328 TEE ICE AGE W NORTH AMERICA. 



mouth of French Creek, at Franklin, Pa., to the mouth of 

 Beaver Creek, twenty-five miles below Pittsburg, a distance 

 of about one hundred and fifty miles, no contributors of gla- 

 cial material enter the Alleghany or the Ohio River, and the 

 course of the river-bed lies wholly in the soft, sedimentary 

 deposits of the coal-measures. Consequently, while this por- 

 tion of the stream contains terraces with northern drift 

 brought into the Alleghany above Franklin, they are of 

 diminishing height, and contain a constantly diminishing 

 amount of material from the glacial drift all the way down 

 to the mouth of the Beaver. 



At the mouth of the Beaver there is a sudden enlarge- 

 ment of the Ohio terrace, and it rises at once to a height of 

 one hundred and twenty feet above the river. Upon the 

 lower side of the Beaver, in the angle between it and the 

 Ohio, down-stream, this terrace is very extensive, and the 

 material very coarse, the terrace being, indeed, largely built 

 up of pebbles and bowlders from a few inches to two feet 

 or more in diameter, and all thoroughly rounded. On the 

 opposite side of the Beaver, in the angle between its mouth 

 and the upper portion of the Ohio, there is, for a limited 

 distance, a terrace of equal height, but of entirely different 

 composition from that upon the lower side. The terrace 

 upon the upper side consists of fine material, being mostly 

 sand and gravel derived from the coal-measures, through 

 which the Ohio itself has cut its way. Pebbles from the 

 northern drift are rare, and the local origin of the material 

 is manifest at a glance. What, now, makes this difference 

 between these terraces upon the opposite sides of this small 

 tributary of the Ohio ? A glance at the map will show.* 



The Beaver River emerges from the glaciated region only 

 a few miles to the north of its junction with the Ohio. The 

 larger part of its drainage-basin lies in portions of northeast- 

 ern Ohio and northwestern Pennsylvania, which are deeply 

 covered with the terminal deposits of the continental ice- 



* See map, p. 145. 



