Above the forest bed and fluvio-marine series comes the well-known unstratified Norwich boulder till, containing immense blocks 6 or 8 feet in diameter, many of which must have come from Scandinavia, and above the unstratified till are a series of contorted beds of sand and gravel. This series may be considered to represent a period of intense glaciation. Above this again comes the middle drift of Mr. Searles Wood, junior, yielding shells which indicate, as is now generally admitted, a comparatively mild condition of climate. Upon this middle drift lies the upper boulder clay, which is well developed in South Norfolk and Suffolk, and which is of unmistakable glacial origin. Newer than all these are the Mundesley freshwater beds, which lie in a hollow denuded out of the foregoing series. In this formation a black peaty deposit containing seeds of plants, insects, shells, and scales and bones of fishes, has been found, all indicating a mild and temperate condition of climate. Among the shells there is, as in the forest bed, the Paludina marginata. And that an arctic condition of things in England followed is believed by Mr. Fisher and others, on the evidence of the “Trail” described by the former observer.
Cave and River Deposits.—Evidence of the existence of warm periods during the glacial epoch is derived from a class of facts which have long been regarded by geologists as very puzzling, namely, the occurrence of mollusca and mammalia of a southern type associated in England and on the continent with those of an extremely arctic character. For example, Cyrena fluminalis is a shell which does not live at present in any European river, but inhabits the Nile and parts of Asia, especially Cashmere. Unio littoralis, extinct in Britain, is still abundant in the Loire; Paludina marginata does not exist in this country. These shells of a southern type have been found in post-tertiary deposits at Gray’s Thurrock, in Essex; in the valley of the Ouse, near Bedford; and at Hoxne, in Suffolk, associated with a Hippopotamus closely allied to that now inhabiting the Nile, and Elephas antiquus, an animal remarkable for its southern range. Amongst other forms of a southern type which have been met with in the cave and river deposits, are the spotted hyæna from Africa, an animal, says Mr. Dawkins, identical, except in size, with the cave hyæna, the African elephant (E. Africanus), and the Elephas meridionalis, the great beaver (Trogontherium), the cave hyæna (Hyæna spelæa), the cave lion (Felis leo, var. spelæa), the lynx (Felis lynx), the sabre-toothed tiger (Machairodus latidens), the rhinoceros (Rhinoceros megarhinus and R. leptorhinus). But the most extraordinary thing is that along with these, associated in the same beds, have been found the remains of such animals of an arctic type as the glutton (Gulo luscus), the ermine (Mustela erminea), the reindeer (Cervus tarandus), the musk-ox or musk-sheep (Ovibos moschatus), the aurochs (Bison priscus), the woolly rhinoceros (Rhinoceros tichorhinus), the mammoth (Elephas primigenius), and others of a like character. According to Mr. Boyd Dawkins, these southern animals extended as far north as Yorkshire in England, and the northern animals as far south as the latitude of the Alps and Pyrenees.[122]
The Explanation of the Difficulty.—As an explanation of these puzzling phenomena, I suggested, in the Philosophical Magazine for November, 1868, that these southern animals lived in our island during the warm periods of the glacial epoch, while the northern animals lived during the cold periods. This view I am happy to find has lately been supported by Sir John Lubbock; further, Mr. James Geikie, in his “Great Ice Age,” and also in the Geological Magazine, has entered so fully into the subject and brought forward such a body of evidence in support of it, that, in all probability, it will, ere long, be generally accepted. The only objection which has been advanced, so far as I am aware, deserving of serious consideration, is that by Mr. Boyd Dawkins, who holds that if these migrations had been secular instead of seasonal, as is supposed by Sir Charles Lyell and himself, the arctic and southern animals would now be found in separate deposits. It is perfectly true that if there had been only one cold and one warm period, each of geologically immense duration, the remains might, of course, be expected to have been found in separate beds; but when we consider that the glacial epoch consisted of a long succession of alternate cold and warm periods, of not more than ten or twelve thousand years each, we can hardly expect that in the river deposits belonging to this long cycle we should be able to distinguish the deposits of the cold periods from those of the warm.
Shell Beds.—Evidence of warm inter-glacial periods may be justly inferred from the presence of shells of a southern type which have been found in glacial beds, of which some illustrations follow.
In the southern parts of Norway, from the present sea-level up to 500 feet, are found glacial shell beds, similar to those of Scotland. In these beds Trochus magus, Tapes decussata, and Pholas candida have been found, shells which are distributed between the Mediterranean and the shores of England, but no longer live round the coasts of Norway.
At Capellbacken, near Udevalla, in Sweden, there is an extensive bed of shells 20 to 30 feet in thickness. This formation has been described by Mr. Gwyn Jeffreys.[123] It consists of several distinct layers, apparently representing many epochs and conditions. Its shells are of a highly arctic character, and several of the species have not been found living south of the arctic circle. But the remarkable circumstance is that it contains Cypræa lurida, a Mediterranean shell, which Mr. Jeffreys, after some hesitation, believed to belong to the bed. Again, at Lilleherstehagen, a short distance from Capellbacken, another extensive deposit is exposed. “Here the upper layer,” says Mr. Jeffreys, “gives a singular result. Mixed with the universal Trophon clathratus (which is a high northern species, and found living only within the arctic circle) are many shells of a southern type, such are Ostrea edulis, Tapes pullastra, Corbula gibba, and Aporrhais pes-pelicani.”
At Kempsey, near Worcester, a shell bed is described by Sir R. Murchison in his “Silurian System” (p. 533), in which Bulla ampulla and a species of Oliva, shells of a southern type, have been found.
A case somewhat similar to the above is recorded by the Rev. Mr. Crosskey as having been met with in Scotland at the Kyles of Bute. “Among the Clyde beds, I have found,” he says, “a layer containing shells, in which those of a more southern type appear to exist in greater profusion and perfection than even in our present seas. It is an open question,” he continues, “whether our climate was not slightly warmer than it is now between the glacial epoch and the present day.”[124]
In a glacial bed near Greenock, Mr. A. Bell found the fry of living Mediterranean forms, viz., Conus Mediterraneus and Cardita trapezia.
Although deposits containing shells of a temperate or of a southern type in glacial beds have not been often recorded, it by no means follows that such deposits are actually of rare occurrence. That glacial beds should contain deposits indicating a temperate or a warm condition of climate is a thing so contrary to all preconceived opinions regarding the sequence of events during the glacial epoch, that most geologists, were they to meet with a shell of a southern type in one of those beds, would instantly come to the conclusion that its occurrence there was purely accidental, and would pay no special attention to the matter.