Page:The American journal of science, series 3, volume 49.djvu/36

18 18 W. ZTpham — Cham/plain Subsidence, etc.

extent inward to the areas on which its waning remnants lingered the latest.

On the Green Mountains of Vermont, the White Mountains region, and indeed probably over a large part of New England, a tract of the departing ice-sheet remained after the access 'of the sea to the St. Lawrence basin left the New England ice as an isolated mass. This is known by the large tribute of strati- fied drift quickly brought by streams from the melting ice of the Green Mountains area and deposited as gravel and sand deltas and offshore clays of the Winooski, LaMoille, and Mis- sisquoi rivers, described by Hitchcock and Baldwin, in the east border of the Champlain arm of the sea. On the west, too, a considerable remnant of the ice-sheet seems to have remained unmelted until this time on the Adirondacks, and to have likewise supplied the deltas and marine clays of the An Sable, Saranac, and Chazy rivers in New York. Deflections of glacial striation down the valleys, with corresponding drift transportation and formation of local moraines across some of the mountain valleys, have been recorded by Hitchcock, Stone, and others, in Vermont and New Hampshire ; but the time allowed for such glacial action, under the warm Champlain climate, was very short. The earlier melting of the ice along the St. Lawrence valley than on these mountain tracts was due on one side to the laving action of the waves of Lakes Iroquois and St. Lawrence, and on the other side to the washing of the ice-cliffs by the fast encroaching sea in the Gulf of St. Law- rence, until at last near Quebec the barrier was severed.

From the Champlain submergence our Atlantic coast was raised somewhat higher than now ; and its latest movement from New Jersey to southern Greenland has been a moderate depression. The vertical amount of this postglacial elevation above the present height, and of the recent subsidence, on all the coast of New Jersey, New England, and the eastern prov- inces of Canada, is known to have ranged from 10 feet to a maximum of at least 80 feet at the head of the Bay of Fundy, as is attested in many places by stumps of forests, rooted where they grew, and by peat beds now submerged by the sea. As in Scandinavia, the restoration of isostatic equilibrium is at- tended by minor oscillations, the conditions requisite for repose having been overpassed by the early reelevation of outer por- tions of each of these great glaciated areas. The close of the Ice age was not long ago, geologically speaking, for equilibrium of the disturbed areas has not yet been restored.