GLACIAL PERIOD, PLEISTOCENE PE RIOD, or ICE AGE. A division of geologic time, comprising the earliest part of the Quater nary period. The term gains its significance from a remarkable episode in which abnormal condi tions of climate were involved. In late Tertiary times there seems to have been a gradual lower ing of temperatures throughout the north tem perate zone, and this change progressed steadily until at the opening of the Glacial period the climate was essentially arctic. Within the conti nental areas enormous glaciers and ice-sheets then formed which advanced southward, filling the river and lake basins, covering the moun tains, and burying the lowlands beneath a vast mer de glace. One field of ice extended over Canada and the northeastern part of the United States. Its northern limits have not yet been defined, but on the east it reached the Atlantic Ocean, and southward it advanced well into New Jersey, Pennsylvania, and the States be tween the Ohio and Missouri rivers. New Eng land, New York, -and the region of the Great Lakes were completely covered by the ice-sheet. In the White Mountains the rocks bear evidence of having been striated and polished almost to the summits of the highest elevations, and the same phenomena have been recorded for the Adirondacks and Catskills, showing that the ice in places was several thousand feet thick. The mountains of Western North America were also the scenes of great glacial activity, of which the snow-fields of the present day are but wasted relics. Glaciers descended from the Rocky Moun tains of Colorado and from the Sierra Nevadas of California far into the river valleys, while those of Alaska and British Columbia were so extensive as to form practically a single field.
The change in temperature seems to have been no less marked in the Old than in the New World. An ice-sheet covered the whole of North ern Europe; it filled up the basin of the Baltic on its way from Scandinavia to the plains of North Germany, and it crossed the North Sea to the Scottish Highlands, whence it moved northward and westward into the Atlantic. The whole of England north of the Thames, as well as Scotland and Ireland, was buried beneath the ice, which attained a thickness in some localities of 5000 feet. On the Continent the sheet spread over Scandinavia, Denmark, Holland, and parts ofGermany, Belgium and Russia, and com prised an area of about 800,000 square miles, or several times larger than the Greenland ice-cap. South of its limits there were smaller snow-fields and glaciers in the Carpathians, Alps, Jura, Pyrenees, and the Central Plateau of France. The present Alpine glaciers are shrunken remnants of the field that covered Switzerland during this period. The high mountain systems of Asia also show evidence of having been glaciated. In the Southern Hemisphere the glaciers or Patagonia were once enlarged so as to extend across the peninsula to the Atlantic shores, and New Zea land was overrun by the ice; but it has not been definitely established that the period of gla ciation here was contemporaneous with that of the Northern Hemisphere.