Page:Popular Science Monthly Volume 49.djvu/185

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171
HOW THE GREAT LAKES WERE BUILT.

their basins (see Fig. 14), but later even the Iroquois Gulf was contracted so as not to occupy even the head of the present Ontario basin (see Fig. 14).

The great deformation of the whole region since the close of the Iroquois episode has from that day to this been slowly raising the northeastern rims of the lake basins so as to cause them to flood more and more the lowlands and valleys at their southwestern extremities, and even to raise the waters so high as to cover some of the deserted shores in those directions. At the same time the waters are leaving their old margins at their northeastern ends, as shown on the map (Fig. 14).

The changes have not been quite simultaneous in the different basins, as the heights of the lake barriers and the rate of terrestrial movements have not been uniform. Thus, in terms of Niagara Falls, it is estimated that the Iroquois Gulf sank below the Iroquois plane about fourteen thousand years ago; but that the

PSM V49 D185 Land tilt from southwest ny to north east end of the adirondacks.jpg

Fig. 15.—Section showing the Tilting of the Iroquois Beach South of Lake Ontario and the St. Lawrence River as far as the Northeast Corner of the Adirondacks.

waters of Lake Huron, which had been emptying by way of the Nipissing Strait for twenty-four thousand years, were turned into Lake Erie only eight thousand years ago. Again, after the waters of the Ontario basin had sunk much below the present western margin of the lakes, they were rising again to near their present height only some three thousand years ago.

Of the absolute amount of rise of the continent we do not know, for the axis of uplift has not been ascertained, but it is evidently in the interior of the continent. The differential rate of elevation varies, being about a foot and a quarter a century in the Niagara district, two feet northeast of Lake Huron, and nearly four feet north of the Adirondacks.

The Future Drainage of the Upper Lakes into the Mississippi River.—With the land rising as at present, it will be only a matter of time until the northeastern rim of Lake Erie will be so high that the drainage must turn into Lake Huron, and thence by way of Lake Michigan and the Chicago Canal into the Mississippi, and Niagara Falls will then end their life history. Some fifteen hundred years ago there was a barrier about a mile north of the present site of the falls that had risen so high in the general regional uplift as to actually cause some of the waters of the upper lakes to overflow where the Chicago Canal is now being built; but, owing to the peculiar buried valley just behind this ridge crossing Niagara River, when the falls had passed the barrier, before the change of outlet of the upper lakes from the Ni-