THE GLACIAL DRIFT. 633 



intimate connection with one another of the systems of erosion of the 

 two regions. The valley of Sugar river, for instance, with itsbranches, 

 is throughout its course worn deeply into the underlying rocks; on 

 its east side it contains morainic drift, proving that it was worn out 

 before the Glacial period, whilst on the west it extends into the drift- 

 less regions. 



O 



We are thus compelled to believe that during the Glacial period 

 the region destitute of drift had the same altitude relatively to the 

 surrounding country as at present. Before the Glacial period por- 

 tions of the drift-bearing region may indeed have been somewhat 

 higher, for in it a considerable amount of material must have been 

 removed from one place to another, by the glacial forces. The only 

 satisfactory explanation remaining then for the existence of the drift- 

 less region is the one I have proposed. We have already seen that 

 the extent of this region to the eastward was marked out by the west- 

 ern foot of the glacier which followed the valley of Green Bay. That 

 it was not invaded from the north is evidently due to the fact that the 

 glacier or glaciers of that region were deflected to the westward by the 

 influence of the valley of Lake Superior. The details of the movement 

 for this northern country have not been worked out, but it is well known 

 that what is probably the most remarkable and best preserved devel 

 opment of morainic drift in the United States exists on the water- 

 shed south of La,ke Superior. Here the drift attains a very great 

 thickness, and the kettle depressions and small lakes without outlet 

 are even more numerous and characteristic than in other parts of the 

 state. The watershed proper lies some 30-40 miles south of the lake, 

 and 800 to 1200 feet above it, but the morainic drift extends 25 to 50 

 miles further southward. On the east side of the state the drift of 

 Lake Superior merges with that of central and eastern Wisconsin, 

 whilst west of the western moraine of the Green Bay glacier, it dies 

 out somewhat gradually, until 125 to 150 miles south of the lake the 

 the drift limit is reached. Much of the country 25 to 75 miles north 

 of the driftless region, though showing numerous erratics, is quite 

 without any marked signs of glaciation; as, for instance, along the 

 valley of the Wisconsin from Grand Rapids north to Wausau. Fur- 

 ther \vest the drift extends more to the southward. The course of the 

 Lake Superior glaciers conveyed them further and further southward 

 as they moved westward. 



Future investigations will imdoubtedly bring out a close connec- 

 tion between the structure of the Lake Superior valley and the glacial 

 movements south of it. Even the facts now at hand seem to point 

 toward some interesting conclusions. Projecting from the south 



