228 Transactions of the Academy of Science of St. Lowis 
PLEISTOCENE HISTORY OF THE: MIDDLE 
MISSISSIPPI RIVER REGION 
The terraces and drift which have been described, are the 
clues to the succession of events which occurred in the Middle 
Mississippi River Region during Pleistocene time. In inter- 
preting these events it is necessary to recognize that the nmber 
of years involved in the Pleistocene period is small in comparison 
with the number involved in earlier geologic periods. On the 
contrary, the time is long in comparison with the time considered 
by the historian or even by the archaeologist. 
Several estimates of the time involved in the successive stages 
of Pleistocene glaciation have been made by various geologists. 
Perhaps the following time scale by Kay’? (Table VII) based 
on the depth of weathering sustained by the various drift sheets 
in Iowa, is as satisfactory as can be found. Daly3° thinks the 
Aftonian interval may be too long. In describing the Pleistocene 
history of the area the nomenclature adopted by Kay and Leigh- 
ton will be used (Table VIII). 
PrE-PLEISTOCENE TOPOGRAPHY AND DRAINAGE 
In the Middle Mississippi River Region the various ice sheets 
apparently moved over a terrain essentially like the one now 
seen. The master streams were incised in a broad plain. During 
the time the ice sheets were in the region, and for a short 
interval thereafter, the changes wrought by the ice sheets un- 
doubtedly appeared great. These changes proved to be ephemeral. 
Only relatively inconspicuous fragments of the changes wrought 
by the ice are discernible today. The unindurated mantle rock, 
which forms a relatively thin veneer over the bedrock in most 
of the region, contains practically all of the evidence of all the 
changes produced by four successive invasions of mighty ice 
sheets, of which at least three covered or approached very closely 
to the Middle Mississippi River Region. 
It has been thought by some, including Leverett,3' that the 
Mississippi River north of the region under consideration 0¢- 
