PLEISTOCENE HISTORY OF LOWER WISCONSIN RIVER 687 



inhering in each. With the advance of the early Wisconsin stage, 

 the outwash valley train was deposited. During the subsequent 

 period of ice withdrawal, the pounded waters of the Fox River 

 Valley flowed across the Portage divide and down the Wisconsin 

 Valley, cutting away a large part of the valley train. This period 

 must have been rather long, or erosion excessively rapid by a large 

 and powerful river, for more erosion took place then than has taken 

 place since the last withdrawal of the ice. This would not appear 

 to be improbable, for, during this partial withdrawal, the ice may 

 have dammed the lake for a much longer period of time than it did 

 in the final deglaciation. Then when the late Wisconsin ice 

 advanced to the region of Prairie du Sac, the outwash partially 

 filled the channel cut below the early fill. Later, as the ice with- 

 drew, the lake was again dammed east of the Portage divide 

 and the waters flowed westward down the Wisconsin Valley, cutting 

 the lower fill to the level of the low terrace. This would involve 

 less cutting at any one stage than the first suggestion, and at the 

 same time would allow the terrace to stand, as it does, at a con- 

 stant elevation above the present river level, for the waters from 

 the lake probably would cut to the same gradient as do those of 

 the present Wisconsin River. 



While the hypothesis of two Wisconsin advances will explain 

 the presence of the high terrace in two of the valleys east of Mazom- 

 anie and its absence from the other two, it will not account for 

 the absence of this high terrace in the tributaries farther down the 

 Wisconsin River. And since it is evident that the former case can 

 be explained on the basis of one advance into this region, the idea 

 of two ice invasions in Wisconsin time may be discarded as needless. 



SUMMARY 



The terraces of Wisconsin age may be best explained on the 

 hypothesis that they are connected with one glacial advance— that 

 of the late Wisconsin ice-sheet— and that the lower terrace was cut 

 from the higher by waters issuing from Lake Jean Nicolet. 



PART III. THE PLEISTOCENE HISTORY 



The first glacial invasion in Pleistocene time advanced on the 

 eastern side of the region to a position somewhat east of the Wiscon- 



