80 
IOWA ACADEMY OP 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 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 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. Io\ya Acad. Sci., Vol. V, 1897, pp. 81-86. 
t Eleventh Annual Report U. S. Geol. Surv., 1889-90, pp. 332, 233, 485, 496, 541, 569. 
