CHANGES OF DRAINAGE IN SOUTHWESTERN INDIANA. 97 



along the present Mississippi bluffs is a few feet lower than the bed of the 

 abandoned channel. This lower altitude along- the Mississippi is due to the 

 incomplete filling of the preglacial channel by drift. 



CHANGES OF DRAINAGE IN SOUTHWESTERN INDIANA. 



The Illinoian invasion produced important drainage modifications in 

 southwestern Indiana, only a part of which have as yet been fully worked 

 out. The present discussion aims to deal chiefly with the changes which 

 have received the most careful attention. The discussion begins with 

 streams in the southwest corner of Indiana. 



By reference to the map, PL VIII, it will be seen that the greater part 

 of Posey County and adjacent portions of Vanderburg and Gibson counties 

 are now drained westward into the Wabash through Big Creek. The 

 South Fork leads almost directly west from near Evansvillb in Vanderburg 

 County to its junction with the North Fork in central Posey County. The 

 North Fork leads northwest from northern Vanderburg across southwestern 

 Gibson into Posey County, and there near the village of Cynthiana turns 

 southwestward and maintains this course to the Wabash. It receives a 

 tributary from the southeast near the crossing of the Peoria, Decatur and 

 Evansville Railway, but no other tributary of importance enters above its 

 junction with the East Fork. An examination of the head-water portion of 

 the North Fork has brought to light decisive evidence that its preglacial 

 line of discharge was westward into Black River and thence to the Wabash 

 at a point 20 miles or more above the present mouth of the creek. A 

 broad valley deeply filled with drift leads from Big Creek westward to 

 Black River, passing just north of the village of Cynthiana. The aban- 

 doned valley is nearly a mile in width, while the new passage is scarcely 

 one-tenth of a mile in width. The creek soon enters another old valley 

 whose head water portion is occupied by the southeastern tributary referred 

 to above, but whose lower course is abandoned. The Peoria, Decatur and 

 Evansville Railway utilizes the abandoned valley between Big Creek and 

 Poseyville. From Poseyville the valley passes northwest to Black River. 

 In the narrow valley near Cynthiana the present stream thus cuts across a 

 low ridge separating two streams, which formerly drained northwestward 

 into Black River. Below the railway crossing Big Creek soon enters a 



MON XXXVIII 7 



