The thickness of the ice was so great that it buried the highest mountains of eastern North America, as is proved by the transported bowlders which have been found upon their summits. If the land then stood at its present height above sea level, and if the average slope of the ice were no more than ten feet to the mile,--a slope so gentle that the eye could not detect it and less than half the slope of the interior of the inland ice of Greenland,--the ice plateaus about Hudson Bay must have reached a thickness of at least ten thousand feet. In Europe the Scandinavian plateau was the chief center of dispersion. At the time of greatest glaciation a continuous field of ice extended from the Ural Mountains to the Atlantic, where, off the coasts of Norway and the British Isles, it met the sea in an unbroken ice wall. On the south it reached to southern England, Belgium, and central Germany, and deployed on the eastern plains in wide lobes over Poland and central Russia (Fig. 360). [Illustration: Fig. 360. Hypothetical Map of the Pleistocene Ice Sheet of Europe] At the same time the Alps supported giant glaciers many times the size of the surviving glaciers of to-day, and a piedmont glacier covered the plains of northern Switzerland. =The thickness of the drift.= The drift is far from uniform in thickness. It is comparatively thin and scanty over the Laurentian highlands and the rugged regions of New England, while from southern New York and Ontario westward over the Mississippi valley, and on the great western plains of Canada, it exceeds an average of one hundred feet over wide areas, and in places has five and six times that thickness. It was to this marginal belt that the ice sheets brought their loads, while northwards, nearer the centers of dispersion, erosion was excessive and deposition slight. =Successive ice invasions and their drift sheets.= Recent studies of the drift prove that it does not consist of one indivisible formation, but includes a number of distinct drift sheets, each with its own peculiar features. The Pleistocene epoch consisted, therefore, of several glacial stages,--during each of which the ice advanced far southward,--together with the intervening interglacial stages when, under a milder climate, the ice melted back toward its sources or wholly disappeared. [Illustration: Fig. 361. Diagram illustrating Criteria by which Different Drift Sheets are distinguished]