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Page:Popular Science Monthly Volume 49.djvu/177

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HOW THE GREAT LAKES WERE BUILT.

Reversals of Ancient Rivers in Pennsylvania and New York.—The great changes in the water way of the Laurentian River had their counterpart in the highlands to the south of the lakes where the ancient streams were tributary to the Laurentian, in place of to the modern Ohio and Susquehanna Rivers. Among the more notable changes, the Alleghany (discovered by Mr. J. F. Carll) flowed to the Erie basin, as did also the upper Ohio (suggested by the writer, and further explained by Dr. P. Max Foshay and Mr. F. Leverett). These and other streams now reversed were tributaries of the Erigan River. In New York the upper Susquehanna and some tributaries descended through the “finger lakes” to the Laurentian River as it passed through the Ontario basin. All the old streams coming from the highlands south of the lake basins flowed through broad, V-shaped valleys, of ancient form, although of considerable depth. These valleys became filled with drift which turned the waters of the Ohio and Susquehanna Rivers to the south. This reversal in the drainage has been further assisted by the recent northward tilting of the land, to be explained later.

How the Ancient Valleys were Obstructed.—The Laurentian Valley and its tributaries were completed before the ice age. Indeed, the high elevation of the continent during the culmination of that period did not last long enough for the deepening of the channels of the main valley, as they could scarcely be affected until a great cañon had been excavated from the continental margin for eight hundred miles to the Ontario basin, which was not the case.

As for the modification of the ancient topography by glacial action, it could have been only slight, and does not appear to have been more than the sweeping of loose geological dust into the valleys, or on to the highlands to the south. The absence of any great plow is shown by the direction of the scratches on the rock surfaces, which lines are everywhere at great angles to the walls and sides of the lake basins, and nowhere parallel to them, as must have been the case if the valleys had been plowed out by ice in any form. This crucial test and many other features had not been applied fifteen years ago, when the writer commenced these researches. Now this fancy of closet geologists has vanished before the application of facts. Yet the work of the ice age was complex, and it is immaterial to the study of the lakes how it was performed. In one way only does it come within the limit of this subject, and that is in the phenomena of the ancient valleys being filled by drift, whether stratified or not. It was this filling of the old channels with drift that closed the ancient drainage of the Laurentian Valley, which at a later date gave rise to the lake basins. But the barriers of the lakes were further exagger-