480 GEORGE W. STOSE 



3 miles in extent occurs at 1,600 feet elevation in the heart of South 

 Mountain east of Montalto, and another at 1,350 feet elevation at 

 Monterey, near the state line. Both of these are cut in the softer, 

 ancient volcanic rocks underlying the quartzites. There are also 

 numerous broad, flat divides at elevations from 1,450 to 1,750 feet. 

 A broad gap near the southern end of North Mountain, at an elevation 

 of 1,550 feet, represents the abandoned outlet of the stream which 

 formerly flowed longitudinally along the North Mountain syncline 

 at this altitude, but was captured by Wilson Run at Franklin Gap. 

 In Path Valley the shale spurs extending out from the mountain on 

 the east are roughly terraced, two of the most prominent benches 

 lying at 1,100 and 1,200 feet. Cowan Gap, in Tuscarora Mountain, 

 through which Little Aughwick Creek formerly flowed into Path 

 Valley, is a broad gap at 1,200 feet. Correlation of these features 

 has not been attempted. 



Mr. Campbell, in his article on the Geographic Development of 

 Northern Pennsylvania and Southern New York,^ states that through- 

 out the great valley (which includes the Cumberland Valley) the 

 limestones which occupy the southeastern side are eroded deeper than 

 the shales which occupy the northwestern portion. This is not the 

 case in the Mercersburg-Chambersburg quadrangles and adjacent 

 areas. The rocks in the eastern portion of the valley are the lower 

 members of the Cambro-Ordovician hmestone, and contain many 

 hard, siliceous beds and several resistant sandstones. Some of these 

 produce prominent hills from 750 to 850 feet in altitude, a few 

 rising to 900 and 1,000 feet. The upper beds of the limestone series, 

 however, are purer and dissolve more readily, so that in the central 

 portion of the valley adjacent to the areas of overlying shale the surface 

 is uniformly lower. Out of this lowland the shale hills rise abruptly, 

 with steep escarpments, forming what may be called low plateaus. 



There are two main belts of shale in the Mercersburg-Chambers- 

 burg quadrangles : one west of Chambersburg and Greencastle, which 

 crosses the quadrangles from north to south, and is cut longitudinally 

 by East Conococheague and Back creeks; the other a smaller north 

 and south belt in the vicinity of Mercersburg. These hills, or 

 plateaus, are in general very level- topped (see Fig. 2), although the 



> Loc cit., p. 283. 



