Page:Popular Science Monthly Volume 49.djvu/183

Rh Niagara Falls and the History of the Great Lakes, by the writer, and published by the Commissioners of Niagara Falls Reservation, under the presidency of Hon. Andrew H. Green.

.—This marks only an episode in the chain of events which are being described, when the waters fell three hundred feet below the Algonquin and Lundy planes. Although the subsidence of the waters was not continuous and left some evidences of temporary pauses, yet the long rest was not reached until they had sunk to the level of the Iroquois beach. By this time the land had risen so high, and, as there were no sufficient barriers, the upper lakes sunk far within their present basins, as is shown on the map (Fig. 14). Still, the waters of these upper lakes discharged by way of the narrow Nipissing Strait.

Lake Erie at this time had its birth, but then it was a very small body of water, as shown on the map. The Niagara district was then covered with a strait expanded into a lakelet, and afterward a river at first without a fall. In the further sinking of the water to the Iroquois level the falls of Niagara commenced their history, and then there was a comparatively long rest, but Ontario was still a gulf, as shown on the map (Fig. 14).

The plane of the Iroquois shore was at identically or nearly the same level as the Nipissing beach (of Taylor) at the outlet of Lake Huron by way of the Ottawa Valley. It is not apparent, and it is theoretically improbable, that the Nipissing River was characterized by more than a gentle slope, for by the time that the land rose high enough to produce a rapid river the water of the upper lakes had changed their outlet into Lake Erie. The proof of these changes rests in the tilting of the beaches, which aggregates several hundred feet (see Fig. 12).

.—Still, the land has continued to rise, and the deformation since the Iroquois episode amounts to more than before that date. The tilting at the head of Lake Ontario becomes an absolute elevation above the sea, amounting to three hundred and sixty-three feet, and at the northeastern corner of the Adirondacks it is fifteen hundred feet, while near the outlet of Lake Ontario it is seven hundred and thirty feet. This warping of the continent is illustrated in Fig. 15, and to it is due the barrier (to a large extent) which retains the waters of the Ontario basin at an elevation of two hundred and forty-seven feet above the sea.

.—The continuing elevation of the