HOW THE GREAT LAKES WERE BUILT. 



165 



exclusion of the sea, of which there is no evidence, some have 

 supposed that these waters of the lake district were held at high 

 altitudes by glacial 

 dams for long ages. 

 This hypothesis, not- 

 ably advocated by 

 Prof. G. K. Gilbert, 

 although based up- 

 on negative evi- 

 dence to explain 

 some difficulties, 

 which are also ap- 

 plicable to southern 

 regions even with- 

 in the tropics, has Fm 8 ._Map of the Western End of Lake Ontario. 

 retarded the re- b, Burlington Beach, separating Burlington Bay from the 



Searches into the his- late ' ^' Burlm g ton Heights, an ancient beach one hundred 



J and eight to one hundred and sixteen feet high, separating 



tory Of the lakes, and Dundas marsh from Burlington Bay. 



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 



Fig. 9. Modern Bowlder Pavement on Georgian Bay, east of the end of Blue Moun- 

 tains 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, 



