One of the proofs of the gradual lowering of temperature toward the close of Pliocene times in Europe is the southward retreat and disappearance of the apes and monkeys; the Upper Miocene gibbon is found as far north as Eppelsheim, near Worms, Germany; in Lower Pliocene times the monkeys and apes are found only in the forests of the south of France; in Upper Pliocene times they are recorded only in the forests of northern Italy; the evidence, so far as it goes, indicates a gradual retreat toward the south.

Finally, at the end of the Pliocene there existed very close geographic relations eastward with the mammalian life of India by way of what was then the isthmus of the Dardanelles and southward with the mammalian life of Africa by way of the Sicilian land bridge. This would indicate that the long lines of eastward and westward migration were open and favorable to the arrival in western Europe of new migrants from the far east, including perhaps the most primitive races of man. There is not the least evidence that Pliocene man or ancestors of man existed in Europe, excepting such as may be afforded by the problematic eoliths, or most primitive flints.

The First Glaciation

In Upper Pliocene times cold marine currents[(6)] from the north began to flow along the southeastern coast of England, with indications of a gradually lowering temperature culminating at a time when the sea abounded in the arctic mollusks, which have been preserved in the 'Weybourn Crags,' a geologic formation along the coast of Norfolk. This arctic current was the herald of the First Glacial Stage.

It does not appear that a glacial cap of any considerable extent was formed in Great Britain at this stage, but about this time the first great ice-cap was formed in British North America west of Hudson Bay, which sent its ice-sheets as far south as Iowa and Nebraska. In the latter State forests of spruce and other coniferous species indicate the appearance of a cool temperate flora in advance of the glaciation. In the Swiss Alps the snow descended 1,200 meters below the present snow-line, and in Scandinavia and northern Germany the first great ice-sheets were formed from which flowed the glaciers and rivers conveying the 'Old Diluvium,' or the 'oldest drift.' Accompanying the cold wave along the eastern coast of England we note, in the famous fossil deposits known as the 'Forest Bed of Cromer,' which overlie the Weybourn Crags, the arrival from the north of the fir-tree (Abies). This is most significant, because it had hitherto been known only in the arctic region of Grinnell Land, and this was its first appearance in central Europe. Another herald of northern conditions was the first occurrence of the musk-ox in England, which is attributed[(7)] to the 'Forest Bed' deposits.

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Fig. 25. The First (Günz) Glacial Stage was far less extensive than that in the above map, which shows Europe in the Second Glacial Stage, during the greatest extension of the ice-fields and glaciers (dots), a period of continental depression in which the Mediterranean, Black, and Caspian Seas were connected. The line from Scandinavia to the Atlas Mountains corresponds with the section shown in Fig. 13, p. 37. Drawn by C. A. Reeds, after James Geikie and Penck.

While Great Britain was less affected at this time than other regions, there is no doubt as to the vast extent of the First Glacial Stage in British America, in Scandinavia, and in the Alps; in the latter region it has been termed 'the Günz stage' by Penck and Brückner. The 'drift' deposits have a general thickness of 98½ feet (30 m.), but they are largely covered and buried by those of the far more extensive Second Glacial Stage. The Scandinavian ice-sheet[(8)] not only occupied the basin of the Baltic but overflowed Scania—the southern part of Sweden—and extended as far south as Hamburg and Berlin. In the Alps the glaciers passed down all the great mountain valleys to the low grounds of the foreland, implying a depression of the snow-line to 4,000 feet below its present level.