HYDROLOGY. I37 



Origin aud Geological Relations of the Lakes of Eastern Wis- 

 consin. Analogous to the topics whicli have been under discussion, 

 is the consideration of the position and cause of the numerous lakes 

 which lend their attractions to the beautiful scenery of this portion 

 of the state. 



It has already been said that the great lalce on the east lies in a 

 basin excavated chiefly from the soft rocks of the Devonian age, and 

 that its western edge rests upon the Upper Silurian limestone along 

 the trend of which it lies. The fact that its bottom lies three hun- 

 dred feet below the level of the ocean, shows that it could not have 

 been eroded by running water in its present position; and its width 

 and the regular contour of its bottom forbids the supposition, even if 

 the proper elevation and slope were given it. The drift on its margin 

 contains material that there is every reason for believing came from 

 its bed, and the polishing and grooving of the rocks that form its 

 rim show that it has been the theatre of powerful glacial action, and 

 to this cause its present regular outline and great depth and breadth 

 are undoubtedly due. That it may have been deeply channeled by 

 running water before the glacial period, and that such channels may 

 have had a directing power over the subsequent and more powerfxil 

 glacial action is not improbable. The fact that it lies within seventy 

 miles — less than its own breadth — of a region where the drift action 

 was very slight, and the preglacial contour of the surface was not 

 more than slightly modified, a region whose present elevation is less 

 than three hundred feet above the lake surface, and beyond which the 

 drift shortly disappears entirely, lends support to this view. 



Green Bay, Lalce Winnebago OMd the former Lake Horicon oc- 

 cupy portions of a similar glacial channel, and owe their origin to 

 slight drift obstructions thrown across the valley. The fact that Lake 

 "Winnebago discharges through a channel having a rocky bottom does 

 not militate against this statement, for the real channel of the valley 

 is nearer Clifton, on the east side of the lake. The drift blocks this 

 up, and the lake pours over a low rock barrier that separates it from 

 the parallel valley of Butte des Morts, which occupies a lower geolog- 

 ical horizon. 



Were the drift removed, a channel between Menasha and Clifton 

 would, without question, drain the lake. 



Lalce Horicon was originally confined by drift, which, in time, by 

 its own outflow, was cut away. Its place was subsequently supplied 

 by an artificial dam, which restored the lake. This, again, was re- 

 moved, and the area is now a marsh. 



Lake Poygan seems to have been excavated by glacial action, 



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