Foshay — Preglacial Drainage of Western Pennsylvania. 401 



valley now occupied by the Ohio below its point of meeting 

 with the Beaver. That this could not have been the outlet is 

 proved by the following facts : — at Smith's Ferry, on the Penn- 

 sylvania Ohio State line, two favorably situated measurements 

 give a maximum depth of drift filling of 30 feet* and at 

 Steubenville, O., the channel could not possibly have been 

 deep enough to drain Spencer River. f The conclusion is thus 

 made imperative, independent of the northward crust move- 

 ments, that this area must have drained northwardly into the 

 Erie basin. This ancient basin would then include the areas 

 now drained by the Lower Allegheny, Clarion, Redbank, 

 Mahoning, Conemaugh, Youghiogheny, Cheat, Monongahela 

 and Little Beaver Rivers. The Monongahela and Allegheny 

 are both known to have buried valleys, the former^: as far as 

 its junction with the Youghiogheny and the latter§ to some- 

 where north of Parker. 



The topography of the a. >fc^ ■ 



Beaver Valley is shown ^ „c«— >r>p-^ 



in fig. 2, which is an X^~CM 

 ideal cross-section. It \ J 



consists first of an old A D 1A i i i i ■ 



, . , , . / * -r»\ AB, old base-level plain. 



base-level plain (AB) CD, outer or rock gorge. 



bounded on either side EF, inner or drift gorge. 



by Slopes rising; slowly to The shaded portion represents the drift filling 



, / -, r i /• , i & , i i 1 t of the old rock gorge with its terraces of erosion. 



the level of the table-land 



which is the basis of the topography of the region ; of a rock 

 gorge (CD) extending from 300 to 350 feet below the level 

 of the plain, which is completely filled with drift for the lower 

 100 feet and partially for the next 125 feet ; and of an inner 

 gorge (EF) in the' drift whose excavation by the modern 

 river gave us the drift terrace system. 



The old base-level plain has more frequently been called the 

 " fourth terrace," though it was known to have no connection 

 with the other terraces. It is a mile or more in width and is 

 covered in all places south of the terminal moraine by a deposit 

 consisting of white or yellowish clay, of variable thickness up 

 to ten feet, which in places contains intermingled pebbles of 

 northern drift, and frequently has sand or gravel above or 

 below it, or both. The maximum observed thickness of the 

 whole deposit is twenty feet. This clay deposit is very con- 

 stant wherever the old base-level plain — -a mere bench often — 

 is found. The plain has in all places, south of the moraine, a 

 rocky scarp on its river side and is always (in the Ohio and 



* Report QQ, I. C. White, Sec. Geol. Survey Pa., 1879, p. 16. 



f Report QQ, I. C. White, Sec. Geol. Survey Pa., 1879, p. 17. 



% Report K, Sec. Geol. Survey Pa., J. J. Stevenson. 1876, p. 20. 



§ Report V, Sec. Geol. Survey Pa., H. M. Chance, 1879, ix, x and 19. 



