234 Transactions of the Academy of Science of St. Louis 
an elevation of about 530 feet. The elevation of the upper sur- 
face of the deposit is usually between 510 and 530 feet. This 
is the Cuivre Terrace. The lake in which the deposit was formed 
came in contact with the front of the ice sheet for only a short 
distance, from the point in St. Louis County north of the St. 
Louis Pumping Station before referred to, for a distance of 
perhaps 20 miles to the east and the northeast. The only place 
where ice-rafted boulders were found in the Cuivre Terrace 
was in the vicinity of this ice front near the Oak Grove School, 
Madison County, Illinois. 
The spillway for this lake while the ice was still present may 
have been over rather low ground west of O’Fallon Park. After 
the retreat of the ice, it would seem probable that the spillway 
was over the moraine. The stratification which can be seen in 
the upper ten feet of the till at Edwardsville certainly suggests 
reworking by water. The elevation of the base of this reworked 
till is 480 feet. The problem of spillways for the rivers north 
of the ice dam presents several interesting points for further 
study, 
In post-Illinoian, or Sangamon, time the morainal dam was 
subjected to erosion. It probably did not take very long for a 
channel to be eroded capable of removing the water from the 
lake; but the complete erosion of the moraine undoubtedly took 
much longer. In fact, it is possible that the drift has not been 
entirely removed up to the present time. It may be that 
the bases of the so-called Indian mounds in the flood plain of 
the Mississippi River near Caseyville are remnants of this 
moraine formed in Illinoian time, though the writer is inclined 
to doubt it. 
During the retreat of the Illinoian ice sheet, streams in Illinois 
whose headwaters were receiving the sediment-laden water from 
the melting ice were aggrading and built up flood plains, which 
later erosion left in the form of terraces whose longitudinal 
surfaces sloped essentially parallel to the profile of the streams. 
