198 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



difference in the relative elevation of the Lake Erie Basin and the Ohio 

 Basin has been produced, by which the slopes toward the Erie Basin have 

 been greatly increased; but of this there is no specific evidence so far as 

 the writer is aware. The available evidence seems to support the view 

 that this diversion of the old Monongahela system to the Middle Ohio 

 system took place as a result of the first glaciation, though it may have been 

 brought about some tim6 before the ice sheet had reached its farthest limits. 

 This diversion seems to have been a permanent one; at least no evidence 

 of a return to the Erie Basin after the earliest glaciation has been noted. 



This earliest glaciation appears also to have thrown the several pre- 

 glacial components of the Allegheny into their present course, and so far as 

 evidence is forthcoming the diversions were permanent ones. 



The diversion of the Middle Ohio or old Kanawha system into the 

 Lower Ohio can not be referred so confidently to glaciation as the diver- 

 sions just mentioned, nor is the available evidence such that any other 

 factors can be cited to have caused the diversion. Further attention is 

 given this matter under the subjects of "Piracy" and "Earth movements." 



The diversions near Cincinnati are probably in large part due to gla- 

 ciation. It is, however, possible that the diversion past the south side of 

 the Walnut Hills resulted from stream piracy prior to the earliest glaciation 

 of the region. Whether the diversion from the old channel past Hamilton 

 to the present direct channel from Cincinnati to the mouth of the Ohio took 

 place as late as the Illinoian stage of glaciation or at an earlier stage has 

 not been determined. 



The changes along the old Kanawha system by which Teays Valley 

 and the valley south of Ashland, Ky., became abandoned are perhaps 

 indirectly due to glaciation. The ponding of waters probably extended 

 into these valleys at each glacial stage down to the lowan. The amount 

 of erosion accomplished after the diversions took place is, however, so great 

 that the diversions seem likely to have occurred as early as the earliest stage 

 of glaciation. 



PlEACr. 



Where one drainage system has an advantage over an adjoining one, 

 shiftings of the divides and even important changes in di-aiuage courses may 

 result. It is probable that such shiftings and changes have been common 

 in early stages of development of drainage systems. They appear also to 

 have taken place to some extent in drainage systems that are somewhat 



