1S7H.J «joJ [Stevenson. 



Surface Geology of South-west Pennsylvania and adjacent portions of West 

 Virginia and Maryland. By John J. Stevenson, Professor of Geology 

 in the University of the City of New York. 



{Read before the American Philosophical Society, Aiigtist 15, 1879.) 

 Introduction. 



I. Benches along Cheat River and vicinity — Along the Monongahela 

 River — George's Creek — Redstone Creek — Along the National Road — Chest- 

 nut Ridge— Youghiogheny River — Westmoreland County, west from Chest- 

 nut Ridge — Ligonier Valley — Somerset County — East from the Allegha- 

 nies. 



II. Resume of the Facts — The Horizontal Benches — The River Terraces 

 — Conclusions. 



The region in which the observations recorded in this paper were made, 

 embraces of Pennsylvania the counties of Fayette, Westmoreland, Wash- 

 ington and Greene, with parts of Alleghany, Beaver and Somerset ; of 

 West Virginia, parts of Monongalia, Preston, Mineral and Hardy ; and of 

 Maryland, the extreme western portion. But the observations were made 

 chiefly in the four counties of Pennsylvania first named and in Mononga- 

 lia county of West Virginia. 



The Alleghany Mountains of Pennsylvania form the eastern boundary 

 of Somerset county. West from these in the same county are Negro 

 Mountain and the Viaduct Axis, which unite with the Alleghanies near 

 the northern line of Maryland to form Prof. W. B. Rogers' Tygart's Val- 

 •ley anticlinal in West Virginia. This great anticlinal is divided by Ty- 

 gart's Valley into two monoclinal ridges, known as Cheat Mountain and 

 Rich Mountain. 



Laurel Ridge, the next west from Negro Mountain in Pennsylvania, is a 

 bold mountain separating Westmoreland and Fayette counties from Cam- 

 bria and Somerset ; it extends into West Virginia and dies out before 

 reaching the Great Kanawha river. Chestnut Ridge passes through West- 

 moreland and Fayette counties at twelve or fifteen miles further west, and 

 is separated from Laurel Ridge by the Ligonier Valley, a canoe-shaped syn- 

 clinal, which disappears southward not far beyond the southern line of 

 Pennsylvania. Chestnut Ridge becomes insignificant soon after entering 

 West Virginia, but in Pennsylvania it is almost as imposing as Laurel 

 Ridge or the Alleghanies. West from Chestnut Ridge, only the Saltsburg 

 anticlinal is strong enough to affect the topography. It forms Brush Ridge 

 in Westmoreland and Fayette counties, but disappears very near the line 

 of West Virginia. 



The drainage west from the Alleghanies belongs to the Ohio river sys- 

 tem and the smaller streams arc tributary to the Conemaugh, the Youghio- 

 glieny, the Cheat, or the Monongahela. The first of these large streams 

 flows into the Alleghany, while the second and third are tributaries to the 

 Monongahela, which unites with the Alleghany at Pittsburgh to form the 

 Ohio. 



