OHIO VALLEY 293 



nut ridge is composed of hard rocks, and rises to an altitude of from 

 2,400 to 2,600 feet above sealevel. Its summit is not level like many of 

 the ridges of the eastern part of the state, but it is probable that in a 

 general way it is a remnant of the Cretaceous peneplain. If this is 

 true, then it is altogether probable that the peneplain below, or that 

 which has been described as standing at an altitude of 1,250 feet, is of 

 early Tertiary age, and consequently corresponds with the Harrisburg 

 peneplain of the eastern part of the state. This correlation is strength- 

 ened by the correspondence in altitude of the peneplain on the two sides 

 of the mountainous region. On the east it has been traced up the 

 Potomac river from an altitude of 500 feet at Harpers Ferry to 1,200 

 feet at Cumberland, and although the Monongahela valley is about 50 

 miles west of Cumberland and separated from it by a mountainous 

 region which was not reduced during the formation of the Harrisburg 

 plain, it seems probable that the projection of this surface would rise 

 somewhat west of Cumberland, and then descend slightly to the Monon- 

 gahela valley corresponding with the surface already recognized at an 

 altitude of 1,250 feet. 



Thus the correlation of the peneplain showing in the Monongahela 

 valley with the Harrisburg plain is based on the similarity of the two 

 features, on the association of surface forms, and the correspondence 

 in altitude along the southern border of the State. 



The peneplain of the Monongahela valley can be traced down that 

 stream to Pittsburg, and from that point to the west along the Ohio river 

 for an indefinite distance. It is well developed at the mouth of Beaver 

 river at an altitude of about 1,200 feet, as shown on the topographic map 

 of the Beaver quadrangle. From this point it can be traced into Ohio, 

 where it has a wide development from Cleveland on the north at least 

 as far as Steuben ville on the south. It ma}^ be seen on the Cleveland, 

 Wooster, Massillon, Canton, Wellsville, Cadiz, and Steubenville atlas 

 sheets. 



In the southwestern corner of Pennsylvania and in the adjacent parts 

 of West Virginia no trace of this peneplain has been observed. The 

 surface of the upland is higher than in the adjacent regions on the east, 

 north, and west. The crests of the ridges are irregular in profile and 

 rise to altitudes of 1,400 or 1,500 feet. This area has the appearance of 

 not having been reduced during the early Tertiary stage of erosion, and 

 consequently now stands above the surrounding region. 



From the above facts it is apparent that this peneplain is nearly flat 

 in southwestern Pennsylvania and adjacent parts of Ohio, but a slight 

 rise is observable toward the northeast, which as one ascends the Al- 

 legheny valley increases to such an extent that it carries tlie peneplain 



XLII— Bull. Geol. Soc. Am.. Vol. 14. 1902 



