The action of a mass of glacier-ice, reaching a thickness of several thousand feet, must necessarily have resulted in extensive erosion of the rocks over which it passed. Everywhere, therefore, throughout the vast area just indicated, we meet with evidence of severe erosion. But, as one should expect, such erosion is most marked in the hilly regions—in those areas where steep slopes induced more rapid motion of the ice, and where projecting crags and hills opposed the advance of the eroding agent. All such prominent obstructions were energetically assailed—abraded, rounded, worn, and smoothed, or crushed, shattered, dislocated, and displaced. The high-grounds of Scandinavia and Finland, formed for the most part of tough, crystalline rocks, or of more or less durable strata, show everywhere roches moutonnées—smoothed and rounded rocks—while innumerable rock-basins have been scooped out in front of prominent crags and hills. In Denmark and other countries, where less durable rocks prevail, the strata have often been broken and disrupted, and pushed out of place. But as regions formed of such rocks are generally gently-undulating, and seldom show abrupt crags and hills, they oppose few obstructions to the advance of an ice-sheet. When the northern ice-sheet flowed into Russia and Germany, it crept over a low-lying and, for the most part, gently-undulating surface; and although here and there the form of the ground favoured glacial erosion and disruption, and extensive displacements of rock-masses took place, yet, upon the whole the low-lying regions referred to became areas of accumulation. The sub-glacial detritus—ground out or wrenched away from the rough Scandinavian plateau and the uplands of Finland—was dragged on underneath the ice, and spread over the great plains lying to the south-east and south, as the gradually attenuated ice-sheet crawled to its terminal line. My friend Dr. Amund Helland, the well-known Norwegian geologist, has made an estimate of the amount of rock-débris derived from Scandinavia and Finland which lies scattered over the low-grounds of northern Europe. According to him, the area in Denmark, Holland, Germany, and Russia (exclusive of Finland), over which northern detritus is scattered, contains about 2,100,000 square kilometres, and the average thickness of the deposits is about 150 feet, of which, however, only two-thirds, or 100 feet, are of northern origin, the remaining third consisting of local materials. Taking, then, 100 feet as fairly representing the average thickness of the rock-rubbish derived from Finland and Scandinavia, the area of which is given as 800,000 square kilometres, there is enough of this material to raise the general surface of those lands by 255 feet. The same amount of material would suffice to fill up all the numerous lakes of Finland and Sweden sixteen or seventeen times over. Or, if tumbled into the Baltic, it would fill the basin of that sea one and a half times. In short, enough northern rock-débris lies upon the low-grounds of northern Europe, which, were it restored to the countries from which it has been taken, would obliterate all the lake-hollows of Finland and Sweden, raise the level of those lands by 80 feet, and fill up the entire basin of the Baltic, with all its bays. And yet this estimate leaves out of account all the material which the ice-sheet carried away from Norway and the British Islands.
Of the glaciation of our own land I need say very little. The configuration of our country necessarily made it a centre of dispersion during the Ice Age, and the ice which covered Ireland, Scotland, and the major portion of England radiated outwards from the dominant elevations of the land. But as the ice creeping outwards from those centres became confluent, the directions which it followed were often considerably modified, especially upon the low-grounds. We know that the British ice-sheet not only covered the land up to near the tops of our higher mountains, but filled up all our seas and extended into the Atlantic beyond the coasts of Ireland and the Outer Hebrides—these latter islands having been glaciated from the east by the ice that flowed outwards from the mainland. Another point upon which we are now well assured is the fact that the British and Scandinavian ice-sheets coalesced, so that the basin of the North Sea completely brimmed over with glacier-ice.
Finally, then, in contemplating the physical conditions that obtained in northern Europe at the climax of the Ice Age, we have to picture to ourselves the almost total obliteration under a vast ice-sheet of all the land-features of the British Islands, Scandinavia, and Finland, and the adjacent low-lying tracts of Denmark, Holland, Germany, Poland, and Russia. If at that distant date a prehistoric man could have stood on the summit of Snaehatten, he would have seen an apparently interminable plain of snow and ice, bounded only by the visible horizon. Could he have followed the plain southwards in hopes of escaping from it, he would have descended its gently-sloping surface by imperceptible gradations for a distance of 700 miles, before he reached its termination at the foot of the mountains of middle Germany. Or, could he have set out upon an easterly course, he would have crossed the Gulf of Bothnia, buried several thousand feet beneath him, and touched the foot-slopes of the Ural Mountains before he gained the terminal front of the ice-cap, a distance of 1600 miles. On the other hand, had he walked south-west in the direction of Ireland, he would have traversed the area of the North Sea at a height of several thousand feet above its bed, and, crossing the British area, would only have reached the ice-front at a point some 50 miles beyond the coast of Ireland. Here he would have seen the ice-sheet presenting a steep face to the assaults of the Atlantic, and breaking away in massive tabular bergs, like those which are calved by the ice-cap of the Antarctic regions.
