742 TE JOOKN ALN OR NGIEOMG GW 
altitudes yet brought to light by borings are along the line lead- 
ing through eastern Iowa, but that the line leading southeastward 
to the Illinois has been less perfectly tested by borings and hence 
may contain a channel equally deep. 
From Muscatine to the Mississippi embayment of the Gulf 
above Cairo, Illinois, the present line of the Mississippi is nearly 
coincident with that of the preglacial line. The most important 
diversion is at the lower rapids just above Keokuk, where for a 
distance of twelve miles the present stream is cutting a new valley 
parallel to and slightly east of the preglacial course. There are 
slight diversions of the present stream into the edge of the bor- 
dering bluffs at two points below St. Louis, similar to the one 
noted above, at Fulton. One of these is at Fountain Bluff and 
the other at Thebes, Illinois. The preglacial course is plainly 
traceable around the rocky points which the present stream cuts 
off, and is utilized at flood seasons in both cases. 
In the district between St. Paul, Minnesota, and the mouth of the 
Illinois the eastern tributaries of the Mississippi, with the excep- 
tion of the Wisconsin River, were very small, owing to the exis- 
tence of a large parallel valley traversing southern Wisconsin and 
western Illinois, as shown below. Western tributaries of the 
Mississippi appear to have drained a wide area but their courses 
have not as yet been worked out, for the region is very heavily 
covered with drift, and a large number of borings will be neces- 
sary to establish, even approximately, the course of preglacial 
drainage. It is certain that the present and preglacial systems 
of drainage are coincident in but a few places. 
The preglacial Rock River drainage basin apparently extended 
to the Mineral Ridge axis on the northwest and to the Niagara 
escarpment on the east. The course of the main line of drain- 
age was probably along the axis of the trough west of the Niag- 
ara escarpment. Whether the drainage of this trough was then 
divided, as at present, between Green Bay and Rock River, 
the writer is not prepared to judge, for this is a district which 
has not been personally examined and the published data are 
scarcely adequate to warrant a decision. 
Rock River follows nearly its preglacial course from the Kettle 
