von Dr. A. Kirchhoff_, Bd. vi., Heft 1.

[BE] For interglacial beds of north Germany see Helland: _Zeitschr. d. deutsch. geol._ Ges., xxxi., 879; Penck: _Ibid._, xxxi., 157; _Länderkunde von Europa_ (Das deutsche Reich), 1887, 512; Dames: _Samml. gemeinverständl. wissensch. Vorträge, von Virchow u. Holtzendorff:_ xx. Ser., 479 Heft; Schröder: _Jahrb. d. k. geol. Landensanst. f._ 1885, p. 219. For further references see Wahnschaffe, _op. cit._ I have not thought it worth while in this paper to refer to the interglacial deposits of our own islands. A general account of them will be found in my _Great Ice Age_, and _Prehistoric Europe_. The interglacial phenomena of the Continent seem to be less known here than they ought to be.

[BF] _Zeitschrift d. deutsch. geolog. Gesellschaft_, Bd. xxxvii, p. 197.

[BG] _Anzeichen einer interglaziären Epoche in Central-Russland_, Moskau, 1891.

No mere temporary retreat and re-advance of the ice-front can account for these phenomena. The occurrence of remains of the great pachyderms at Rixdorf, near Berlin, and the character of the flora met with in the interglacial beds of north Germany and Russia are incompatible with glacial conditions in the low-grounds of northern Europe. The interglacial beds, described by Dr. C. Weber[BH] as occurring near Grünenthal, in Holstein, are among the more recent discoveries of this kind. These deposits rest upon boulder-clay, and are overlaid by another sheet of the same character, and belong, according to Weber, to "that great interglacial period which preceded the last ice-sheet of northern Europe." The section shows 8 feet of peat resting on freshwater clay, 2 feet thick, which is underlaid by some 10 feet of "coral sand," with bryozoa. The flora and fauna have a distinctly temperate facies. It is no wonder, then, that Continental geologists are generally inclined to admit that north Germany and the contiguous countries have been invaded at least twice by the ice-sheets of two separate and distinct glacial epochs. This is not all, however. While every observer acknowledges that the Diluvium is properly divided into an upper and a lower series, there are some geologists who have described the occurrence of three, and even more boulder-clays--the one clearly differentiated from the other, and traceable over wide areas. Is each of these to be considered the product of an independent ice-sheet, or do they only indicate more or less extensive oscillations of the ice-front? The boulder-clays are parted from each other by thick beds of sand and clay, in some of which fossils have occasionally been detected. It is quite possible that such stratified beds were deposited during a temporary retreat of the ice-front, which when it re-advanced covered them up with its bottom-moraine. On the other hand, the phenomena are equally explicable on the assumption that each boulder-clay represents a separate epoch of glaciation. Until the stratified beds have yielded more abundant traces of the life of the period, our judgment as to the conditions implied by them must be suspended. It is worthy of note in this connection, however, that in North America the existence of one prolonged interglacial epoch has been well established, while distinct evidence is forthcoming of what Chamberlin discriminates as "stages of deglaciation and re-advancing ice."[BI]

[BH] _Neues Jahrbuch f. Mineralogie, Geologie, u. Palæontologie_, 1891, Bd. ii., pp. 62, 228; 1892, Bd. i., p. 114.

[BI] _Sixth Annual Report, U. S. Geol. Survey_, 1884-5, P. 315.

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]

[BS] For Scottish post-glacial glaciers see J. Geikie: _Scottish Naturalist_, Jan., 1880; _Prehistoric Europe_, pp. 386,407; Penck: _Deutsche geographische Blätter_, Bd. vi., p. 323; _Verhand. d. Ges. f. Erdkunde, Berlin_, 1884, Heft 1; Hinxman: _Trans. Edin. Geol. Soc._, vol. vi., p. 249.

[BT] "Die Eiszeit in den Pyrenäen": _Mitth. d. Vereins. f. Erdkunde_, Leipzig, 1883.

[BU] Kerner: _Mitth. k. k. geograph. Ges. Wien_, 1890, p. 307; _Sitzungsb. d. kais. Akad. d. Wissensch. in Wien_, Bd. c., Abth. i., 1891; Brückner: _X. Jahresbericht d. geograph. Ges. v. Bern_, 1891.

