290 CAMPBELL — NORTH PENNSYLVANIA AND SOUTH NEW YORK 



representing the altitude of the peneplain. This determination is, in a 

 measure, corroborated by the altitude of an abandoned water gap through 

 Wills mountain, just west of Cumberland. While such features as this 

 can not be regarded as positive evidence of the existence of a peneplain, 

 still it seems probable that the abandonment of this gap occurred after 

 a period of baseleveling, when the streams were revived and erosion was 

 most active.* The altitude, therefore, of the Harrisburg peneplain in 

 the vicinity of Cumberland is probably about 1,200 feet. 



From the above figures it is seen that this peneplain has a decided 

 eastward slope from Cumberland to Harpers Ferry, and while it is cer- 

 tain that some of the slope is original and due to the grade of the stream 

 while the peneplain was forming, it is probable that at least 90 per cent 

 is due to subsequent deformation. 



The Shenandoah Valley plain, which is so well developed about 

 Harpers Ferry, with difficulty can be traced up the Potomac to Cum- 

 berland. Since it is a feature which is developed only on the softest 

 rocks of the region, it is not reasonable to expect any extended develop- 

 ment on the hard rocks which characterize the Potomac valley in this 

 region, but in a few places in the immediate vicinity of the river where 

 drainage conditions were most favorable, there are traces of a lower level 

 which presumably corresponds with the Shenandoah plain. Its altitude 

 can not be definitely fixed, but in the vicinity of Cumberland it appears 

 to range from 700 to 800 feet. Since the two surfaces are separated by 

 about 100 feet at Harpers Ferry and 400 to 500 feet in the vicinity of 

 Cumberland, it is apparent that considerable deformation ensued after 

 the development of the Harrisburg peneplain and before the formation 

 of the Somerville plain. As a consequence of this deformation the two 

 surfaces diverge toward the northwest and converge toward the south- 

 east, and it is probable that they coincide somewhere between Harpers 

 Ferry and Washington. 



UPPER SUSQUEHANNA VALLEY 



Along the Susquehanna river the Harrisburg peneplain is fairl}^ well 

 developed and shows features similar to those observed along the Potomac 

 river. From an altitude of 500 feet at Harrisburg it rises steadily up- 

 stream to about 600 feet in the northern part of the Harrisburg quad- 

 rangle, and to 800 feet in the vicinity of Sunbur3^ From this point it 

 can be traced up the main river to an altitude of from 1,200 to 1,300 feet 

 in the vicinity of Pittston, where it shows a remarkable development on 

 the Devonian shale hills west of the river. The valley in which the 



*Abbe: Op. cit., p. IGO. 



