GENERAL PHYSICAL FEATURES. H7 



Till.] 



toward the east, so that at last they become continuously rock-bound. The 

 drift fades out, on the north, toward the rock-bound shore of lake Superior, 

 as remarkably evinced along the international boundary, and on the south 

 toward the ancient equally rocky valley formed by the St. Croix and the 

 Mississippi. 



The diversified nature of the drift cannot be so briefly described. It 

 may be divided under three general distinctions, viz., till, stratified sand 

 and gravel, and stratified clay. 



Till. In general the entire drift-sheet might be said to consist of till, 

 that confused mixture of sand, gravel and clay which is believed to be the 

 product of glaciers, or land ice, since the other parts are insignificant in 

 amount and area compared to it, and since they have been derived from it 

 by the assorting and distributing action of water. When two or more of 

 these parts exist at the same place the till always lies at the bottom. 

 Where the drift prevails the most of the surface is till, but it fades out in 

 the southeastern corner of Minnesota, and in its place is found a water- 

 deposited fine clay or loam. This covers Houston, Winona and Wabasha 

 counties, and the eastern portions of Goodhue, Olmsted and Fillmore. Its 

 western area is underlain by till which increases in amount toward the west 

 and finally rises to and forms the surface. This belt, occupied by the van- 

 ishing western edge of the loam, crosses Fillmore, Olmsted, western Good- 

 hue, western Dakota and Washington counties. In a similar manner, but 

 from a different cause, the till is found wanting in the northeastern corner 

 of the state, but hqre no loam takes its place. This driftless region is found 

 to the north and east of Vermilion lake and Net lake. The rocks in this 

 part of the state are bare, and as they consist of the crystalline terranes, the 

 depressions hold numerous lakes which are connected with each other by 

 streams that plunge from one rock shelf to another in their descent to 

 Rainy lake or to lake Superior. Another variation and exception to the 

 almost uniform till surface in Minnesota is found in the northwestern cor- 

 ner of the state. This differs from the northeastern and the southeastern 

 in having an unusually thick and uniform mantle consisting of both till and 

 loam, the latter overlying and separated from the till by a sudden and dis- 

 tinct line of demarcation. This area of loam-covered till not only occupies 

 the valley of the Red river of the North, from lake Traverse to St. Vincent, 



