528 THE ILLINOIS GLACIAL LOBE. 



deflections worthy of note have been observed. The South Fork follows 

 nearly the glacial boundary throughout much of its length, but apparently 

 occupies a preglacial channel. 



CACHE RIVER. 



A change of some consequence has occurred in the Tertiary lowland 

 in southern Illinois. The Ohio at one time discharged either wholly or in 

 part through the "Cache Valley," which crosses southern Illinois a few miles 

 north of the present course of the Ohio. Its point of connection with the 

 Cache Valley was immediately north of Metropolis, Illinois, where for a 

 distance of 4 or 5 miles a clay deposit has accumulated in the line of the 

 old valley. The surface of this clay deposit stands only about 75 feet 

 above the present stream and is much lower than the surface of the Tertiary 

 deposits on either side. Wells indicate that the clay has sufficient depth to 

 extend to river level, and it may extend much lower. The surface of this 

 clay deposit presents much less erosion than that of the bordering- Tertiary 

 lowland and evidently is of far more recent date. Judging from the 

 amount of erosion displayed it is no older than the Illinoian drift sheet. It 

 may possibly be as recent as the white clay of southern Illinois, which 

 seems referable to the Iowan stage of glaciation. It is not known as yet 

 whether this channel formerly constituted the sole line of discharge for the 

 Ohio. Possibly the river divided its waters between the Cache channel and 

 its present channel. The cause for the filling which led to the abandon- 

 ment of this valley by the Ohio is not clearly understood. 



The Ohio River falls within the limits of a district covered by another 

 report, hence it is not taken up here. 



WABASH RIVER DRAINAGE BASIN. 



The large drainage basin of the Wabash River, with an area of about 

 33,000 square miles, extends from western Ohio westward across the central 

 portion of Indiana and thence southward to the Ohio, embracing on the 

 west side of its watershed a considerable portion of southeastern Illinois. 

 About one-half of this drainage area was covered by the Illinois glacial 

 lobe, and many important changes have resulted from its occupancy of the 

 region. Indeed, there appears to be very little similarity of outline between 

 th; present watershed and the watershed which in preglacial times had its 



