754 LLL i OOLINATE OLN GIZOLE OGN A 
the Mississippi. The fall in that stream in the 222 miles from 
the mouth of the Hlinois to Cairo, being 133 feet, or about 7.18 
inches per mile.* We seem to have in the lower Illinois Valley 
a partially obliterated lake in which the outlet has worked its 
head up the valley to the vicinity of Peoria, with a decrease in 
slope headwards. Then comes Peoria Lake, between Peoria and 
Chillicothe, at the head of which is a silt-filled district, in which 
the river shows an increase in slope headwards to Peru, the old 
head of the pool or lake. The only available data concerning 
the amount of filling is obtained at the Santa Fé bridge at Chil- 
licothe, where it is probably much lighter than nearer the head 
of the old pool. The bridge foundations are reported to have 
reached the bottom of the river silts at a depth of thirty feet 
below the bank, or about twenty feet below the bed, there being 
coarse sand and gravel at lower depths.* A series of test bor- 
ings along the valley is needed to bring out satisfactorily the 
condition of the valley bed at the beginning of the present 
period of deposition. 
It is a question whether the lake outlet was adequate to pro- 
duce a channel with such extremely low grade as this valley 
presents between Hennepin and its mouth. Another possible 
factor should be considered, that of a warping of the valley sub- 
sequent to this period of excavation. 
This leads us to the discussion of the attitude of the floor of 
the preglacial valley. By reference to the above table it will be 
observed that the valley floor at Princeton is but 270 feet A. T., 
while the lowest known point in the valley bottom opposite St. 
Louis, forty miles below the mouth of the Illinois and 260 miles 
below Princeton, is 284 feet A. T. or fourteen feet higher than at 
Princeton. It is barely possible, but scarcely probable, that there 
exists, opposite St. Louis, a channel sufficiently deep to givea 
fair gradient for the valley floor from Princeton to St. Louis. 
A gradient of but three inches to the mile would require a 
* The distance we have given is about that of the high water stage. COOLEY’S 
report to the Chicago Drainage Commission gives 238 miles as the distance along the 
river channel. This would reduce the fall per mile to 6.70 inches. 
? CoOLEY’s Report to Chicago Drainage Commission, p. 58. 
