38 TWELFTH REPORT. 



south, to nortli .so that the earliest discharge was along the face of the 

 south bluff. It is probable also that after the main valley had been 

 uncovered ice lobes protruded into it from northern tributaries and 

 blocked it up sufficiently to hold the waters considerably above the level 

 of the valley bottom. Features pointing to such a history ar^e found 

 in the occurrence of weak shore lines in the Huron, Michigan and Su- 

 perior basins at levels too low to be included in the Algonquin beaches 

 and too high for the Xipissiug shore. There are two such shore lines, 

 the higher of which is known as the Battlefield beach, because well de- 

 veloped on the battle grounds on Mackinac Island, while the lower is 

 termed the Fort Brady beach, it being well develo])ed at Fort Brady near 

 Sault Ste. Marie. 



NIPISSIXG GREAT LAKES. 



The Nipissing Great Lakes began with the complete lowering of the 

 lake waters to the level of the low pass that leads from Georgian Bay 

 eastward past Lake Nipissing into the Ottawa valley (Fig. 8). The 

 present small lake east of Georgian Bay, known as Lake Nipissing, lies 

 near the head of the old outlet. The term Nipissing Great 

 Lakes is applied because of the close association of the Nipissing shore 

 at its latest stage with the shore of the modern lakes in the Michigan, 

 Huron, and Superior basins. 



At the outlet by Georgian Bay and in the extreme northeast jiart of 

 the Superior basin uplift has been sufficient to give the beach an alti- 

 tude about 120 feet above the modern lakes. It falls to only 15 feet 

 above the lake at the southern end of Lakes Huron and Michigan and 

 drops slightly below the level of Lake Superior in the western i)ortion 

 of its basin. 



At the time the Nipissing Great Lakes began eastward discharge, the 

 altitude of the outlet ai)i)ears to have been considerably lower than that 

 of the Chicago or St. Clair outlet, and the lake was drawn down to a 

 somewhat smaller area than the present area of Lakes Michigan and 

 Huron. As the uplift of the outlet progressed the lake became extended 

 with the rise of the waters until it resumed an outHow through the 

 St. Clair River. The beach known as the Nipissing beach, on the borders 

 of the Huron and Michigan basins, and also of the portion of the Sui)e 

 rior basin south of an isobase running through the outlet, was formed 

 after the lake had become expanded sufficiently to have a discharge 

 through the St. Clair as well as the Ottawa outlet. It is, therefore, 

 the beach of a two outlet stage. The part of the Nipissing shore north 

 of an isobase running through the head of the Ottawa outlet exposes a 

 series of l)eaches formed prior to the two outlet stage, as well as the 

 beach of that stage. The continuation of these earlier beaches is sub- 

 merged in the portion of the lake area south of the isobase running 

 through the head of the outlet. 



The direction of tilting of the Nipissing shores does not coincide 

 precisely with the tilt lines of the Algonquin shores, being in places a 

 few degrees to the east. The amount of ui)lift is very much less than 

 that affecting the Algonquin beaches. The horizontal portion of the 

 Nipissing beach extends over about the same area as that of the 

 horizontal ]>ortion of the Algonquin and includes the southern 



