954 MINNESOTA BOTANICAL STUDIES. 



and Rainy lake the glacial body of water was bounded by mo- 

 rainic shores, and Upham has traced the various beaches for 

 many miles in Minnesota, Dakota and Canada. 



The somewhat abrupt disappearance of Lake Agassiz, not 

 far from 7000 years ago, left much of its basin to the drainage 

 of the Red and Rainy rivers. Clearly the water, as it aban- 

 doned its ancient bed, must have early laid open the southern 

 portion of the valley to plant immigration, and the immigrants 

 must have followed in northward extension the receding inland 

 sea. Since the ice-barrier apparently extended along the 

 eastern shores of what is now Rainy lake, a very sharp distinc- 

 tion arose in those earlier times, and is still to some extent 

 perpetuated between the region within Lake Agassiz and the 

 uninundated, though strongly glaciated area outside its shore- 

 line. Here, it would seem, may be in part the explanation of 

 the rather remarkable dissimilarity between the plant-popula- 

 tion of the north shore of Lake Superior, the Mesabi and Giant 

 ranges on the one hand and that of the Rainy lake and Lake of 

 the Woods region on the other. In the latter region many 

 species of plants of southern range are conspicuous, although 

 often dwarfed, or occupying peculiar localities. Indeed the 

 flora of the Lake of the Woods is essentially similar to that of 

 the Red river valley of the Dakotas, and may be regarded as a 

 forest modification of the general north-bound group of plants 

 which established themselves in the bed of the ancient lake. 

 But the plant population of the north shore of Lake Superior — 

 and the formation extends westward to the region south of Gun- 

 flint lake and about Vermilion lake — is quite as distinctly an 

 originally northern and south-bound group of plants relatively 

 free from the infiltration of southern forms such as the Solidagos 

 and Apocynums. 



Character of the country rocks and drift.— A detailed 

 account of the geological formations of the Lake of the Woods 

 district would be out of place in this paper as it is rather the 

 physical and chemical character of the soil-components that 

 are of importance, than their f aultings and foldings, stratifica- 

 tions and correlations. A full account of the complex geology 

 6f the region is given by Lawson 4 and this may be referred to 

 for amplification of the facts brought forward here. The 

 region is one almost entirely of Archaean rocks comprising 

 diorites, augites, felsites, quartzites, granites, diabasic rocks, 



4 Lawson: Lake of the Woods. Rep. Geol. Surv. Can. 1887 



