78 IOWA ACADEMY OF SCIENCES. 



above the upper rajDids may have trained southeastward 

 through the Green river basin to the Illinois. Hirshey has 

 suggested a northward discharge for the headwater portion of 

 the basin,* a suggestion that awaits adequate investigation. 

 The preglacial valley, which passes the lower rapids on the 

 west, is nearly coincident with the present Mississippi, from 

 the head of these rapids up to Muscatine, but its position 

 farther north has not been ascertained, nor has the size of 

 its drainage basin been even approximately determined. It is 

 probable, however, that much of eastern Iowa was tributary to 

 this preglacial line. 



DATE OF THE DEPLECT[ON ACROSS THE LOWER RAPIDS. 



In previous years attention has been called, both by Mr. 

 Fultz and myself, to evidence that the region around the lower 

 rapids presents a complicated glacial history, f It has been 

 shown that one ice field extended southward from Kewatin, in 

 the Dominion of Canada, across Manitoba, Minnesota and Iowa 

 into Missouri 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, Mo. 

 Two invasions may have been made by that ice field with an 

 intervening deglaciation interval of some length, as indicated 

 by Bain]:. The later, and probably the more extensive, 

 advance is referred to the Kansan stage of glaciation. 



It has also been shown that subsequent to the Kansan stage 

 of glaciation an ice field extended from Labrador and the 

 heights south of Hudson bay southwestward 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 

 displaced the stream to a much greater extent than the deflec- 

 tion past the rapids, as indicated below. The invasion from 

 Labrador, 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 



* AmericaQ Geologist, Vol. XX, 1897, pp. 2t6-26:s. 



+ F. M. Fultz, Proc. lowi Acad, of Sciences for 1895, Vo'. H, pp. 209-213. 

 J bid. 1898, Vol. Ill, pp. 60-62. 

 Franli Loverett, Science, .Ian. 10, 1896. 



American Geologist, Feb.. 1896. 

 Bull., No. 2, Chi. Acad. Scl.. May, 1897. 

 Froc. Iowa Acad. Sci., 1897, Vol. V, pp. 71-74. 

 :|: Proc. Iowa Acad. Sci. for 1897, Vol. V, pp. 88-101. 



