6 FRANK LEV E RETT 



and that it spread eastward beyond the valley of the Mississippi, 

 from near the southern end of the Driftless Area of the upper 

 Mississippi to the vicinity of Hannibal, Missouri. Two invasions 

 may have been made by that ice-field with an intervening degla- 

 ciation interval of some length, as indicated by Bain. 1 The 

 later and probably the more extensive advance is referred to the 

 Kansan stage of glaciation. It has also been shown that subse- 

 quent to the Kansan stage of glaciation an ice-field extended 

 from Labrador and the heights south of Hudson Bay, southwest- 

 ward across Michigan, the Lake Michigan Basin, and Illinois 

 into southeastern Iowa. 



The Kewatin ice-field not only covered the preglacial valley 

 near the lower rapids, but also the district which the stream 

 traverses in passing the rapids. It was thus liable to have dis- 

 placed the stream to a much greater extent than the deflection 

 past the rapids, as indicated below. The invasion from Labra- 

 dor, on the other hand, appears to have barely reached to the 

 rapids and may not have interfered seriously with drainage 

 across them, though it greatly disturbed the course of the Mis- 

 sissippi above the rapids. It did not reach the section of the 

 preglacial valley west of the rapids. The deflection from the 

 preglacial channel must, therefore, be due to the Kewatin ice- 

 field. 



But since the Kewatin ice-field may have twice invaded this 

 region it is necessary to inquire into the probable effect of each of 

 its two invasions. If it be found that the earlier invasion extended 

 beyond the line of the preglacial valley and deposited sufficient 

 material to prevent the reestablishment of the river along the 

 preglacial line, some deflection at this early date must have 

 occurred. The deflection, however, need not necessarily have 

 thrown the stream into its present course across the rapids. 

 That course may have been taken as a result of the later invasion 

 of the Kewatin ice-field, if not -as a result of the still later 



Vol. Ill, pp. 60-62 ; Frank Leverett, Science, January 10, 1896 ; American Geolo- 

 gist, February 1896; Bull. No. 2, Chi. Acad. Sci., May 1897; Proc. Iowa Acad. Sci., 

 1897, Vol. V, pp. 71-74. 



'Proc. Iowa Acad. Sci. for 1897, Vol. V, pp. 86-101. 



