102 GEOLOGY OF EASTERN WISCONSIN 



cilitated bj numerous springs that issue from the upper surface of 

 the clay and shale, keeping them soft and assisting in wearing them 

 down. The dip of the rocks tended to keep the stream hard against 

 the soft stratum, and so hastened its removal. 



There is abundant reason for believing that this process had far 

 advanced before the glacial period, and had already formed so consid- 

 erable a valley as to influence the glacial movement. During the 

 latter part of that period, at least, the ice mass moved southward up 

 the valley, modifying its sides and polishing its rock bottom in the 

 most beautiful manner. Subsequently the valley was partially filled 

 with red clay, without however affecting its general features. The 

 strati graphical relations of this valley are then most conspicuous. 

 If, keeping this prominently in mind, we follow up the valley, we are 

 led southward from Lake Winnebago to what was formerly Lake 

 Horicon, now drained to a marsh, where the head waters of the Rock 

 river gather together and flow southward to the Mississippi. We 

 have then crossed the watershed between the great St. Lawrence and 

 Mississippi basins; and yet on the very divide itself we find the pe- 

 culiar character of the valley still conspicuous. The watershed in 

 the trough of the valley is, in round numbers, 200 feet above Lake 

 Michigan, while on either side the surface rises to more than twice 

 that elevation. 



The Horicon basin is only a repetition of that of Lake Winnebago, 

 overlooked by the same line of cliffs on the east, and scarcely confined 

 by the gentle rise on the west. Descending the Rock river valley, 

 these features are still discernible, but become more and more ob- 

 scured by the heavy drift accumulation of this region, until in the 

 southern tier of counties they are almost entirely concealed by a great 

 drift ridge, a glacial moraine, hereafter to be described, which 

 stretches entirely across the valley and, combined with other elements, 

 gives it a new character. The river has removed from the eastern 

 side of the valley to a more central position, and from it there arise 

 diversified undulating slopes on either side. 



The course of the Rock river through this valley is interesting and 

 for convenience may here be considered, in lieu of its appropriate 

 place. From Horicon marsh southward, it follows, as closely as the 

 drift accumulations will permit, the ledge of JSTiagara limestone above 

 mentioned until about opposite Oconomowoc, when it turns abruptly 

 and flows to the northwest, until it reaches Watertown, where it bends 

 again suddenly to the southward and follows this course, bearing 

 westward, till it leaves the state. The sigmoid flexure thus formed is 

 apparently due to drift accumulations. Were these removed, there is 



