Problems, Middle Mississippi River Region, Pleistocene Time 173 
In addition to these three extensive terraces which may 
be found in numerous tributaries to the master streams, there 
are a few terraces found in some of the streams in Illinois 
south of St. Louis which appear to be independent of one 
another and of the larger terraces. These are called in this 
report terraces of the Kaskasia River Type. 
THEORETICAL CONSIDERATIONS 
In attempting to reconstruct the original surfaces of deposits 
laid down in a great river systum, a few simple facts should 
be horne in mind. If the deposit were formed by an unob- 
structed stream in its flood plain, the initial surface of the de- 
posits would have a longitudinal slope equal to the gradient of the 
stream at the time it was depositing the sediment. If, on the 
contrary, the deposit were made in a portion of the stream that 
was temporarily dammed, thereby forming a lake, then the in- 
itial surface of the deposit would be essentially horizontal. 
If a delta were deposited in such a lake, the topset beds would 
have been horizontal, but the foreset beds would have had a 
very gradual slope away from the source of the sediment. 
In attempting to reconstruct the initial surface of a terrace, 
care must be taken to eliminate all flat surfaces produced sub- 
sequently to the formation of the initial surface, whether by 
erosion, by rain wash, or by eolian deposition. By properly plot- 
ting the terrace fragments on the longitudinal profile of a river, 
the nature of the original deposit may be ascertained. This is 
brought out in Figure 2. 
METHODS OF STUDY 
In the specific region under investigation the structure of the 
terraces was observed in recent road cuts and in places where 
Streams have undercut their banks, thereby exposing clear sec- 
tions. Occasionally a soil auger was used. The elevation of 
the tops of the terraces was recorded, using a Paulin System 
altimeter, checked as frequently as possible against established 
bench marks. In some instances where elevations determined 
