Page:Canadian Alpine Journal I, 2.djvu/116

256 glaciers in the Rockies and Selkirks were, in recent geological time, encased in such deposits of ice and snow, with only the higher peaks and ridges protruding.

(d) Continental Ice-sheets. During the so-called Pleistocene stage of the earth's history conditions were favorable for the formation of glaciers over the entire region between the Rockies and the Pacific and from the International Boundary to Alaska. These conditions resulted from an increased precipitation over the region and a reduction in the mean annual temperature. In the way above noted local ice-caps developed wherever favorable conditions existed and later were completely buried in snow and their outlines obliterated. With the submergence of the higher ridges the filling of the intervening valleys would go on slowly and at one stage the entire western portion of the Dominion was heavily encased in ice. The movement was mainly to the north, west and south, but piedmont glaciers of great magnitude developed along the eastern margin of the Rockies and reached out for many miles over the plains. In our imagination we may apply the same characteristics to this great ice-sheet, with its complex of submerged glaciers, that were noted for the local ice-cap. Climatic conditions finally changed and this continental type of glacier was slowly resolved into its components, only relatively few of which still remain to grace the landscape. Two similar ice-sheets developed further eastward, either simultaneously or subsequently, one centering to the west of Hudson's Bay and the other in Labrador. Existing glaciers of this type are found in Greenland and the Antarctic region.

Within the sphere of their activity glaciers may become powerful geological agents, destroying or modify-