I have elsewhere traced the history of the succeeding stages of the post-glacial period, and brought forward evidence of similar but less strongly-marked climatic changes having followed upon those just referred to, and my conclusions, I may add, have been supported by the independent researches of Professor Blytt in Norway. But these later changes need not be considered here, and I shall leave them out of account in the discussion that follows. It is sufficient for my present purpose to confine attention to the well-proved conclusion that in early post-glacial times genial climatic conditions obtained, and that these were followed by cold and humid conditions, during the prevalence of which considerable local glaciers reappeared in certain mountain-valleys.[BV]

[BV] For a full statement of the evidence see _Prehistoric Europe_, chaps. xvi., xvii.

We speak of Pleistocene or Glacial and of Post-glacial periods as if the one were more or less sharply marked off from the other. Of course, that is not the case, and in point of fact it would be for many reasons preferable to include them under some general term. Taken together they form one tolerably well-defined cycle of time, characterised above all by its remarkable climatic changes--by alternations of cold and genial conditions, which were most strongly contrasted in the earlier stages of the period. It is further worthy of note that various oscillations of the sea-level appear to have taken place again and again both in the earlier and later stages of the cycle.

We may now proceed to inquire whether the phenomena we have been considering can be accounted for by movements of the earth's crust--a view which has recently received considerable support, more especially in America. I need hardly say that the view in question is no novelty. Many years ago, while our knowledge of the Pleistocene phenomena was somewhat rudimentary, it was usual to infer that glaciation had been induced by elevation of the land. This did not seem an unreasonable conclusion, for above our heads, at a less or greater elevation, according to latitude, an Arctic climate prevails. One could not doubt, therefore, that if a land-surface were only sufficiently uplifted it would reach the snow-line, and become more or less extensively glaciated. But with the increase of our knowledge of Pleistocene and post-glacial conditions, such a ready interpretation failed to satisfy, although not a few geologists have continued to defend the "earth-movement hypothesis," as accounting fairly well for the phenomena of the Glacial period. By these staunch believers in the adequacy of that view, it has been pointed out that elevation might not only lift lands into the region of eternal snow, but, by converting large areas of the sea-bed into land, would greatly modify the direction of ocean-currents, and thus influence the climate. What might not be expected to happen were the Gulf Stream to be excluded from northern regions? What would be the fate of the temperate latitudes of North America and Europe were that genial ocean-river to be deflected into the Pacific across a submerged Isthmus of Panama? The possibility of such changes having supervened in Pleistocene times has often been present to my mind, but I long ago came to the conclusion that they could not account for the facts. Moreover, I have never been able to meet with any evidence in favour of the postulated "earth-movements." Having carefully studied all that has been advanced of late years in support of the hypothesis in question I find myself more than ever constrained to oppose it, not only because it is grounded on no basis of fact, but because it altogether fails to explain the conditions that obtained in Pleistocene and post-glacial times.

There are various forms in which the hypothesis has appeared, and these I shall now consider seriatim, and with such brevity as may be. It has been maintained, for example, that at the advent of the Glacial period vast areas of northern and north-western Europe, together with enormous regions in the corresponding latitudes of North America, stood several thousand feet higher than at present. But when we ask what evidence can be adduced to prove this we get no satisfactory reply. We are simply informed that a glacial climate must have resulted from great elevation, and that the latter, therefore, must have taken place at the beginning of the Glacial period. Some writers, however, have ventured to give reasons for their faith. Thus Mr. W. Upham, pointing to the evidence of the fiords of North America, and to the fact that drowned river-valleys have been traced outwards across the 100-fathoms line of the marginal plateau to depths of over 3500 feet, maintains that the whole continent north of the Gulf of Mexico stood at the commencement of the Glacial period some 3000 feet at least higher than now. Of course he cites the fiords of Europe as evidence of a similar great upheaval for the northern and north-western regions of our Continent. Mr. Upham even favours the notion that during glacial times a land-connection probably existed between North America and Europe, by way of the British Islands, Iceland, and Greenland. When "this uplifting attained its maximum, and brought on the Glacial period," he says, "North America and north-western Europe stood 2500 to 3000 feet above their present height."[BW]

[BW] _American Geologist_, vi., p. 327.

