PIRIEGILACHAUIL, WAILLIGIES (UF IEE MHL SSH SSI 745 
Borings have been made at frequent intervals westward and 
northwestward from this trough across northwestern Indiana and 
northeastern Illinois and none of them show so low a rock 
surface as this line presents. They usually enter rock above the 
level of Lake Michigan. There seems, therefore, no ground for 
suggesting a southward or southwestward outlet of the Lake 
Michigan basin further west than the line which leads from the 
head of the lake to La Fayette, Indiana. Furthermore this 
seems the most probable line for a channel, since it follows 
nearly the western edge of the soft Devonian shales, where 
degradation would naturally proceed more rapidly than in the 
firm and resistant limestone ledges to the west. 
The tributaries of the Wabash in its lower portion follow, to 
a large extent, their preglacial courses. The eastern tributaries 
drain a driftless region in Indiana, while the western drain a 
region thinly clad with drift in southern Illinois. From the 
latitude of Terre Haute and St. Louis southward, the Mississippi 
and Wabash seem to have divided the drainage of southern 
Illinois in preglacial times, about as they do at present. 
The Ohio River seems to have been greatly enlarged by the 
ice invasion. But little study has been given the lower course of 
this stream. It is probable, however, that from southern Ohio 
to its mouth it follows nearly the preglacial line, since that 
portion was encroached upon but little by the ice-sheet. The 
upper course from eastern Ohio northward seems to have dis- 
charged to the Lake Erie basin.* The Muskingum Valley of 
eastern Ohio is thought by Professor W. G. Tight, to have dis- 
charged northwestward in preglacial times, instead of southward 
into the Ohio.?, The data collected by Professor Tight make it 
appear quite probable that this valley was a tributary to the 
Scioto basin. The view that it continued across that basin into 
the Wabash seems less fully sustained. 
*For a presentation of the evidence see Amer. Jour. Sci., Vol. XLVII., 1894, pp. 
247-283. The paper referred to contains references to the literature of the subject, 
which is already quite extensive. 
2 A contribution to the knowledge of the Preglacial Drainage of the Ohio, W. G. 
TicHT, Bull. Sci. Lab., Denison University, Vol, VIII., pp. 35-61. With two plates. 
Granville, Ohio, 1894. 
