UPPER POTOMAC VALLEY 289 



peneplain recognized by Keith is the Somerville plain, and that he 

 failed to appreciate the fact that the highe3t land in the valley corre- 

 sponds in altitude with the general surface of the shale hills to the north, 

 and therefore in all probability is a remnant of the Harrisburg pene- 

 plain. 



Abbe,* in his study of the surface features of Maryland, recognized 

 the Harrisburg peneplain in the shale belt of the Hancock quadrangle, 

 and he traced it west as far as Cumberland, at the foot of the Allegheny 

 Front, but in attempting to extend it to the east he apparently has con- 

 fused it with the Somerville plain, which occurs at least 100 feet lower 

 than the Harrisburg peneplain. Abbe gives its altitude across the lime- 

 stone belt of the valley as 600 feet, on both the Potomac river and Cono- 

 cocheague creek, but he failed to observe that just north of the state line 

 the two peneplains are well developed in the vicinity of Chambersburg, 

 Pennsylvania, with a vertical interval between them of at least 100 feet. 



Thus it seems probable that the Harrisburg peneplain stands at an 

 altitude of 700 feet in the southeast corner of the Hancock quadrangle, 

 600 feet in the shale belt west of Harpers Ferry, and 500 feet in the 

 eastern part of the Harpers Ferry quadrangle. 



The deposits of gravel which Keith f regards as belonging to the Lafa- 

 yette formation presumably lie upon remnants of the Harrisburg pene- 

 plain, and their presence affords corroborative evidence regarding the 

 early Tertiary age of this topographic feature. 



UPPER POTOMAC VALLEY 



In passing west from the Hancock quadrangle the surface is seen to 

 become more rugged, the rocks are composed of harder material, and 

 the reduction was evidently not so complete as farther east. There are, 

 however, belts of weak rocks showing in the Paw^paw quadrangle, on 

 which the Harrisburg peneplain was fairly well developed. In the east- 

 ern part of the quadrangle it has a general altitude of about 800 feet, and 

 on the western side about 900 feet above sea-level. In the Flintstone 

 quadrangle the rocks are of such a character that the reduction was not 

 very complete, but the general surface on the softer rocks seems to cor- 

 respond on the eastern side with the 900-foot level observed in the quad- 

 rangle to the east. In the vicinity of Cumberland the evidence is not so 

 conclusive regarding the altitude of this plateau, but along a belt of weak 

 rocks in the southeastern corner of the Frostburg quadrangle the altitude 

 of the hills does not exceed 1,200 feet. This is taken provisionally as 



* Physiography of Maryland, Maryland Weather Service, vol. 1, part 2, pp. 157-161. 

 t Op. cit., pp. 366-369. 



