343 



THE PREGLACIAL DRxilNAGE OF THE BASIN OF THE WABASH 

 AND LOWEK OHIO. 



About one-half of the drainage basin of the Wabash river is so deeply 

 buried under glacial deposits that there is A^ery little similarity between the 

 modern watershed and the watershed of the preglacial streams that dis- 

 charged through the lower course of the Wabash. The preglacial rock sur- 

 face was probably very rough, for the drift varies within short distances 

 from a few feet to over 200 feet in depth. 



The Wabash river at Lafayette is flowing in its original channel. 

 Below Lafayette the preglacial channel runs westward and then southward, 

 meeting the present Wabash at Covington. Below Covington the present 

 river follows the ancient channel. Nothing is known of the upper portion 

 of the preglacial Wabash above Lafayette. A study of the drift covered 

 rocks reveals a divide extending south along the west side of Lake Mich- 

 igan, and curving to the east into Indiana. It is from 100 to 200 feet above 

 Lake Michigan and is deeply sculptured by preglacial streams and thor- 

 oughly drift covered. It has been suggested that the Lake Michigan basin 

 was the headwater portion of the Wabash in preglacial time. On this point 

 Leverett says (62) : "The headwater portion of the Wabash stream form- 

 ing 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 both 

 North Judson [497 ft], Winamac [490 ft.] and Mouticello [407 ft.], Indiana, 

 situated near the line connecting the heads of Lake Michigan with the pre- 

 glacial valley at Lafayette, 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 can not have, in the vicinity of Mouticello, a breadth of moi-e 

 than ten miles. Mouticello is situated near the middle of the trough. The 

 probabilities are, therefore, against the existence of a much deeper channel 

 in it." 



Leverett (65) suggests that the old channel which passes into Grant 

 County from Ohio may be a headwater purtion of the preglacial Wabash. 

 The modern Wabash has not completely excavated the ancient valley to its 

 full width above Tcrrc Haute, but below that city the excavation is more 

 nearly complete. 



