744 THE JOURNAL OF GEOLOGY. 
Southward from Hennepin the drift deposits are so heavy that it 
has been found impossible to map out the courses of preglacial 
drainage. A basin or expansion of the valley just north of the 
mouth of the Sangamon River, and a very low altitude of rock 
surface for some distance to the east of this basin is thought to 
indicate the point where a large preglacial tributary enters from 
the east. The Sangamon drains a region which probably then, 
as now, discharged to the north, perhaps joining this large eastern 
tributary. South from the Sangamon River the preglacial trib- 
utaries were about the same size as the present, there being a 
well-defined rock divide between the headwaters of the south 
fork of the Sangamon and the lower portion of the Illinois. The 
western tributaries of the Illinois were probably small as were 
the present ones, owing to its nearness to the Mississippi. 
The present Wabash River follows a preglacial drainage line 
from its bend near Covington, in western Indiana, southward to 
the Ohio River. The stream is in a new valley for a few miles 
above Covington but is again in a preglacial valley in the vicinity 
of La Fayette. It is probable that this preglacial valley leads 
westward past Oxford, Indiana, and thence south to the pre- 
glacial Wabash near Covington. From near Delphi, Indiana, to 
its source the Wabash is mainly in a new course. The head- 
water portion of the streams forming the preglacial Wabash may 
prove to have been in the Lake Michigan basin. But if so the 
connection with the Wabash is through a very much narrower 
trough than that occupied by Lake Michigan. Borings at North 
Judson, Winamac and Monticello, Indiana, situated near the line 
connecting the head of Lake Michigan with the preglacial valley 
at La Fayette, go to a level about 100 feet below the surface of. 
Lake Michigan before entering rock. But within a few miles 
east of this line, rock ledges have an altitude as great as the 
surface of Lake Michigan, while immediately west of this line 
they rise 90-125 feet above that level. This trough cannot have, 
in the vicinity of Monticello, a breadth of more than ten miles. 
Monticello is situated near the middle of the trough. The 
probabilities are, therefore, against the existence of a much 
deeper channel in it. 
