94 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



auce to both the northeast and the southwest system, and assigns to piracy 

 no more than might be expected of it, in view of the fact that the southwest 

 system was at least 100 feet the lower. 



In the portion of the Ohio Valley between Moundsville and the mouth 

 of the Beaver there are very few rock shelves or remnants of old fluvial 

 plains at levels below those which appear to belong to the old north-flowing 

 system, but on the tributaries rock shelves are present at all levels as 

 incidents of the cutting down of their valleys. The general absence of 

 rock platforms or ten-aces along the main valley may be assigned to the 

 increased volume of the united stream. The most conspicuous of the rock 

 terraces formed after reversal is found north of New Cumberland, W. Va. 

 It stands about 100 feet lower than any of the gradation plains of the 

 north-flowing system in that vicinity, being only 200 feet above the river 

 and 836 feet above tide. It is preserved for a distance of nearly a mile 

 with a width of 30 to 40 rods, and carries a few feet of gravel on its sur- 

 face. In gullies which cut into this gravel Unio shells in large numbers 

 occur, which have the weathered appearance to be expected if they are 

 native to this old river bottom.^ The gravel was evidently deposited by a 

 stream flowing southward in the present direction of the Ohio, for it con- 

 tains material of glacial derivation brought down from the glaciated districts 

 to the north. 



THE NOBTHWARD OUTLET. 



The question of a northward outlet for the Upper Ohio, Monongahe^a, 

 and much of the Allegheny drainage through the Beaver was raised by 

 Spencer nearly twentj' years ago, being presented as a woi'king hypothesis 

 in connection with a general discussion of the origin of the lower Great 

 Lakes.^ Evidence in support of this hypothesis was brought forward by 

 Foshay in 1890.^ He called attention to the great breadth of the main 

 gradation plain on the Beaver, to the apparent northward slope of the rock 

 floor, and to the occurrence of potholes on this rock floor which appear to 

 have been formed by a, north-flowing stream. His discussion was limited 



' For notes on the occurrence of Unio shells on a terrace of the Monongahela at a similar height 

 above the stream, see Stevenson's paper in Am. Jour. Sci., 3d series, Vol. XV, 1878, pp. 245-250. 



^Proc. Am. Philos. Soc, Vol. XIX, 1882, pp. 330-337, with maps; also published as an appendix 

 to Kept. Q* of Second Geol. Survey Pennsylvania, 1881, pp. 357-406. See pages .387 and 405-406 for this 

 refei-ence. 



Mm. Jour. Sci., 3d series, Vol. XL, 1890, pp. 397-403. 



