618 FRANK LEVERETT 
of the bed, so its slope conforms only in a general way with that of 
the stream which occupied it. 
On the line of the old valley leading from Clinton southeast- - 
ward to the Illinois River at Hennepin the rock floor falls to about 
340 feet, as shown by borings at Princeton and Bureau Junction, 
near Hennepin. The Illinois River at Hennepin is 432 feet A.T. 
or about go feet above the rock floor. 
The Illinois River has a fall of only 27 feet in 207 miles from 
Hennepin to the junction with the Mississippi River, or 1.56 inches 
per mile. From the mouth of the Illinois River to St. Louis, a 
distance of 41.5 miles, the Mississippi falls 21 feet, or 6 inches to 
the mile. The rock floor, as shown by borings and bridge excava- 
tions opposite St. Louis, is about 100 feet below the low-water 
level of the river, or 280 feet A.T. This gives a fall of 60 feet from 
Hennepin to St. Louis in 248 miles, or about 3 inches per mile. 
The distance by the course of the present Mississippi from 
Clinton to St. Louis is about the same as along the line to Hennepin 
and down the Illinois, being not far from 300 miles. The fall is 
about 180 feet, of which 48 feet is over rock rapids in a distance of 
24 miles. The remaining 132 feet is at the rate of less than 6 
inches per mile. The rock floor in the valley below Muscatine is 
known to be too feet or more below low water. It has been best 
tested at Fort Madison, and found to be 120 to 135 feet below low 
water, or 365 to 380 feet A.T. ‘There is thus a fall of nearly 100 
feet between Fort Madison and St. Louis in about 200 miles, or 
a rate very similar to that of the present stream, aside from the 
rapids. 
The fact that these buried valleys have beds roo to 180 feet 
below the low-water level of the present streams has led certain 
geologists to infer that the altitude must have been higher than 
now when they were excavated. There are, however, certain 
other conditions that have had influence in causing this difference. 
One of these conditions is a marked lengthening of the lower course 
of the Mississippi, or extension of its mouth gulfward, in the long 
period since the river was flowing on this rock bed. Warren 
Upham has presented evidence from a study of old maps of the 
lower course of the Mississippi that the delta has made a marked 
