478 GEORGE W. STOSE 



average elevation of 2,100 feet, but rises to the height of the anticHnal 

 ridge near their junction. The rocks become steeper toward the 

 north, and the ridge contracts correspondingly in size and altitude. 



South Mountain, on the east side of the valley, is more massive 

 than the western range. The strata composing it are several thousand 

 feet of interbedded Cambrian quartzites and shales, overlying older 

 volcanics. The quartzites form the higher ridges, and the shales 

 and volcanics are covered by a thick mask of the quartzite bowlders 

 which prevent the erosion of deep valleys. The ridges are not straight, 

 parallel, and even-topped like those of the western range, but are 

 offset by cross folds and faults, and are cut through by transverse 

 drainage, so that the crest is composed of numerous round-topped, 

 elongate knobs and short ridges. Consequently they do not present 

 so level a crest line as the western range. 



In the heart of the mountains extending beyond the eastern 

 border of the Chambersburg quadrangle, two high and very- 

 level tracts occur. Sandy Ridge and Snowy Mountain, south- 

 east of Montalto, are broad and level-topped, and have a general 

 altitude of about 2,000 feet, with a small knob rising to 2,100 feet. 

 They are composed of nearly horizontal quartzite, forming the flat 

 top of the anticlinal uplift of South Mountain. The other tract is 

 Big Flat, occupying a large area on the top of the mountains north- 

 east of Fayetteville. This also is the flat crest of an antichne developed 

 to the west of the main axis of the mountain, and producing a promi- 

 nent offset in the mountain front opposite Fayetteville. This plateau 

 extends for 7 or 8 miles beyond the limits of the quadrangle at a 

 general altitude of 2,000 feet, attaining an elevation of 2,100 feet 

 at two points. These two level tracts are undoubtedly remnants of 

 the old peneplain, preserved at a height of 2,000 feet. The mono- 

 cHnal ridges along the front of the mountain, which once stood at this 

 same altitude, have been reduced by erosion to 1,700 and 1,900 feet. 



This peneplain, observed in both South Mountain and the Tusca- 

 rora Mountain group, has long been recognized in this part of the 

 Appalachians, and has been described by several geologists. Davis^ 

 named it the Schooley peneplain from its characteristic development on 



^ "Geographic Development of Northern New Jersey," Proceedings of the Boston 

 Society of Natural History, Vol. XXIV, p. 377. 



