THE NIPISSING GREAT LAKES 



the site of Detroit to the small Lake Erie, which was then ten 

 to twelve feet below its present level. Submerged old river 

 channels can be traced in the western part of the Erie basin, 

 indicatini; the land conditions of Nipissing time. The bays 

 along the southern shore of Lake Erie are the drowned river 

 channels of this time. The caves at Put-in-Bay were all above 

 water during the Nipissing stage of Lake Erie, although now 

 parts of them are submerged. 



During this time only the waters of the greatly-reduced 

 Lake Erie poured over Niagara halls and cut the narrow gorge 

 between the upper part of the Eddy Basin and the railroad 

 bridges — the three-fourths-of-a-mile-Iong gorge of the 

 Whirlpool Rapids. (Eigure 12.) Above the bridge the gorge 

 is wider, marking the beginning of the present Great Lakes 

 stage. 



When uplift of the continent had changed the shore lines 

 and outlets of the lakes and the entire discharge of the Nipis- 

 sing lakes passed through the col at North Bay, the stage en- 

 dured for a long time. But the end came for the Nipissing 

 lakes as once again uplift in the north spilled the waters south- 

 ward. The old Chicago outlet was put in operation again but 

 for only a short time, for the lake level soon fell three or four 

 feet so that entire discharge was finally poured through the 

 outlet past Port Huron, reviving St. Clair River and spreading 

 the overflow into the St. Clair basin to form Lake St. Clair; 

 then the current quickened in the newl\ -formed Detroit 

 River. The waters of the broad shallow strait connecting the 

 Superior and Huron basins across the eastern end of the St. 

 Mary's peninsula were withdrawn into St. Mary's River, and 

 the two lakes become independent; Lake Superior was held 

 back in its higher basin b>' the rim of the old Cambrian sand- 

 stone "bowl," which crosses the river at the falls mu\ rapids 

 of the St. Mary's. So recently did this separation take place 



77 