That fiords are simply submerged land-valleys has long been recognised: that they have been formed mainly by the action of running water--just in the same way as the mountain-valleys of Norway and Scotland--has been the belief for many years of most students of physical geology. But it is hard to understand why they should have been cited by Mr. Upham in support of his contention, seeing that their evidence seems to militate strongly against the very hypothesis he strives to maintain. No one acquainted with the physical features and geological structure of Scotland and Norway can doubt that the valleys which terminate in fiords are of great geological antiquity. Their excavation by fluviatile action certainly dates back to a period long anterior to the advent of the Ice Age. And a like tale is told by the fiords and drowned valley-troughs of North America, which cannot be referred to so recent a period as post-Tertiary times. Those who are convinced that our continental areas have persisted throughout long æons of geological time, and that rivers frequently have survived great geological revolutions--cutting their way across mountain-elevations as fast as these were uplifted--will readily believe that some of the submarine river-troughs of North America, such as that of the Hudson, may belong even to Secondary times.[BX] It would be hard to say at what particular date the excavation of the Scottish Highland valleys commenced--but it was probably during the later part of the Palæozoic era. The process has doubtless been retarded and accelerated frequently enough, during successive movements of depression and elevation, but it was practically completed before the beginning of Pleistocene times, and that is all that we may trouble about here. Precisely the same conclusion holds good for Norway: and such being the case it is obvious that the question of the origin and age of the fiords has no bearing on the problem of the glacial climate and its cause. In point of fact the evidence, as already remarked, tells against the "earth-movement hypothesis," for it shows us that, during a period when Europe and North America stood several thousand feet higher, and extended much further seawards, rivers, and not glaciers, were the occupants of our mountain-valleys. It was not until all those valleys had come to assume much the appearance they now present that general glaciation supervened.

[BX] Professor Dana inclines to date the erosion of the Hudson trough so far back as the Jura-Trias period.--_American Journ. Science_, xl., p. 435.

We are not without direct evidence, however, as to the geographical conditions that obtained in the ages that immediately preceded the Pleistocene period. The distribution of the Pliocene marine beds of Britain entitles us to assume that at the time of their accumulation our lands did not extend quite so far to the south and east as now. The absence of similar deposits from the coast-lands of North America is supposed to support the view of great continental elevation in pre-glacial times. All it seems to prove, however, is that in Pliocene times the North American continent was not less extensive than it is at present. It is even quite possible that in glacial times pre-existing Pliocene beds may have been ploughed out by the ice, just as seems to have been the case in the north-east of Scotland. But without going so far back as Pliocene times, we meet with evidence almost everywhere throughout the maritime regions of the glaciated areas of Europe and North America, to show that immediately before those tracts became swathed in ice the geographical conditions were much the same as at present. The shelly boulder-clays in various parts of our islands, and the similar occurrence of marine and brackish-water shells in and underneath the Diluvium of north Germany, etc., prove clearly enough that just before the coming-on of glacial conditions neither Britain nor the present maritime lands of the Continent were far removed from the sea. It is true that the buried river-channels of Scotland indicate a pre-glacial elevation of some 200 or 300 feet above the existing sea-level, but it is quite certain that the Minch, St. George's Channel, the Irish Sea, the North Sea, and the Baltic were all in existence at the commencement of the Glacial period. And we are led to similar conclusions with regard to the geographical conditions of North America at that time, from the occurrence of marine shells in the boulder-clays of Canada and New England. We note indeed that there is abundant evidence of land-submergence during glacial times. Indeed, we may say that the Pleistocene marine deposits of northern latitudes are almost invariably indicative of colder conditions than now obtain.

If it be true that cold climatic conditions were contemporaneous in our latitude with submergence, it is equally true that an extensive land-surface in north-west Europe has, sometimes at least, co-existed with markedly genial conditions. In Tertiary times, for example, as the Oligocene deposits of Scotland, the Faröe Islands, Iceland, and Greenland testify, a land-connection existed between Europe and the North American continent. Again, it has been shown that during the interglacial phase of the Pleistocene period Britain was continental, and enjoyed at the time a peculiarly genial climate. And somewhat similar geographical and climatic conditions again supervened in post-glacial times. In other words, when the land was more elevated and extensive than now, it enjoyed a warmer climate. Nor can we escape the conclusion that the excavation of the fiord-valleys of northern latitudes, which is a very old story (far older than the Pleistocene), was the work not of glaciers but of running water, at a time when north-western Europe and the corresponding regions of America were much more elevated than they are now.

