Problems, Middle Mississippi River Region, Pleistocene Time 235 
Such terraces may be traced along the Kaskaskia River and in 
some other tributaries of the Middle Mississippi on the Illinois 
side of the river, south of St. Louis. 
ELporRAN EpocH 
It is held by geologists that a stream flowing from a melting 
glacier is ordinarily an aggrading stream. The Middle Missis- 
Sippi trough was apparently aggraded to an elevation of about 
50 feet above the present floodplain during the retreat of the 
Wisconsin ice. This aggradation produced the Festus Terrace. 
While the main stream was thus being aggraded, tributaries 
on both sides of the river were dammed, producing lakes in 
their lower courses. At times of flood the sediment-ladened 
waters of the Mississippi carried deposits into the mouths of 
the tributary streams, thereby producing delta-like deposits. 
As the Wisconsin ice sheet retreated, there came a time when 
the waters from the retreating ice passed through several es 
of the Glacial Great Lakes. These lakes provided settling basins 
for the great volumes of water from the melting Ape Pai 
Mississippi River, having deposited most of its load, again be- 
came capable of erosion. It was, in a sense, a rejuvenated 
Stream. As the main trough was cleared of sediments, most of 
the tributary streams cut through the delta-like deposits at their 
mouths. The vast volumes of the Mississippi River caused 1t 
to remove the sediment it had deposited more rapidly sites pele 
completely than was the case in most of the tributaries. This 
is more specifically true for the Middle Mississippi pepe: 
than above it. Remnants of the old floodplain depo se we ese 
to be found as terraces bordering the Illinois River in a 
County. 
Similar terraces may be traced in both the Missouri and 
Mississippi rivers. But below this triple confluence, I 
that can be definitely identified as due to aggradation during 
Wisconsin time may be pointed out in the trough of the river. 
