The interglacial period of marine depression in Britain is represented in Switzerland by the lignite beds of Dürnten, Utznach, and Pfaffikon, the last of which rests upon and is covered by the boulder drift. The fossil remains from Dürnten, identified by Dr. Falconer and Prof. Rütimeyer, prove that two southern animals, Elephas antiquus and Rhinoceros megarhinus, inhabited the district in the interval between the retreat of one set of glaciers and the advance of another. They probably migrated from the plains of Lombardy, where they abounded in the pleistocene age.
Europe invaded by Pleistocene Mammals before the Glacial Period.
What is the precise relation of the pleistocene mammals to these two periods of cold? Did they invade northern and central Europe during the first or the second, before or after, the marine submergence indicated by the “middle drift?” We might expect, à priori, that as the temperature became lowered, the northern mammalia would gradually invade the region occupied before by the pleiocene forms, and that the reindeer, the mammoth, and woolly rhinoceros would gradually supplant the southern Rhinoceros Etruscus and Elephas meridionalis. Traces of such an occupation would necessarily be very rare, since they would be exposed to the grinding action both of the advancing glacial sheet, and subsequently to that of the waves on the littoral zone during the depression and re-elevation of the land. At the time also that the greater part of Great Britain was buried under an ice-sheet, it could not have been occupied by animals, although they may have been, and most probably were, living in the districts farther to the south, which were not covered by ice. The labours, however, of Dr. Bryce, Prof. Archibald Geikie, and others prove that one at least of the characteristic pleistocene mammalia—the mammoth—lived in Scotland along with the reindeer before the deposit of the lower boulder-clay; while Mr. Jamieson has pointed out that it could not have occupied that area at the same time as the ice, and therefore must be referred to a still earlier date.[266] The teeth and bones discovered in the ancient land surface at Selsea, under the boulder drift, also very probably indicate that the mammoth lived in Sussex before the glacial submergence, although they were never admitted by Dr. Falconer to be of the same age as the remains of Elephas antiquus from the same preglacial horizon. The animal also occurs in the preglacial forest-bed of Norfolk and Suffolk. On a careful examination of the whole evidence, I am compelled to believe, with Mr. Godwin-Austen and Prof. Phillips, that the à priori belief that the pleistocene mammalia occupied Great Britain before the period of the ice-sheet and submergence is fully borne out by the few incontestable proofs that have been brought forward of the remains being found in preglacial deposits. And the scanty evidence on the point is just what might be expected from the rare accidents under which the bones in superficial deposits could have withstood the grinding of the ice-sheet, and the subsequent erosive action of the waves on the coast-line. It may therefore be concluded, that the pleistocene mammalia arrived in Europe before the temperature had reached its minimum in the glacial period. On the other hand, the occurrence of mammaliferous river strata, either in hollows of the boulder-clay as at Hoxne, or in valleys excavated after its deposition as at Bedford, prove that the characteristic animals occupied Britain after the retreat of the ice-sheet, and after the re-emergence of the land from beneath the glacial sea.
Mammalia lived in Britain during the Second Ice or Glacial Period.
The distribution of the animals in the river deposits gives us a clue to the physical geography during the second ice period. In an essay read before the Geological Society in 1869, and in a second printed in the “Popular Science Review” in 1872, I showed that there was a singular irregularity in the contents of the river strata, and that while the fossil mammalia were abundant throughout the area (marked with dots in the map, [Fig. 126]), there were certain districts in which they had not been met with. One of these barren areas comprises (plain in the map, [Fig. 126]), nearly the whole of Wales. A second includes a large portion of Lancashire, Yorkshire, Cumberland, Westmoreland, and the whole of Scotland (if the preglacial mammals in the low district between the Frith of Forth and Frith of Clyde in the map be omitted), and a third is represented by nearly the whole of Ireland. These areas are remarkable for the absence of the mammalia from the river deposits. They are also characterised by the freshness of the ice marks which they present. Nearly every valley has its own system of grooves and its own set of moraines; and the mounds of clay and marl left behind by the local glacier, as it slowly retreated to higher levels till it finally disappeared, are to be observed in great abundance. If we bring these facts into relation, the barrenness of the areas may be reasonably explained by the presence of glaciers, while the pleistocene mammals were living in the south and east (see map, [Fig. 126]). A barrier of some kind may reasonably be inferred to have prevented their range over those districts, and its nature is indicated by the ice marks. It is very probable that these glaciers had not passed away before the close of the pleistocene age: for in that case the characteristic animals would be discovered in the river gravels, which are later than the deposits of local glaciers in those districts.
The Glacial Period does not separate one Life-era from another.
The lowering of the temperature which culminated in the glacial period has left palpable traces behind in the changes which it caused in the European fauna. As the pleiocene climate became colder, the animals unfitted to endure the cold, such as the deer of the Indian types of Axis and Rusa, either migrated to the south or became extinct, while their feeding-grounds were invaded by the dwellers in the temperate zone, the stag, roe, bison, and other animals. These in their turn were pushed forward by the arctic group of animals, the musk-sheep, lemming, reindeer, and others, the progress being in the main steadily to the south while the cold was increasing, and the retreat being steadily to the north while it was decreasing. It will follow from this, that the same district in central or north-western Europe would be traversed by these migratory bodies of animals, both in their southern advance in preglacial and glacial times and their northern retreat in postglacial times, and that, therefore, their fossil remains cannot afford a means of fixing the preglacial, glacial, or postglacial, age of the deposit in which they are found, where it is not marked by traces of glaciation. Sir Charles Lyell’s view, that the glacial period cannot be taken as a landmark in the classification of the European pleistocene deposits, is fully borne out by the facts, and still less can it be taken as a hard and fast line between one fauna and another. It cannot be considered a life-era like the eocene, meiocene, pleiocene, or prehistoric divisions of the tertiary period.
Bone-caves inhabited before and after Ice Period.
If we allow that the lowering of the temperature was the principal cause of the presence of temperate and arctic animals, in a region before inhabited by species fitted to live in a comparatively warm climate, it will follow that bone-caves cannot be said to be either pre- or postglacial, by an appeal to their fossil mammalia. If they were open before the minimum of temperature was reached, they would afford shelter to the animals then in the neighbourhood, and they would continue to be occupied in the south during the vast period of time represented by the enormous physical changes in the region north of the line of the Thames, during the development of the ice-sheet, the submergence and the re-elevation of nearly the whole of Britain and Ireland. As, however, the cold increased, the percentage of arctic animals would also increase, and the more temperate species be weeded out. For these reasons it has seemed to me, that the machairodus of Kent’s Hole, and the Rhinoceros megarhinus of Oreston, represent an early stage of the pleistocene period, before the arctic mammalia were present in full force in the caves. It is very probable that vast herds of reindeer lived in the south of France, while northern Britain lay buried under the ice-sheet, as well as during the two succeeding physical changes.