Page:Popular Science Monthly Volume 49.djvu/373

Rh five hundred and seventy-five thousand square miles, and rises with average slopes of one hundred feet or more per mile to a central height, along its axial portion, of eight to ten thousand feet, or almost two miles measured vertically, above the sea level. The ancient ice sheets had a similar altitude and thickness. From the directions of outflow of the North American ice fields as shown by the transportation of the glacial drift, and from the observed upper limits of glaciation on high mountains, Prof. James D. Dana estimated the thickness of the ice formerly accumulated above the Laurentide highlands, between the St. Lawrence River and Hudson Bay, to be fully two miles. It probably varied in thickness from one to two miles across Labrador, the Laurentide highlands, James Bay, Lake Winnipeg, Reindeer and Athabasca lakes, to the Rocky Mountains, in the region of the Peace River, where their summits, lower than southward, were probably buried beneath the ice expanse. In British Columbia, according to Dr. George M. Dawson's observations of glacial strise and drift on mountains, the ice sheet exceeded a mile in depth.

In all directions from its thick central areas the vast continental glacier flowed outward, carrying its drift from Hudson Strait, Labrador, and Newfoundland easterly beyond the present coast line; from the provinces of Quebec and Ontario southeasterly across New England, and southerly and southwesterly across the basins of the Laurentian lakes; from Manitoba and the Saskatchewan region southerly into Minnesota, Iowa, the Dakotas, and Montana; from British Columbia into Idaho and Washington on the south, into the edge of the Pacific Ocean on the west, and down the Yukon Valley on the north; and from the great northern Barren Grounds northerly down the Mackenzie and across the islands of the Arctic Sea.

Northern Europe and the present basins of the Irish, North, Baltic, and White Seas were covered by an ice sheet which attained an extent of two million square miles, being half as large as that of North America; and its maximum depth above Sweden and the beds of the Baltic Sea and Gulf of Bothnia was one mile, or more probably two miles. The high, much eroded, and channeled Scandinavian plateau even now has numerous local ice fields, varying in size up to five hundred square miles, which are doubtless remnants of a continuous glaciation through all the centuries since the vast European ice field of the Glacial period