Page:The American journal of science, series 3, volume 49.djvu/27

Rh Beelevation of the St. Lawrence river hasm. 9

present St. Clair and Detroit rivers, by which the earliest out- flow of the old glacial lake probably passed southward and thence ran east as a glacial River Erie, at first tributary to Lake Lundy. As soon as that very briefly existing glacial lake was drained away, the river followed the lowest part of the shallow bed of the present Lake Erie along all its extent, which then had an eastward descent of probably 200 feet, allowing no lake or only a very small one to exist in the deepest depression of the basin ; and north of Buffalo it coin- cided with the course of the Niagara river.

Gilbert, Wright, and Spencer, have thought that for a long time the outflow of the three great lakes above Lake Erie passed by the way of Lake Nipissing to the Mattawa and Ot- tawa rivers. It seems to me far more probable, however, that the epeirogenic uplift of the Nipissing region, which had ele- vated it already about 400 feet during the existence of Lake Warren, continued so fast that both the Trent and JNipissing- Mattawa passes were raised the additional 50 feet needed to place them above the level of Lake Algonquin before the glacial retreat uncovered the country east of them so that out- lets could be obtained there.

With the continuance of the uplift of the Lake Superior basin after the formation of the Algonquin beach, the mouth of Lake Superior and the Sault Ste. Marie came into existence ; and this movement allowed the lake level at Duluth to fall probably 40 or 50 feet beneath the Algonquin and present shore line. Subsequent differential elevation of the eastern and northern parts of the basin, as compared with Duluth, has again brought the west end of the lake up to the Algonquin shore, but not until the St. Louis river, while the water sur- face stood considerably lower than now, had deeply eroded its broad channel through the very gently sloping expanse of till from Fond du Lac to the harbor of Duluth and Superior.

The differential uplift of the Algonquin beach, as compared with Chicago and the previous mouth of Lake Warren, has been about 60 feet near the mouth of Lake Huron and at Duluth ; 110 feet at the mouth of Lake Superior ; 200 feet at Lake Xipissing ; and 240 to 290 feet at Barrie, Lorneville, and Orillia, on Lake Simcoe. A broad lobe of the waning ice sheet, terminating on the highland area between the south end of Georgian bay and the west end of Lake Ontario, appears to have delayed the elevation of that district, so that subsequent to the formation of the Algonquin beach more uplifting took place there than at the north side of Georgian bay and about Lake Xipissing. The ascent of the Algonquin beach in nearly 200 miles from the mouth' of Lake Huron northeasterly to Lake Simcoe averages about a foot per mile ; and thence in