338 



will probably be found to be the aucient course of the Mississippi river. 

 That it is the position of a preglacial valley is indicated by a deep well 

 at the St. Paul Harvester Works, situated in the present topographical de- 

 pression, which penetrated rock at 235 feet beneath the surface or 628 feet 

 above the sea, which is o5 feet beneath the present low-water level of the 

 Mississippi river at St. Paul. The lake Phalen depression is separated 

 from the head of the Mississippi canon valley by a moraine which is evi- 

 dently based on a comparatively low surface, for it does not rise nearly as 

 high as the drift to the east or west. As seen from the opposite side of 

 the valley, its escarpment or bluff at the head of the old caiiou valley shows 

 such topography as is usually produced by the erosion of drift. In short, 

 all the evidence favors this lake Phalen depression as the position of the 

 pre-glacial continuation of the Mississippi canon valley." (46: 263.) 



From the southeastern corner of St. Paul to Leclair, Hershey believes 

 with other geologists that the valley is pre-glacial. In the vicinity of Du- 

 buque, however, he thinks that the valley is proportionately too small for 

 the stream which it carries, that the preglacial stream flowing past Du- 

 buque could not have been larger than the present Rock river, or possibly 

 no larger than the Pecatonica. The valley is caiion shaped and narrow and 

 the rock floor is about 300 feet below a deep filling of drift. The divide 

 is suggested to be somewhere between La Crosse and Prairie du Chien, par- 

 ticularly where "military ridge" is traversed by the present river. 

 (46:266.) 



Hershey believes that the stream north of this supposed divide flowed 

 toward central Minnesota instead of away from it, but that the reversal 

 came early, before the Ice Age, probably at the end of the Ozarkian, by an 

 uplift in the north, or, as an alternative view, it may have "resulted from 

 the disturbance of other drainage systems by the accumulating northern 

 ice. For instance, it is quite possible that the Kansan ice-sheet had ad- 

 vanced across the outlet of the supposed northwardly flowing ancestor of 

 the upper Mississippi river, obstructing its flowage, and after the produc- 

 tion of a great extra-glacial lake, turning the drainage of the entire region 

 over the lowest point on tlie divide which intervened between it and the 

 headwaters of the southwai'dly flowing central Mississippi river, long before 

 it glaciated the country soutli of the 'driftless area.' " (40:207.) 



