288 CAMPBELL — NORTH PENNSYLVANIA AND SOUTH NEW YORK 



the belt just described the interval between them varies from 300 feet at 

 Easton to a little over 100 feet near the Maryland state line. On ac- 

 count of this convergence the two features are not so easily differentiated 

 in the Potomac valley as they are in the Delaware or Susquehanna val- 

 leys. In fact, along Conococheague creek the post-Harrisburg uplift 

 was so slight that the deeply weathered surface of the Harrisburg pene- 

 plain on the shale belt was reduced practically to the same level as the 

 adjacent limestone valleys. 



POTOMAC VALLEY 



The topographic maps recently prepared of the territory along the 

 Potomac river show admirably the character of the Harrisburg pene- 

 plain. In the Hancock quadrangle the rocks are sandy shale, belonging 

 to the Devonian period. A casual inspection of the map shows that it 

 is an extremely hilly country, but upon closer examination it is found 

 that the hilltops show great regularity in altitudes, ranging from 600 or 

 700 feet in the southeastern corner to about 800 feet in the northwestern 

 corner of the quadrangle. If the irregularities due to recent erosion 

 were eliminated by filling the sharp V-shaped stream valleys to the heights 

 mentioned, the surface would be very regular, and certainly would be 

 classed as a peneplain. It is interrupted here and there by monadnocks 

 which stand above the general level, but in all such cases the monad- 

 nocks represent harder rocks than the average and their presence tends 

 to confirm the idea that the surface is a peneplain and due to subaerial 

 erosion. The geologic structure of the region consists generally of broad, 

 open folds, with many minor wrinkles to complicate the structure. The 

 planes of stratification are generally inclined, and the peneplain, which 

 is represented by the tops of the hills, cuts across the beds of rock at all 

 angles. It seems impossible to account for this general accordance in 

 altitude on any other theory than that of peneplanation. 



Unfortunately there is no detailed map of the Shenandoah valley by 

 which to compare the topography of the shale belt with that developed 

 on the limestone belt to the east. If, however, the Harpers Ferry map 

 is consulted, it will be seen that the valley floor is gently rolling and it 

 ranges in altitude from a little less than 500 feet near the major drain- 

 age lines to about 600 feet on the shale belt in the central part of the 

 valley. Keith * endeavored to account for this variation in the floor of 

 the valley on the supposition that it represents incomplete reduction 

 to a baselevel, which presumably now stands at an altitude of 500 feet. 

 This is probably true as far as it goes, but it seems probable that the 



♦Geology of tlie Catoctin belt, U. S. Geol. Survey, Fourteenth Annual Report, pp. 374-376. 



