ECOLOGY OP ISLE ROYALE. 



33 



■wall continued to retreat toward the northeast, these ponded waters 

 increased in area and formed the highest beach lines now preserved on 

 the north shore of Lake Superior. The evidence for this is found on 

 Mt. Josephine, located on this shore just north of the southwestern end 

 of Isle Eoyale, which reaches an elevation of about 800 feet above the 

 Lake. Par up on its slopes, according, to Lawson ('93, p. 253), evidence 

 of beach lines are found, about which he says: "These two terraces at 

 685 and 607 feet are remarkable for being the highest strand lines which 



#j?r^:;s^'^\ .rV 





Fig. 51. Showing origin of the Glacial Lakes, their relation to the ice sheet and their Miss- 

 issippi drainage. 



have thus far been observed on the coast of Lake Superior." Under such 

 conditions it seems that Isle Eoyale would undoubtedly have been sub- 

 merged. The ice retreat continued and finally Isle Eoyale, freed from 

 the ice and in part from the lake waters, emerged as a narrow rocky 

 ridge — the crest of the Greenstone Eange which today rises, at the 

 northeastern end of the island, to a maximum elevation of about 550 

 feet. As the ice wall retreated the Michigan and Huron basins became 

 confluent, and an outlet to the east (Trent valley, Ontario) at first, 

 later the Port Huron and possibly the Chicago outlet (Goldthwait, '09, 

 p. 65) became available, Fig. 52, and at about this time the Glacial Great 

 5 



