80 IOWA ACADEMY OF SCIENCES 



of greenstone rocks. These differences have naturally led to 

 the suspicion that two quite distinct sheets of till are present 

 and this suspicion is confirmed by the occasional occurrence of 

 a black soil at the surface of the blue-black till. Such 

 exposures are rare compared with those of the Yarmouth soil 

 found between the Kansan and Illinoian till sheets,* but their 

 rare occurrence may not demonstrate that the interval of 

 deglaciation is of minor importance. From conversations with 

 Calvin, Norton and Bain, I am led to think that a large part of 

 the buried soils reported by McGee, from eastern Iowa, f 

 occupy a horizon corresponding to the junction of the blue- 

 gray and blue-black tills of southeastern Iowa. This being 

 true, the interval of deglaciation between the blue-gray and 

 blue-black tills becomes of much importance. 



The sheet of blue-black till has been found to occur at 

 points farther east than the lower rapids. It occurs in the 

 Mississippi valley in the vicinity of Ft. Madison, Iowa, and in 

 Hancock and Adams counties, Illinois, east and southeast of 

 the rapids. There is little doubt, therefore, that during the 

 deposition of this till the Kewatin ice field was sufficiently 

 extensive to force the Mississippi out of the preglacial channel 

 which passes west of the lower rapids. 



It is not certain, however, that the auiount of flliiu/ in that 

 valley was sufficient to prevent the return of the stream to its 

 preglacial course in the interval between the deposition of the 

 blue-black till and the blue- gray till. The blue-black till in 

 the vicinity of Ft. Madison is found to rise to a height of only 

 sixty to seventy-five feet above the present stream, or nearly 

 seventy- five feet less than would probably have been necessary 

 to throw the stream from the preglacial channel into its 

 present course across the rapids. This may possibly have 

 been sufficient to throw the drainage of the portion above the 

 lower rapids eastward into the Illinois, either by way of the 

 Green river basin or by some line farther south that is now 

 completely concealed by the later sheets of drift. But it 

 seems quite as probable that the stream returned to its pre- 

 glacial course. 



The blue-gray till seems to be fully as extensive a sheet as 

 the underlying blue-black till. It extends eastward into 



*See Proc. Iowa Aead. Sci., Vol. V, 1897, pp. 81-86. 



tEleventh Annual Report U. S. Geol. Surv., 1889-90, pp. 233, 238, 485, 496. 541, 



