TEE GENESIS OF LAKE AGASSIZ 637 



cause of the differential uplift which is shown to have taken place 

 by the deformation of the shore lines of the lake. 



It is known that the whole of the southern portion of the Lake 

 Agassiz basin was affected by uplift but that the region south of the 

 southern outlet of the lake was unaffected, for the abandoned shore 

 line of Lake Dakota in this region is apparently nearly horizontal. 1 

 That is, there is a sort of "hinge-line" here. The location of this 

 "hinge-line" was not due to "quick recovery of the crust by uplift" 

 following removal of the ice from the immediate neighborhood, for 

 the ice border was at least 250 miles north of the location of the 

 "hinge-line." 



The question also arises whether, as Chamberlin suggested, the 

 land was being depressed during the time of advance of the latest 

 ice sheet. It would be possible to determine this if the present 

 altitude with respect to sea-level of the beaches which were made 

 during the rising stage of the lake could be determined. It was 

 found in Rainy River district that the strongest beach of Lake 

 Agassiz apparently marks a long stand of the waters during the 

 rising stage and again during the subsiding stage; for the beach 

 deposits show evidence of having been partly eroded and spread out 

 by the rising waters and beach ridges having a slightly different 

 trend were later built on the older deposits. This would seem to 

 show that the land was already depressed during the rising stage 

 of the lake, but the evidence is not very conclusive. In the case of 

 the "fossil" shore line seen in sections along the south shore of 

 Lake of the Woods (see Fig. 1), it was found that the beach main- 

 tains the same altitude in a direction corresponding to the trend of 

 the isobases of the beaches formed during the subsiding stage of 

 Lake Agassiz. It is not known whether it rises toward the north- 

 east, for unfortunately no record of its occurrence could be found 

 in the northern part of Lake of the Woods. 



The evidence suggests, but does not prove, that if, as seems 

 probable, the uplift of the land was due to isostatic readjustment 

 following the removal of the burden of the ice sheets, there was no 

 close sympathetic relation; but that uplift lagged 2 considerably 



1 U.S. Geol. Survey, Monograph 25, p. 267. 



2 J. Le Conte, Bull. Geol. Soc. Amer., II (1891), 329-30; W. B. Wright, The 

 Quaternary Ice Age, 19 14. 



