DRAINAGE OF THE GLACIAL PERIOD. 327 



instructive to warrant a special description. The upper part 

 of the Ohio River, between Pittsburg and New Kichmond, 

 in the vicinity of Cincinnati, lies entirely outside of the gla- 

 ciated area, while nearly all of its northern tributaries rise 

 within that area. In the happy phraseology of Professor 

 Dana, the Ohio River becomes, therefore, the great distribu- 

 ter^ while its northern tributaries are the principal contribu- 

 tor*, of terrace material. Now, it is observable that, wherever 

 a large contributor of drift material comes in from the north, 

 there is a great increase in the extent and height of the ter- 

 races for some distance below, and the material of which the 

 terrace is composed is coarser at these points. For example, 

 in the neighborhood of Cincinnati, the Ohio is joined by the 

 Little Miami, and twenty miles below, at Lawrenceburg, by 

 the Great Miami. Throughout their entire course these 

 tributary streams flow through a region deeply covered with 

 glacial deposits, As a consequence, the terraces here are of 

 great height and width. At Cincinnati, the upper terrace 

 upon which the original city is built is one hundred and 

 twenty feet high : and at Lawrenceburg the valley, from 

 three to four miles wide, is nearly "filled to a height of one 

 hundred and twelve feet above the flood-plain, with a ter- 

 race deposit clearly derived from the glacial floods of the 

 Great Miami and its tributaries. Below this point the ter- 

 races of the Ohio gradually diminish in height, and the ma- 

 terial becomes finer, and more and more water-worn. Aboye 

 Cincinnati there is a marked development of the terraces at 

 the mouth of the Scioto, at Portsmouth ; and again, below 

 Marietta, at the mouth of the Muskingum, where, opposite 

 Blennerhassett Island, the terraces are in the neighborhood 

 of one hundred feet above the present low-water mark. It 

 is to be noted that both of these streams were so situated as 

 to be among the largest contributors to the Ohio, both of 

 glacial floods and glacial debris. 



But the most instructive place for the observation of this 

 class of phenomena is to be found in Pennsylvania, at the 

 junction of Beaver Creek with the Ohio River, From the 



