Page:The American journal of science, series 3, volume 49.djvu/34

16 16 W. Ujpham — Champlain Subsidence and

mum height at Montreal somewhat exceeding 500 feet above the present sea level. Earlier than that time of occupa- tion of the depressed broad valley by the sea, it was filled from Lake Ontario to near Quebec, by a great glacial lake, held on its northeast side by the receding continental ice-sheet. The directions of the glacial striae and transporta- tion of the drift in the St. Lawrence valley, running south- westward at Montreal and onward to the great lakes, but east- ward from Quebec down the shores of the Gulf of St. Law- rence, and southeast across Nova Scotia and New Brunswick, show that the latest remnant of the ice barrier blockading this valley was melted away in the neighborhood of Quebec, then admitting the sea to a large, low region westward. Until this barrier was removed, a glacial lake, which here for convenience of description and citation is designated as the Lake St. Law- rence, dating from the confluence of Lakes Iroquois and Hud- son-Champlain and growing northward and eastward, spread over the Ottawa valley probably to the mouth of the Mattawa, and down the St. Lawrence, as fast as the ice-front was melted back.

When Lake Iroquois ceased to outflow at Eome and, after intervening stages of outlets existing for a short time at suc- cessively lower levels north of the Adirondacks, began to oc- cupy the Champlain basin and the St. Lawrence valley north- ward, changing thus to the Lake St. Lawrence, its surface fell by these stages about 250 feet to the glacial Lake Hudson- Champlain, which had doubtless reached northward nearly to the St. Lawrence. After this reduction of the water body in the Ontario basin, it still had a dqpth of about 150 feet over the present mouth of Lake Ontario, as shown by a beach traced by Gilbert, which thence rises northeastward but declines toward the south and southwest. Its plane, which is nearly parallel with the higher Iroquois beaches, sinks to the present lake level near Oswego, N. Y. Farther southwestward the shore of the glacial lake at this lower stage has been since submerged by Lake Ontario. The Niagara river was then longer than now, and the lower part of its extent has become covered by the present lake. From the time of the union of Lakes Iroquois and Hudson-Champlain, a strait, at first about 150 feet deep, but later probably diminished on account of the rise of the land to a depth of about 50 feet, joined the broad expanse of water in the Ontario basin with the larger expanse in the St. Lawrence and Ottawa valleys and the basin of Lake Cham- plain. At the subsequent time of ingress of the sea past Que- bec the level of Lake St. Lawrence fell probably 50 feet or less to the ocean level. The place of the glacial lake so far westward as the Thousand Islands was then taken by the sea,