8 ERA NK LE VERE TT 



the buried soils, reported by McGee from eastern Iowa, 1 occupy 

 a horizon corresponding to the junction of the blue-gray and 

 blue-black tills of southeastern Iowa. This being true the inter- 

 val 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 Mis- 

 sissippi out of the preglacial channel which passes west of the 

 lower rapids. 



It is not certain, however, that the amount of filling 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 east- 

 ward 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 preglacial course. 



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

 the underlying blue-black till. It extends eastward into Illinois 

 beneath the Illinoian till sheet an undetermined distance. The 

 tendency to break into rectangular blocks often serves to dis- 

 tinguish it from the overlying Illinoian till, as well as from the 

 underlying blue-black till, though the Illinoian in places takes on 

 this phase of fracture. Probably the most extensive of the 



'Eleventh Annual Report, U. S. Geol. Surv., 1889-90, pp. 232, 233, 485-496, 

 54i> 509- 



