Page:Popular Science Monthly Volume 49.djvu/179

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165
HOW THE GREAT LAKES WERE BUILT.

exclusion of the sea, of which there is no evidence, some have supposed that these waters of the lake district were held at high

PSM V49 D179 Map of western end of lake ontario.jpg
Fig. 8.—Map of the Western End of Lake Ontario. b, Burlington Beach, separating Burlington Bay from the lake; h, Burlington Heights, an ancient beach one hundred and eight to one hundred and sixteen feet high, separating Dundas marsh from Burlington Bay.

altitudes by glacial dams for long ages. This hypothesis, notably advocated by Prof. G. K. Gilbert, although based upon negative evidence to explain some difficulties, which are also applicable to southern regions even within the tropics, has retarded the researches into the history of the lakes, and had it been followed would have prevented the discovery of some of the greatest changes in the geography of northeastern America. Indeed, the greater portion of the modern elevation of the region has been

PSM V49 D179 Bowlder pavement on georgian bay.jpg

{{c|Fig. 9.—Modern Bowlder Pavement on Georgian Bay, east of the end of Blue Mountains of Collingwood, Ontario.

recorded in the tilting of the beach lines recognized by all. But the period of glacial conditions was prior to the lacustrine.