GLACIAL AND POSTGLACIAL HISTORY OF GREAT LAKES REGION. 331 



The relation in time of Lakes Algonquin and Iroquois is a matter of great importance in 

 connection with the study of the deformation of the land. Certain facts seem to show that Lake 

 Iroquois had already been established when the Kirkueld outlet was opened. The scoured bed 

 of the outlet river of Lake Algonquin, called by Spencer Algonquin River, is strongly marked 

 between the small lakes of the Trent Valley. At Peterboro an expanded part of it, which 

 stood so near the level of Lake Iroquois that i-t may have been a land-locked bay of that lake, is 

 filled with a great deposit of gravel and sand. This appears to be a delta deposit of Algonquin 

 River and seems to show conclusively that this river emptied into Lake Iroquois. But the facts 

 now available seem to show that Lake Algonquin existed also for some time after Lake Iroquois 

 had fallen, for the scoured bed of Algonquin River appears to extend down to the Bay of Quinte 

 at Trenton, and in all probability reaches below the present level of Lake Ontario. It even passes 

 below the probable level of the marine waters that entered the Lake Ontario basin. But below 

 Peterboro it seemed to the writer to show less scour than above and in some places seems to 

 suggest a smaller stream. At present the relation of this lower part to the lake history is quite 

 problematic, for no other fact indicating so low a level for the waters of the Lake Ontario 

 basin at that stage is known. It may be that while Algonquin River was still flowing the ice 

 front withdrew far enough to allow Lake Iroquois to be drained off for a brief time, and then 

 readvanced, restoring the lake for another relatively long period. 



POSTGLACIAL HISTORY OF THE GREAT LAKES REGION. 



POSTGLACIAL LAKES. 

 NIPISSING GREAT LAKES. 



With the establishment of the outlet of the upper three lakes at North Bay a new order of 

 things began. The ice sheet had disappeared from the Great Lakes region and no longer served 

 as a dam or retaining barrier for any of its waters. This event may be taken as the place of 

 division between the Pleistocene and Recent epochs of geologic time; the lakes (with their de- 

 posits) before this time belonging to the Pleistocene, those after it to the Recent. 



At the beginning of this stage the Port Huron outlet had been abandoned. The new lake 

 level, with its outlet at North Bay, lay in a plane which passed a little, though perhaps only a 

 little, below the outlets at Port Huron and Chicago. During the later differential uplifting of 

 the land, which occurred while the North Bay outlet alone was active, the isobase passing 

 through that outlet became a nodal line on which the water plane swung. As elevation pro- 

 gressed the strands to the north of this line were abandoned and left dry, and those to the south 

 were progressively submerged by the backing up of the water. 



This order of changes continued until the North Bay outlet was raised as high as that at 

 Port Huron. Then both outlets became active with the discharge divided between them. 

 The beach made at this two-outlet stage has been called the Nipissing beach. In a strict sense, 

 however, it is not the original Nipissing beach, made when the North Bay outlet alone was active, 

 but a transitional beach made during the two-outlet stage. It might be called the Nipissing 

 transition or two-outlet beach. The original Nipissing beach may be seen now only in the small 

 part of the Lake Superior basin that lies north of the North Bay isobase or nodal line. This 

 area, however, is so small and so close to the nodal line that the measurements thus far made 

 have not yet disclosed any difference in the attitude of the old water plane on the two sides of 

 the line. Theoretically there should be some difference and there probably is. South of the 

 line the visible beach is the Nipissing transition or two-outlet beach, the original beach being 

 everywhere submerged. But as no measureable change of plane has been found in crossing 

 the nodal line the name Nipissing beach has generally been applied to the whole extent of the 

 two-outlet strand. 



The Nipissing beach thus denned has been traced with substantial continuity around all 

 three of the upper lake basins and is, especially in the northern part of the area, much the strong- 

 est abandoned beach of the Great Lakes region. Its plane extends with almost perfect uni- 

 formity and with no certainly discoverable warping over all the northern parts of the basins. 



