Page:The American journal of science, series 3, volume 49.djvu/19



accompanying map (Plate I) shows the maximum area covered by the ice-sheet in the St. Lawrence basin and adjoining portions of the United States and southern Canada, with approximate outlines of the glacial boundary at successive stages of its retreat. The Champlain epoch or part of the Pleistocene period including these stages of glacial recession was begun and ended, respectively, by downward and upward epeirogenic movements. It comprised the time of departure of the ice-sheet, with many small and large glacial lakes temporarily formed by its receding barrier, and with marine submergence to hundreds of feet above the present shore lines. The Late Glacial subsidence appears to have been principally completed before the retreat of the ice and deposition of the Champlain lacustrine and marine beds; but the following uplift was in progress, advancing as fast as the ice receded, from the beginning to the end of Champlain time. Indeed, considerable parts of the glaciated areas of North America and Scandinavia are still undergoing small and slow oscillatory movements, not having yet, during the short Postglacial period, fully reached isostatic repose.