PLEISTOCENE HISTORY OF LOWER WISCONSIN RIVER 677 



2. MID-COURSE DRIFT 



a) Origin. — The cross-bedding at Blue River makes it evident 

 that the drift in the mid-course was brought in from the east 

 (Fig. 3). This being the case, the upper surface of these terrace 

 deposits should slope conspicuously toward the west, for, while 

 large bowlders so common in most of the exposures may have been 

 -carried in bergs, the mass of the material is fluvial and must have 

 been transported by a glacial river — a river having powerful current 

 and fairly steep gradient. A steep gradient may well be postulated 

 for such a glacial river, for there appears, if the bedrock floor of 

 the valley be considered, to have been down-warping at the eastern 

 end of the region. The bedrock in the valley bottom near Prairie 

 du Sac has an altitude of something less than 500 feet,^ while at 

 Prairie du Chien, near the mouth, its altitude is 490 feet, making 

 a gradient of only two inches to the mile (Fig. 2). The preglacial 

 river with so low a gradient as this could not have eroded so deep 

 and narrow a valley as the rock bottom of this part of the Wis- 

 consin Valley appears to be. The unavoidable inference is that the 

 eastern end of the region must have been higher in preglacial and 

 possibly early glacial times, and must have subsided before the last 

 glacial advance. Data from neighboring regions also suggest that 

 such warping has taken place.^ 



b) Age. — Old drift has been described on the eastern margin 

 of the driftless area by Leverett, Weidman, and Alden,^ and called 

 by them pre-IlHnoian in age. The absence of calcareous material in 

 the old drift in the mid-course of the Wisconsin Valley, even where 

 seen 10 and 12 feet below the surface, suggests that it is as old as 

 Kansan and probably older, i.e., the outwash from the first ice 

 advance. 



3. WESTERN DRIFT 



a) Origin. — The drift on the Bridgeport terrace must be either 

 glacial or fluvio-glacial in origin. The large number of striated and 



' W. C. Alden, U.S. Geol. Surv. Prof. Paper 106 (1918), Plate II. 



" W. C. Alden, op. cit.; F. Leverett, Journal of Geology, Vol. Ill (1895), p. 740; 

 E. W. Shaw, Bull. Geol. Soc. Amer., Vol. XXVI (1914), P- 67. 



3 F. Leverett, U.S. Geol. Surv. Monograph 38 (1899), pp. 109-10; S. Weidman, 

 Wis. Geol. Surv., Btill. 16 (1907), p. 433; W. C. Alden, op. cit., p. 168. 