I must now pass rapidly in review the facts relating to the glaciation of the mountainous regions which lay outside of the area covered by the northern ice-sheet. The glaciers of the Alps of Switzerland, about which so much has been written, and the study of which first gave Venetz, Charpentier, and Agassiz the clue to the meaning of striated rocks, boulder-clay, and erratics, are, as is well known, the puny descendants of former gigantic ice-flows. At the culmination of the Ice Age all the mountain-valleys of Switzerland and northern Italy were choked with glaciers that streamed out upon the low-grounds. Along the northern slopes of the Alps, as in Bavaria and Würtemberg, these glaciers coalesced to form a considerable ice-sheet, and so likewise did the glaciers that descended from Switzerland, Savoy, and Dauphiny, into the great valley of the Rhone. Even in north Italy the same was the case with the glaciers that occupied the valleys in which now lie Lakes Orta, Maggiore, Varese, Lugano, and Como—the united ice-flows of those valleys forming a glacier which deployed upon the plains of the Po, with a frontage of not less than 40 miles.
To the north of the Alps, the Vosges Mountains and the Black Forest, the Harz, the Erz Gebirge, the Riesen Gebirge, and the Böhmer-Wald—all had their perennial ice and glaciers, although none of those elevated tracts now reaches the snow-line. It was the same with the Carpathians and the Urals, amongst which we meet with relics of much larger ice-streams than any that now exist in the Alps. Considerably further south were the glaciers of the Despoto Dagh of Roumelia. Great glaciers also in former times descended from the Caucasus, and in many hilly regions of Asia Minor indubitable traces of similar large ice-flows have been detected. The high-grounds of central France, and the mountains of Beaujolais and Lyonnais supported considerable glaciers, while from the Pyrenees numerous glaciers of the first class flowed out upon the low-grounds of France, and considerable ice-streams occupied the mountain-valleys on the Spanish side. Other Peninsular chains—the Serra da Estrella, the Sierra Guadarama, and the Sierra Nevada—had likewise their snow-fields and ice-streams. The same was the case with the Apennines and the Apuan Alps of Italy, the traces of former glacial action being conspicuous over a considerable part of Tuscany. Even in Corsica we encounter the same evidence of glaciation—striated rock-surfaces and moraines—which point to the former descent of considerable glaciers from Monte Rotondo.
But rock-striæ and moraines are not the only proofs of former cold and humid conditions having prevailed over middle and southern Europe at the climax of the glacial period. The limestone-breccias of Gibraltar have been described by Professor Ramsay and myself, and we have shown that these could only have been formed under the influence of excessive frost and melting snows. The limestone of the Rock has been broken up along the ridge, and its fragments showered down the slopes, at a time when these were more or less thickly covered with snow. Resting upon and imbedded in this snow, the rock-rubbish would be carried downward and outward during the gradual melting that took place in summer. And in this way immense accumulations of débris were borne forwards over the low-grounds that extended from the base of the Rock into regions which are now partially submerged. Breccias which have probably had a similar origin occur also in Corsica, Malta, and Cyprus, and doubtless they will yet be recognised in many other places. Again, over wide areas in northern France and the south of England, we meet with extensive sheets of earthy clay and rock-rubbish, which have certainly been heaped up under very different conditions of climate than obtain now. This stony earth has evidently travelled down the gentle slopes of the land, under the influence of frost and melting snow, in much the same way as ice-driven rock-rubbish and soil move slowly down the slopes of such dreary regions as Patagonia and certain low-lying tracts within the Arctic Circle.
II.
Changes of Climate in Europe during the Ice Age.