Thus there appears to be no evidence either direct or indirect in favour of the view that glacial conditions were superinduced by great continental elevation. But it may be argued that even although no evidence can be cited in proof of such elevation, still, if the glacial phenomena can be well explained by its means, we may be justified in admitting it as a working hypothesis. Movements of elevation and depression have frequently taken place--the Pleistocene marine deposits themselves testify to oscillations of the sea-level--and there can be no objection, therefore, to such postulations as are made by the hypothesis under review. All this is readily granted, but I deny that the conditions that obtained in Pleistocene times can be accounted for by elevation and depression. Let us see how the desiderated elevation of northern lands would work. Were north-western Europe and the corresponding latitudes of North America to be upheaved for 3000 feet, and a land-passage to obtain between the two continents by way of the Faröe Islands, Iceland, and Greenland, how would the climate be affected? It is obvious that under such changed conditions the elevated lands in higher latitudes might well be subjected to more or less extensive glaciation. Norway would become uninhabitable and glaciers might well appear in the mountain-valleys of Scotland. But it may be doubted whether the climate of France and Spain, or the corresponding latitudes of North America, would be much affected. For were a land-passage to appear between Britain and Greenland no Arctic current would flow into the North Atlantic, while no portion of the Gulf Stream would be lost in Arctic seas. The North Atlantic would then form a great gulf round which a warm ocean-current would circulate. The temperature of that sea, therefore, would be raised and the prevailing westerly and south-westerly winds of Europe would be warmer than now. However much such warm moist winds might increase the snow-fall in North Britain and Scandinavia, we cannot suppose they could have much influence in central and southern Europe, and in North Africa; and still less could they affect the climate of Asia Minor and the mountainous regions of the far east, in most of which evidence of extensive glaciation occurs. And how, we may ask, could the postulated geographical changes bring about the glaciation of the mountainous tracts on the Pacific sea-board? In fine, we may conclude that however much the geographical changes referred to might affect north-western Europe and north-eastern America, they are wholly insufficient to account for the glacial phenomena of other regions. The continuous research of recent years has shown that the lowering of temperature of glacial times was not limited to the lands which would be affected by any such elevation as that we are considering. A marked and general displacement of climatic zones took place over the whole continent of Europe; and similar changes supervened in North America and Asia. Are we then to suppose that all the lands within the Northern Hemisphere were extensively and contemporaneously upheaved?

We may now consider another form of the "earth-movement hypothesis." It has frequently been suggested that our glacial phenomena may have been caused by the submergence of the Isthmus of Panama, and the deflection of the Equatorial Current into the Pacific. But it may be doubted whether a submergence of that isthmus, unless very extensive indeed, would result in more than a partial escape of Atlantic water into the Pacific basin. The Counter Current of the Pacific which now strikes against the isthmus might even sweep into the Caribbean Sea, and join the Equatorial on its way to the Gulf of Mexico. But putting that consideration aside, what evidence have we that the Isthmus of Panama was submerged during the glacial epoch? None whatsoever, it may be replied. It is only a pious opinion. Considerable movements of elevation and depression of the islands in the Caribbean Sea would seem to have taken place at a comparatively recent date, but those movements may quite well belong to Pliocene times. Whether they be of Pliocene or Pleistocene age, however, no one has yet proved that the Isthmus of Panama was sufficiently submerged, either at the one time or the other, to permit the escape of the Atlantic Equatorial into the Pacific basin. But let it be supposed that the isthmus has become so deeply submerged that the Equatorial Current is wholly deflected, and that no Gulf Stream issues through the Straits of Florida to temper the climate of higher latitudes. What would result from such an unhappy change? Can any one conversant with the geographical distribution of the glacial phenomena imagine that the conditions of the Glacial period could be thus reproduced? Norway might indeed become a second south Greenland, and perennial snow and ice might appear in the mountainous tracts of the British Islands. The climate of Hudson's Bay and the surrounding lands might be experienced in the Baltic and its neighbourhood, and what are now the temperate latitudes of Europe, north of the 50th parallel, would possibly approach Siberia in character. But surely these changes are not comparable to the conditions of the Glacial period. The absence of a Gulf Stream would not sensibly affect the climate of south-eastern Europe and Asia, and could not have the smallest influence on that of the Pacific coast-lands of North America.