756 THE JOURNAL OF GEOLOGY. 
The available data seem, therefore, to point quite strongly to 
a depression in the portion of the Illinois valley north from 
Beardstown. To fully establish this depression, it will be neces- 
sary to make certain that there is no narrow gorge leading down 
the Illinois, with a rock floor sufficiently low to correspond with 
that at Princeton. If the absence of such a gorge were demon- 
strated by a full series of test borings across the valley at points 
below Princeton, there would still remain an element of uncertainty 
in the fact thata local deepening of the valley through glacial agen- 
cies may have been produced at Princeton during the ice invasion. 
We would remark, however, that the valley has a trend transverse 
to the direction of ice-movement and, therefore, unfavorable for 
erosion by the ice, or by subglacial streams. 
The probability of northward depression in the Illinois valley 
being so strong, we will refer briefly to a possible cause for such 
a depression. To the general cause for northward depression, 
found in the weight of the ice-sheet, we have here an additional 
cause in the immense amount of drift deposited along the course 
of the old valley. This valley, from the vicinity of Pekin north- 
ward, past Princeton, is bordered by a very bulky moraine, 
which, for some distance northward from Princeton occupies the 
supposed old course of the Rock-Illinois. The drift in this 
region is rarely less than 200 feet, and probably reaches a depth 
of between 500 and 600 feet north of Princeton, where the 
moraine occupies the old valley. This weight, unlike the ice- 
sheet, still continues an obstacle to the return of the valley floor 
to its former altitude, if not as a direct cause of depression. It 
may be found that some depression has been produced by it since 
the date of the lake outlet. Ifso the very low gradient of this 
portion of the Illinois would be explained only in part by the 
greater volume of the lake outlet. Recent warping of the valley 
may become an important factor. 
The Wabash Valley—The few borings of which we have. 
record along the Wabash below Covington, are situated in the 
middle of the valley and probably test its deepest portion. The 
altitudes of the valley floor and present stream at points where 
these borings have been made are as follows: 
