Geography of Ohio 389 



general principles in connection with drainage evolution in Ohio 

 are understood, even though the complete history cannot yet be 

 written. It is known that the relation of our continent to the 

 ocean has shifted several different times since the Permian period. 

 The altitude of the Ohio area has varied with these shifts. Follow- 

 ing an upward movement of the land, stream work was acceler- 

 ated; following a depression, the stream velocity was checked. 

 Under the latter condition, flood plains would appear in the val- 

 leys; whereas the former movement would trench and remove 

 material from the flood plains, and lengthen the rivers through 

 piracy. Furthermore, it is generally felt that other periods of 

 fairly static relationship between land and water existed ; during 

 such intervals, the streams and valleys tended to develop a com- 

 plete drainage cycle. One such period, the Cretaceous, is said 

 to have witnessed, in some parts of eastern North America, a 

 base-level cycle. At the close of this period, the land area of 

 North America was much diminished, and the epi-continental 

 part of the ocean was very wide. The succeeding oscillation of 

 the shoreline must have produced many changes in the streams. 

 From this time, the drainage history of Ohio appears to admit of 

 more satisfactory study. 



Former river courses. So much of the state bears a deep mantle 

 of glacial deposits that a long time may elapse before the buried 

 river channels will have been located to such an extent as will 

 warrant a satisfactory reconstruction of the old drainage of Ohio. 

 In many counties, even where the land surface is level, well- 

 borings have disclosed an irregular rock surface, sometimes cut 

 up by the gorges of young rivers, sometimes showing the gentle 

 slopes of old-age valleys. In the unglaciated area of the state 

 the former drainage lines are more easily deciphered. 



The problem of figuring out the river valleys of the past is 

 further complicated by the fact that much of the glaciated part 

 of the state appears to have been, from early times, a lowland 

 toward which the rivers flowed from the non-glaciated area. 

 Many abandoned valleys, conspicuous outside the drift sheet, 

 have been followed into the drift-buried area, and their further 

 tracing rendered impossible save where well-borings are numerous. 

 For these several reasons, the map (fig. 1), gi^ ing a reconstructed 

 drainage of Ohio, is largely hypothetical. Corrections will be 

 made in time, particularly if activity in sinking deep wtILs should 



