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HISTORICAL GEOLOGY 



has been slowly rising, as is shown by the beach lines of the ancient 

 lakes which are now higher at the north than they are further south — 

 in some cases 400 feet or more. The surfaces of the lakes are held 

 up by rock and drift barriers to levels several hundreds of feet above 

 their rock beds, while the bottoms of all the lakes, except Lake Erie, 

 are below sea level. 



Great Lakes Stages. 1 — The Great Lakes had their inception when 

 the ice sheet had retreated across the higher land which turns some of 



Fig. 568. — An early (first) stage in the history of the Great Lakes. 

 (After Taylor and Leverett.) 



the water to the north and some to the south. The melting waters 

 being prevented from flowing north, accumulated between it and the 

 ice front, gradually enlarging upon the further recession of the ice. 

 At first there were doubtless many small lakes which had temporary 

 outlets to the south. As the ice retreated still further these lakes 

 coalesced into larger ones. The brief and incomplete history of the 

 Great Lakes which is given below has been learned from a study of 

 the beaches, sand bars, deltas, and outlets made at former lake levels. 

 I he history of the Great Lakes may be considered as beginning 

 after the ice had retreated to such an extent that a large lake (glacial 



1 Taylor, F. B., — The Glacial and Postglacial Lakes of the Great Lake Region : Smithsonian 

 , 1 91 2, pp. 291-327. 



