PHYSIOGRAPHIC STUDIES IN PENNSYLVANIA 475 



peneplains exist in this area, and similar features in the adjacent 

 portions of Maryland have been described by Abbe.' The moun- 

 tains rise abruptly out of the valley 1,000 to 1,500 feet. The western 

 range consists of a series of parallel ridges composed of quartzite 

 and sandstone of Medina and CHnton age. The crests of these 

 ridges form a very even and level sky line, which suggests at a glance 

 that they are remnants of an old peneplain. As one approaches 

 from the east North Mountain comes first into view, but extends 

 into the area only about 6 miles. It is a double or canoe-shaped 

 mountain, ending at the south in Parnell Knob. It is a closely 

 folded syncline of quartzite, inclosing overlying shale in the center. 

 The rocks are nearly vertical, and the quartzite ridges are conse- 

 quently narrow and sharp. Although at a distance their tops fall 

 in line with the crests of the more distant ridges and present a level 

 sky line (see Fig. 2), on closer examination irregularities appear. 

 The eastern hmb of the syncHne is cut through by water-gaps at several 

 points, and its original height has been reduced so that the crest is 

 irregular. Its highest points are slightly over 1,800 feet. The 

 western hmb is more even, but is cut by a wind-gap at the southern 

 end. Its top lies between 1,850 and 1,950 feet elevation, except near 

 the northern border of the area, where it is locally 2,040 feet. At 

 the southern end of the syncline two knobs, whose summits are broad 

 and rounded, rise to 2,000 feet, and they may well represent portions 

 of the old peneplain but slightly eroded. 



The next ridge to the west is another canoe-shaped mountain, 

 less compressed than the North Mountain synchne. It ends in 

 Jordan Knob, a companion peak to Parnell Knob, and extends 

 about the same distance into the quadrangle. The eastern limb of 

 this fold is very steep and the ridge narrow and sharp-topped. In 

 the western limb the dips are flatter and the mountain is broader 

 and more massive. The eastern ridge has an altitude of about 1,950 

 feet. The general altitude of the western ridge is about 2,000 feet, 

 but the highest peak rises to an altitude of 2,250 feet near the northern 

 boundary of the quadrangle. The higher summits of this ridge are 

 rounded, and in this respect possess the characteristics of an old 



3 "Physiography of Maryland," Maryland Weather Service, Vol. I, Part 2, pp. 

 157-61. 



