IOWA ACADEMY OP SCIENCES. 
67 
extensive deposits of water- laid gravels. It was at once 
assumed that the two drift sheets at Af ton J unction were the 
upper and lower till of McGee. Within the past year or so 
Mr. Bain, of the Iowa Survey, studied the Afton deposits and 
became convinced that the till above the gravels and soil bed 
was equivalent to McGee’s lower till, that the upper till was 
not present in that part of Iowa, and that the lower bed at 
Afton is distinct from any ol the drift sheets recognized in 
northeastern Iowa. The locality was afterward visited in com- 
pany with Professor Chamberlain and others and Bain’s con- 
clusions were fully confirmed. Here is a drift sheet older than 
McGee’s lower till. In the meantime a lobe of drift, crossing 
the northern boundary of the state with a width reaching from 
Worth to Dickinson counties and narrowing toward its apex at 
Des Moines, was recognized as younger than the upper till of 
McGee. This youngest drift has been named Wisconsin by 
Chamberlin, McGee’s upper till Chamberlin calls Iowan, and 
the lower till Kansan. The drift beneath the Aftonian soil and 
gravels is so far unnamed, but it is provisionally called sub- 
Aftonian. Mr. Leverett has recently shown that a bed of till 
occupying a small area in southeastern Iowa was deposited by 
glaciers coming from the northeast through Illinois. These 
glaciers spread a characteristic sheet of till over a large part 
of the state last named, and this drift sheet, which is younger 
than the Kansan and older than the Iowan, is called the Illinois. 
There is therefore in Iowa a record of five ice invasions 
separated from each other by interglacial periods of consider- 
able duration. The drift sheets corresponding to the several 
ice invasions are named in the order of age: 1, sub- Aftonian; 2, 
Kansan; 3, Illinois; 4, Iowan; 5, Wisconsin. The interglacial 
deposits between the first and second are called Aftonian. 
Kespecting the length of the interglacial periods it may be 
shown that many of them were many times longer than the 
period that has elapsed since the retreat of the Wisconsin ice. 
The Oelwein cut to which reference is made in the papers 
under discussion is particularly interesting for the reason that 
it shows three of these drift sheets, the sub-Aftonian, Kansan 
and Iowan, in their normal relations. The first and second are 
separated by the peat bed which represents the Aftonian inter- 
glacial period. The second and third are separated by a zone 
of oxidation. The Iowan drift at the top of the cut is thin, but 
it contains boulders fresh as when they left the parent ledge. 