We come next to the very interesting question of alternations of climate during the Ice Age. The evidence under this head has accumulated to such an extent within recent years as to convince most students of Pleistocene geology that very extensive changes of climate characterised the glacial period. How many such changes took place we are not yet in a position to say, but we know that the intensely arctic condition of things which has just been described was interrupted more than once by what have been termed “interglacial epochs,” during which a mild and genial climate prevailed over middle and northern Europe. For some time it was believed that such “interglacial epochs” had only a local significance, that they bespoke mere transitory retreats of the ice-fields, such as are known to have taken place within historical times in the glacier-valleys of the Alps. But increased observation and reflection have shown that this explanation of the phenomena of “interglacial beds” will not suffice. It is impossible to enter here upon details, but I may briefly state that the evidence in question is two-fold. First, we have the stratigraphical evidence. We have ascertained the existence, over wide areas in this and other glaciated countries, of several successive sheets of boulder-clay, which are often separated from each other by fossiliferous aqueous strata. It has been demonstrated that each of these sheets of sub-glacial detritus is the accumulation of a separate and distinct ice-flow. Second, we have the evidence of fossil organic remains. We find, for example, that the flora which covered the low-grounds of middle and temperate Europe during a certain stage of the glacial or Pleistocene period, consisted of plants which are now restricted to the tops of our mountains and to northern Scandinavia. The characteristic fauna associated with that flora embraced the reindeer, glutton, mammoth, woolly rhinoceros, Arctic fox lemming, chamois, and so forth. We know, indeed, that man hunted the reindeer and the mammoth in the south of France. Similar testimony to the coldness and humidity of the climate is borne by the land- and freshwater shells which occur in certain Pleistocene deposits in Italy, Corsica, southern France, Switzerland, Germany, etc. That this flora and fauna were contemporaneous with the great glaciation of our Continent has been as well ascertained as the fact of the Roman occupation of Britain. But if the evidence of organic remains strongly confirms and supports that supplied by the distribution of glacial deposits in Europe, no less forcibly does it corroborate the physical evidence as to the former existence of a warm and genial interglacial climate. During interglacial times a most abundant mammalian fauna roamed over all temperate Europe—a fauna comprising such animals as Irish deer, urus, bison, horse, stag, saiga, brown bear, grisly bear, several species of elephant, rhinoceros, and hippopotamus, hyæna, lion, leopard, etc. A like tale of genial conditions is told by the land- and freshwater shells, which occur in some of the Pleistocene deposits of England, France, Belgium, Germany, Switzerland, and Italy. The testimony of the associated flora is just as striking. How genial and equable must have been the climate which permitted plants like the Canary laurel, the Judas-tree, the fig-tree, and others to flourish side by side in the north of France, with such forms as the hazel, willow, ash, and sycamore! The most noteworthy additions to our knowledge of interglacial conditions which have recently been made are the results obtained by M. Gaudry in the valley of the Seine, and by Dr. Penck in Bavarian Tyrol, the latter of whom has shown that there have been at least three great advances of the Alpine glaciers, separated by long-continued mild conditions, during which the glaciers receded far into the mountains.
It is interesting to observe that we have, especially in our own islands, good evidence to show that during the glacial period considerable oscillations of the relative level of land and sea took place. Thus, it has been ascertained, that just before the latest epoch of extensive glaciation, the British Islands were largely submerged in the sea. To what depth this remarkable submergence was carried we do not know, because any marine deposits which may have been accumulated at that time over the drowned country were for the most part obliterated by the action of the ice-sheet which subsequently covered and reglaciated our lands.[L] But the few fragments of such marine deposits as have been preserved show us that the depression reached more than 500 feet in Scotland (i.e., measured from the present sea-level), and exceeding 1000 feet in Wales and Ireland. We note, then, in passing, that the only great Pleistocene submergence of these lands of which geologists have any knowledge took place before the appearance of the last general ice-sheet that overflowed our low-grounds. The submergences of a later date were of inconsiderable importance, hardly exceeding 100 feet or thereabouts below the present sea-level. The latest occupant of our islands and of northern Europe was not the sea, but ice. The “Palæocrystic Sea,” which we have been recently assured would account for our glacial phenomena, is of “the stuff that dreams are made of.” There is not a jot or tittle of evidence for the former existence of such a sea over any part of Britain or the continent of Europe.