LOWER OHIO DRAINAGE SYSTEM. 109 



ward, though not so great as at New Martinsville, and he is inclined to refer 

 the expansion there to the increased dip of the strata. This enlargement of 

 the valley seems to have antedated its filling with glacial gravel of AVis- 

 consin age. Since the filling occurred the stream has been shifting its course 

 over the gravel bottom, and at present, in both the Moundsville and the 

 New Martinsville expansion, it is following the west bluff instead of the east. 



LOWER OHIO SYSTEM. 

 PROBABLE EXTENT. 



Under this name will be discussed the portion of the Ohio below the 

 old divide near Manchester, together with such of its tributaries as are 

 concerned in the drainage and glacial history of the region. The Ten- 

 nessee and Cumberland rivers, which now enter the Ohio near its mouth, 

 are practically independent of the Ohio, since their mouths are within the 

 Tertiary valley of the Mississippi. Grreen, Salt, Kentucky, and Licking 

 rivers are the main large southern tributaries. These and the small south- 

 ern tributaries have apparently suffered very little disturbance by g'lacia- 

 tion. The northern tributaries. Little and Great Miami rivers, and the 

 Wabash, with its main affluents, White and East White rivers, have had 

 their drainage systems greatly modified b)^ glaciation, so that it is difficult, 

 if not impossible, to restore the preglacial system. It is probable, however, 

 that a lai'ge part of the present drainage basins of these rivers was tributary 

 to the Lower Ohio in preglacial time. Attention is called below to the 

 question of a former northward discharge of part of the Ohio drainage basin 

 through the Great Miami Basin. 



RELATION TO TOPOGRAPHIC FEATURES. 



In the portion of the Ohio below the old divide near Manchester 

 several rock formations are crossed which have yielded very unequally 

 to subaerial degradation, and now present a series of escarpments and 

 basins that are more impressive as topographic features than the valley of 

 the river. These topographic featiires, however, exert but little iiifluence 

 upon the course of the Ohio and its tributaries. They trend in line with 

 the strike, while the Ohio takes a course more nearly in harmony with the 

 dip of the rock formations. The river passes from the Cincinnati arch 

 across the low Niagara escarpment, formed by the Lockport limestone, 

 down to the basin formed in the Devonian shale and tlience on through 



