DISCRIMINATION OP AGE OF GLACIAL DRIFT SHEETS 639 



Character of this drift and reasons for regarding the drift exposed at the 

 surface throughout this area as belonging to one and the same sheet. The 

 lithological composition and its significance, directions of ice movement, ab- 

 sence of intercalated weathered zones, soils, or vegetable deposits. 



Differences in the apparent amounts of surface modification of this drift in 

 different parts of the area which might be regarded as indicating differences in 

 age: 



1. Topographic relations and amount of erosion. 



2. Weathering, leaching, and oxidation. The occurrence in places of thor- 

 oughly disintegrated drift or residual till ; in others, of drift but moderately 

 leached and oxidized ; in others, of perfectly fresh unmodified drift at the sur- 

 face or immediately below the loess. 



The reasons for these differences : 



1. Influence of pre-Glacial topography on drainage slope and upland of the 

 drift. Influence of the Saint Peter sandstone on the pre-Glacial topography. 

 Relations of surface wash to the apparent amount of surficial leaching and 

 oxidation. 



2. The post-Illinoisan diversion of Rock river below Rockford, Illinois, and 

 the consequent retardation of erosion due to the work of cutting new rock 

 gorges at several cols. Removal of the weathered drift from slopes with 

 preservation on the uplands. 



Necessity for caution in the discrimination of distant drift sheets in the ab- 

 sence of marked differences in lithological composition or of sections showing 

 overlapping drift with intercalated soils, vegetable deposits, or weathered 

 zones. 



The two papers together were discussed by F. Leverett. 

 Then was read 



LAKE OJIBWA, LAST OF THE GREAT GLACIAL LAKES 

 BY A. P. COLEMAN 



[Abstract] 



As the Labrador ice-sheet retreated north to the watershed between the 

 Great lakes and James bay, the waters now flowing northward were impounded, 

 first as a narrow bay of lake Algonquin opening south past Sudbury, after- 

 wards as a separate lake with an outlet down the Ottawa valley. This lake 

 probably existed during the time of the Nipissing Great lakes, and was the last 

 of the ice-dammed lakes. The elevation of its outlet is now 900 feet, but was 

 then much lower. In its bed the "clay belt" of northern Ontario and Quebec 

 was deposited, having an extent of over 25,000 square miles. The maximum 

 area covered by its waters must have been greater than that of lake Superior ; 

 though probably its extent varied greatly in accordance with the position of 

 the ice front. 



This paper was discussed by F. B. Taylor and A. P. Coleman. 



LXI — Bull. Geol. Soc. Am., Vol. 20, 1908 



