341 



aloug the Minnesota valley, that in large jnirt its erosion was effected in 

 preglacial time and dnring stages of retreat and readvance of the ice-sheet 

 previous to the final departure." In an earlier article he says that the 

 valley was eroded in the Lower Magnesian and Calciferous formations, 

 before the Cretaceous subsidence, was re-elsvated, and, in the first prin- 

 cipal epoch, of glaciation, covered with a "thick, unbroken, moderately 

 undulating expanse of till" and partly re-excavated by an interglacial 

 stream which, guided by the slope determined by preglacial erosion, coin- 

 cides along much of its way with the old valley eroded in these strata 

 before the Ice Age (119:109). 



The St. Croix river has been discussed by Berkey, R. T. Chamberlin 

 (IS), Elftman (30), and Upham (121, 1S2) and others. Chamberlin thinks 

 the preglacial course from the Dalles of the St. Croix was east to the pre- 

 glacial Apple river ; while the other writers would have it to the westward. 

 The streams of the driftless area in Wisconsin, Minnesota, Iowa and 

 Illinois are preglacial (20, 45-47, 64). 



The Wisconsin river is in the preglacial channel below Prairie du Sac. 

 Below Kilbourn City, according to Salisbury and Atwood (89), the pre- 

 glacial course is east of the present stream, through the Lower Baraboo 

 Narrows, and the Devils Lake Gap of the quartzite ridges on either side 

 of Baraboo, Wisconsin. According to Fenneman (33), a preglacial tribu- 

 tary of the Wisconsin passed northwest through Kegonsa and Mendota 

 lakes. Fenneman finds sections of preglacial channels marked out by the 

 lake basins in southeastern Wisconsin, as shown on the map. 



In Illinois, outside of the thick Wisconsin drift which obscuz'es the 

 preglacial valleys of the northeastern part of the State, the preglacial val- 

 leys can be fairly easily traced. Outside of the triangular area whose 

 vertices are at Clinton, Hennepin and Rockford, the directions of the pres- 

 ent and the preglacial drainage systems are coincident. In this triangular 

 area, the changes have been considerable (62, 64). The preglacial valley 

 of Rock river from .Janesville, Wisconsin, to the edge of the Wisconsin 

 drift southward is easily traced, but beyond that the drift rises 100 feet 

 above the preglacial bluffs and its course can be traced only by borings. 

 Its bed is found to be a little lower than that of the Mississippi to the west, 

 descending 210 feet in a distance of 100 miles south from the Wisconsin 

 border. It probably was tributary of the preglacial Mississippi if that 

 river joined the present Illinois. 



