GEOLOGICAL HISTORY OF THE LA URENTIAN BASIN. 397 



The conclusion that the region under consideration has been 

 glaciated is well established; it remains, therefore, to determine 

 what topographic forms, if any, due to pre -glacial stream erosion 

 can be recognized. As an example of this kind of evidence 

 desired, attention may be directed to the northward facing rock 

 escarpments which follow the southern shores -of lakes Erie and 

 Ontario for a large part of their courses and at varying distances 

 up to several miles. These escarpments are composed of the edges 

 of nearly horizontal strata, mostly of Paleozoic limestone, and 

 their bases are buried beneath glacial debris and stratified clays so 

 deeply that in some instances, at least, they do not reveal half of 

 their actual height. These escarpments not only have Pleistocene 

 deposits banked against them, but their faces and summits are pol- 

 ished and grooved, showing how stubbornly they resisted the inva- 

 sion of the ice which impinged against them from the north. South 

 of lake Ontario especially, the trend of the escarpment referred 

 to is directl}' athwart the course of the ancient glaciers. The 

 entire history of these escarpments cannot be discussed here, as 

 my desire is simply to call attention to the fact that they existed 

 before the Glacial epoch, and are relics of a strongly accented 

 pre -glacial topography. They are within the southern border 

 of the Laurentian basin, and hence afford means of determining, 

 in part, what was the form of that basin before it was modified 

 by ice action. Other similar escarpments exist in the northern 

 and western portions of the same great basin, and as this study 

 progresses it is to be expected that still other features of the 

 pre -glacial land will be revealed. It is perhaps too earl}- to 

 decide what were the special topographic forms which gave 

 character and expression to the St. Lawrence basin before the 

 ice invasion, but the Erie and Ontario escarpments and some 

 other similar features now recognized, suggest that in Tertiary 

 times it resembled the present condition of the upper portion of 

 the Mississippi valley, where bold, rock escarpments border 

 wide stream -worn depressions. 



Deep drift -filled channels are known to cut across the Erie 

 and Ontario escarpments. These seem to have been formed 



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