Problems, Middle Mississippi River Region, Pleistocene Time 233 
only fragments of it are preserved in the tributaries of the 
Missouri River. 
CENTRALIAN Epocu 
The Tllinoian ice sheet originated in the Labrador peninsula. 
In the Middle Mississippi River Region the border of the ice 
was essentially parallel to the river itself as far south as Mur- 
physboro, Apparently a terminal moraine was built up by the 
ice in some places. 
So far as the drainage of the Middle Mississippi River is 
concerned, the most important event of the Illinoian age was 
the crossing of the river by a tongue of ice from the main ice 
lobe. This tongue probably extended westward between Edwards- 
ville in Madison County, and Caseyville in St. Clair County, Th- 
nois. As it moved slowly westward, the course of the Mississippi 
River was deflected westward around the ice front, The river 
impinged upon the relatively unresistant Pennsylvanian clastic 
sediments and eroded a scallop in the west bank of the stream. 
Eventually the ice front reached the western side of the trough 
of the river. The northern edge of the tongue of ice rested 
against the cliffs at a point about two miles north of the pump- 
ing station of the water works at Chain-of-Rocks, St. Louis. 
The southern edge of the ice rested upon the present site of 
O’Fallon Park. It is probable that there was a slight reentrant 
in the tongue of ice and that some of the water-lain sediments 
in the Vicinity of Chain-of-Rocks may have been deposited in 
this reéntrant, 
This extension of the ice seems to have formed, at least 
eventually, an effective barrier to the Mississippi above Chain- 
of-Rocks. In the lake thus formed extensive alluvial deposits 
were made. They are so extensive that it would seem quite 
Probable that the tongue of ice built a moraine which formed a 
competent barrier for the waters even after the ice had retreated 
from the region. The upper surface of the lake must have had 
