438 PLEISTOCENE OF INDIANA AND MICHIGAN. 



found possible so far to tell whether these are all Battlefield or Fort Brady, or are divided between 

 the two. In the stretch between Mackinac Island and St. Ignace on the north and Cross Village 

 on the south, although accurate measurements are not available for the latter place, the impres- 

 sion that the Fort Brady beaches lie hi planes sensibly parallel with the Nipissing plane is strong 

 enough to lead to the provisional conclusion that the Fort Brady beaches, or at least the upper 

 ones, do not pass down under the Nipissing plane. This would mean that the opening of the 

 eastward outlet for Lake Algonquin to the Ottawa Valley occurred at the end or very near the 

 end of the Fort Brady substage of the lake. 



The convergence of the planes of the lower members of the upper Algonquin group suggests 

 that during that part of the uplift a hinge line may have existed in the vicinity of Traverse City. 

 Similar suggestions of a subsidiary hinge line occur near Tawas City and on the east side of 

 Lake Huron. This line runs parallel with the main Algonquin hinge line and about 25 miles 

 north of it. The uplift related to this line appears to have taken place sometime after the 

 formation of the highest Algonquin beach but before the Nipissing beach, for the latter appears 

 not to have been affected by it. The data in hand at present, however, seem insufficient to settle 

 the questions suggested. (See fig. 8.) 



OTTAWA OR CLOSING TRANSITIONAL STAGE. 



It seems certain that an outlet for Lake Algonquin to the Ottawa Valley opened before the 

 beginning of the Nipissing Great Lakes and the full eastward discharge over the col at North 

 Bay, but the facts at present available are not decisive. The opening of a lower outlet could 

 hardly have resulted from elevation alone. Something must have given way or have been 

 withdrawn to afford a lower passage. 



This early drainage to the Ottawa Valley may have prevailed during all or a part of the 

 formation of the Fort Brady beaches or it may have begun with the abandonment of the last. 

 In any case it marked the last stage of Lake Algonquin and the transition to the Nipissing Great 

 Lakes. 



GLACIAL BARRIERS OF LAKE ALGONQUIN. 



Although from first to last Lake Algonquin endured for a relatively long time, certainly 

 much longer than did any of the Huron-Erie lakes that preceded it, the precise position of its 

 ice barriers at any of its stages is not known from observation. 



The position of the barrier for Early Lake Algonquin, however, seems fairly well determined, 

 only slight uncertainty remaining as to the correlation of the moraines on the two sides of 

 Lake Huron. 



In Michigan the mam moraine of the Port Huron morainic system turns northwest on the 

 north side of Au Sable River, about 20 miles west of Greenbush in Alcona County. In Bay 

 and Tuscola counties the next later moraine is the Bay City. This passes 5 or 6 miles north- 

 west of Tawas, but it has not been surely identified in the jumbled and broken area northwest 

 of Harrisville. Still, from its lower altitude west of Tawas, it seems certain that it lies lower 

 down on the slope north of Au Sable River. The Tawas moraine passes close along the lake 

 shore at Alabaster and Tawas and seems to be continued in the prominent ridge that passes 

 close west of Greenbush and Harrisville. From Black River it is continued in the chain of 

 knolls that run northwest and along the base of which lies the Algonquin beach. South and 

 west of Rogers the Algonquin beach runs along the inner base of a high morainic mass which 

 rises 40 to 100 feet above it. From the vicinity of Rogers the divide between the drainage basins 

 of Thunder Bay River on the southeast and the Ocqueoc and Black rivers on the northwest 

 runs southwest with no passage across it as low as the Algonquin beach. Hence, the waters 

 of the Lake Huron basin in the time of Early Lake Algonquin, or even after this stage, could 

 not have found open connection with the waters of the Lake Michigan basin until the ice 

 front had withdrawn somewhat from this high morainic mass. 



