230 MAN AND THE GLACIAL PERIOD. 



touching it at Salamanca, N. Y., Franklin, Pa., and Cin- 

 cinnati, Ohio. But throughout its whole length the ice- 

 front was approximately parallel to the valley, and occu- 

 pied the head-waters of nearly all its tributaries. Now, 

 wherever tributaries which could be fed by glacial floods, 

 enter the trough of the main stream, they brought down 

 an excessive amount of gravel, and greatly increased the 

 size of the terrace in the trough itself, and from the mouth 

 of each such tributary to that of the next one below there 

 is a gradual decrease in the height of the terrace and in 

 the coarseness of the material. 



This law is illustrated with special clearness in Penn- 

 sylvania between Franklin and Beaver. Franklin is ujdou 

 the Alleghany Eiver, at the last point where it was reached 

 directly by the ice, Below this point no tributary reaches 

 it from the glaciated region, and none such reaches the 

 Ohio after its junction with the Alleghany until we come 

 to the mouth of Beaver Creek, about twenty-five miles be- 

 low Pittsburg. 



But at this point the Ohio is joined by a line of drain- 

 age which emerges from the glaciated area only ten or 

 twelve miles to the north, and whose branches occupy an 

 exceptionally large glaciated area. Accordingly, there is at 

 Beaver a remarkable increase in the size of the glacial ter- 

 race on the Ohio. In the angle down-stream between the 

 Beaver and the Ohio there is an enormous accumulation of 

 granitic pebbles, many of them almost large enough to be 

 called boulders, forming the delta terrace, upon which the 

 city is built and rising to a height of 135 feet above the 

 low-water mark in the Ohio. In striking confirmation of 

 our theory, also, the terrace in the Ohio Valley upon the 

 upper side of Beaver Creek is composed of fine material, 

 largely derived from local rocks and containing but few 

 granitic pebbles. 



From the mouth of Beaver Creek, down the Ohio, the 

 terrace is constant (sometimes upon one side of the river 



