DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 231 



and sometimes upon the other), but, according to rule, 

 the material of which it is composed gradually grows finer, 

 and the elevation of the terrace decreases. According to 

 rule, also, there is a notable increase in the height of the 

 terrace below each affluent which enters the river from the 

 glaciated region. This is specially noticeable below Mari- 

 etta, at the mouth of the Muskingum, whose head- waters 

 drain an extensive portion of the glaciated area. From the 

 mouth of the Little Beaver to this point the tributaries of 

 the Ohio are all small, and none of them rise within the gla- 

 cial limit. Hence they could contribute nothing of the gra- 

 nitic material which enters so largely into the formation 

 of the river terrace ; but below the mouth of the Mus- 

 kingum the terrace suddenly ascends to a height of nearly 

 one hundred feet above low-water mark. 



Again, at the mouth of the Scioto at Portsmouth, there 

 is a marked increase in the size of the terrace, which is 

 readily accounted for by the floods which came down the 

 Scioto Valley from the glaciated region. The next marked 

 increase is at Cincinnati, just below the mouth of the 

 Little Miami, whose whole course lay in the glaciated re- 

 gion, and whose margin is lined by very pronounced ter- 

 races. At Cincinnati the upper terrace upon which the 

 city is built is 120 feet above the flood -plain. 



Twenty-five miles farther down the river, near Law- 

 renceburg, these glacial terraces are even more extensive, 

 the valley being there between three and four miles wide, 

 and being nearly filled with gravel deposits to a height of 

 112 feet above the flood -plain. Below this point the ter- 

 races gradually diminish in height, and the material be- 

 comes finer and more water-worn, until it merges at last 

 in the flood-plain of the Mississippi. The course of the 

 Wabash Eiver is too long to permit it to add materially to 

 the size of the terraces which characterise the broader val- 

 ley of the Ohio below the Illinois line. 



It is in terraces such as these just described that we find 



