PLEISTOCENE HISTORY OF MISSISSIPPI VALLEY 617 
followed by the preglacial Mississippi, for other preglacial valleys 
converge toward it, one followed by the lower course of the Rock 
River, reversed, and another by Duck Creek, and its continuation 
in an abandoned valley to the east of Rock Island. The breadth 
of the valley leading to the Illinois also appears to be greater than 
that of the one to the southwest. <A different stream from that 
which opened the upper Mississippi Valley probably developed the 
valley now utilized by the Mississippi below Muscatine. It is 
likely to have taken the drainage of much of eastern Iowa and 
drained the region now tributary to Iowa River, though it does 
not follow the course of that stream. It may, perhaps, be appro- 
priately called the preglacial Iowa River. 
The two drainage lines came together at the mouth of the 
Illinois River. The western one departed slightly from the present 
Mississippi in the southeast corner of Iowa, the old course being 
southwest from Fort Madison to the mouth of the Des Moines 
River, while the present stream turns southward across the Des 
Moines rapids to Keokuk, and joins the old valley at Warsaw, 
Illinois. 
The present river follows essentially the course of the pregla- 
cial valley from the mouth of the Illinois River to Thebes in south- 
ern Illinois, though it has cut off points from the west bluff near 
Grand Tower, which in high-water stages stand as islands in the 
valley. The Mississippi turns into a rock gorge at Thebes, and a 
few miles below comes into the old Ohio Valley, while the old 
valley runs southwestward, and unites with the Ohio Valley farther 
down. 
RELATION OF PRESENT STREAM BED TO BURIED ROCK FLOOR 
The present stream falls from 683 feet A.T. at St. Paul to 566 
feet at Clinton, or 117 feet in about 300 miles. This section includes 
Lake Pepin which holds a level of 664 feet for 25 miles. Aside 
from this lake the stream has an average fall of about 5 inches per 
mile. The fall of the buried rock floor is very similar, for it stands 
480 to 500 feet A.T. near St. Paul and is below 400 feet in the 
vicinity of Clinton. The rock floor has more or less inequality 
because of variations in the scour of the stream, and in the hardness 
