Glacial Period

Glacial Period, the earlier portion of the Pleistocene (q.v.) period, during which a gradual refrigeration of climate, of which we have evidence in the mollusca of the Pliocene, became intensified in the northern hemisphere until an ice-cap descended from the pole into the centres of Europe and North America, to Saxony in the one case and to about lat. 39° N. in the other. The underlying rocks, where hard enough to retain them, show not only polished surfaces, but striee, evidently the work of land-ice, mainly in one direction in each district. In the thick accumulation of "till" or unstratified boulder-clay (q.v.), which commonly rests on these polished and striated surfaces, the boulders derived from a distance are found to have come from the direction indicated by the striee. The ice-cap seems to have been from 6,000 to 7,000 feet thick in Norway, filling the Baltic, the German Ocean, and the Irish Sea; 5,000 feet in the northwest of Scotland; and nearly 1,500 feet in the Harz. Scandinavian boulders are frequent in the boulder-clay of East Anglia. The southern margin of the sheet seems in Europe to have passed from the neighbourhood of Nijni Novgorod and Kieff across Galicia and Silesia to the Riesen-Gebirge and Erz-Gebirge, the Harz, Hanover, Holland, the Thames, and the Bristol Channel. That of the American sheet is marked by a series of mounds or kames that have been traced from the coast of Massachusetts for over 3,000 miles across the continent.

Beds of peat and stratified sands and clays occurring at various levels in the boulder-clay, and containing the bones of land animals, point apparently to various prolonged episodes of a more genial climate known as interglacial periods. Thus, whilst owing to the cold we find the arctic species of birch and willow far south of their present limits, Pecten islandicus and other arctic shells in Scotland, the woolly mammoth (q.v.) in Italy, the reindeer in Switzerland, and the musk ox in the Pyrenees, during these warmer periods we find plants of temperate latitudes migrating to Siberia, and the hippopotamus, lion, hyaena, and porcupine travelling northwards into Central Europe.

Whilst during the culmination of the Great Ice Age or period of first glaciation, much of northwest Europe seems to have been at a higher level relatively to the sea than it has ever occupied since, it seems afterwards to have been lowered beneath an ice-laden sea, until shells could be deposited at least 1,350 feet above present sea-level on Moel Tryfaen in North Wales. Then an upward movement to existing levels took place with long pauses, during which the various lines of raised beaches (q.v.) were formed, which fringe the coasts of Scotland and Norway. Before the close of the Great Ice Age man seems to have appeared in Europe, for his flint implements occur with the bones of arctic animals in Central France and beneath glacial deposits in various parts of England.