S20 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1912. 



sudden and rapid. Almost all the deformation occurred during and 

 since the later part ol the life of Lake Algonquin. 



(4) Closing transition stage. — At its very end Lake Algonqum 

 appears to have been held up by a relatively small glacial barrier at 

 some point in the Ottawa Valley east of Mattawa. (Fig. 6, ice border 

 position A'.) Wlien this last dam broke out or shrank back north- 

 ward the waters rushed eastward from Lake Algonqum through the 

 Mattawa Valley to lakes in the Ottawa, Petawawa, and Madawaska 

 Valleys (fig. 6, ice border at position A"), and thence to the Cham- 

 plain Sea, and came to a settled level in the upper lake basins only 

 when the eastward flowing outlet had been established on the col 

 at North Bay. 



Tlie relation in time of Lakes Algonquin and Iroquois is a matter 

 of great importance m comiection with the study of the deformation 

 of the land. There are certain facts that seem to show that Lake 

 Iroquois had aheady been established when the Kirkfield outlet first 

 opened. The outlet river of Lake Algonquin has been called by 

 Spencer the Algonqum Kiver. The scoured bed of this great river is 

 quite e\ddent between the smaU lakes of the Trent Valley. At Peter- 

 boro an expanded part of the valley which stood so near the level 

 of Lake Iroquois that it may have been a land-locked bay of that 

 lake is filled with a great deposit of gravel and sand. This appeal's 

 to be a delta deposit of the Algonquin Kiver and seems to show 

 conclusively that this river emptied mto Lake Iroquois. But the 

 scoured channel of the Algonquin River does not stop at Peterboro. 

 It appears to contmue down the Trent Valley to Trenton and in all 

 probability passes below the present level of Lake Ontario. (Fig. 6.) 

 It even passes below the probable level of the marme waters that 

 entered the Lake Ontario basin. But below Peterboro it seems. to 

 show decidedly less scour than above and m some places suggests a 

 smaller stream. At present the relation of this lower part to the Lake 

 history is quite problematical, for no other fact indicating so low a 

 level for the waters of the Lake Ontario basin at that stage is kno-vvti. 

 It may be that while the Algonquin River was flowing the ice front 

 withdrew far enough to allow Lake Iroquois to be drained off for a 

 brief time, only to be restored again for another relatively long 

 period by a readvance of the ice. There were a number of episodes 

 of this kind earlier in the lake history. 



POST-GLACIAL LAKES OF THE GREAT LAKES REGION. 



THE NIPISSING GREAT LAKES. 



With the estabhshment of the outlet of the upper three lakes at 

 North Bay, a new order of things was inaugurated. The ice sheet 

 had disappeared from the Great Lakes region and no longer served as 



