PHYSIOGRAPHIC STUDIES IN PENNSYLVANIA 481 



rocks composing them are broadly folded and sharply plicated, and 

 vary from fissile shale to soft but tough sandstone, which are alike 

 planed off. 



The northern portion of the larger shale belt, lying between East 

 Conococheague and Back creeks, is the highest of the shale tracts. 

 It is a narrow plateau extending from the junction of the two creeks 

 to beyond the northern limits of the quadrangle. Along its axis it 

 has a nearly uniform elevation of 750 feet, but rises at one point in 

 the north to 780 feet, and decreases to 700 feet near its southern apex 

 and on its margins. It has been considerably dissected along its 

 borders, especially on the western side, where the tributaries of Back 

 Creek have trenched it deeply, but the intervening spurs still retain 

 their level character and to some extent their original height. The 

 descent from the plateau level to the stream bottoms on either side 

 is very steep and abrupt. This is especially pronounced on the east 

 side, where the valley is a limestone lowland. On the west the shale 

 tract extends beyond Back Creek, and the plateau character con- 

 tinues at a somewhat reduced altitude, but attains 720 feet elevation 

 at the northern border of the quadrangle. 



There are no other extensive level tracts at this altitude in the area, 

 but there are several scattered terraces and hilltops which approxi- 

 mate this height. West of Mercersburg, near Cove Mountain, the 

 shale hills rise to 700 feet, and a little farther north, where an inter- 

 montane stream debouches at Cove Gap, an apron of mountain rock 

 has been spread over these hills at an altitude of 740 feet. The same 

 is true of a small 720-foot terrace just east of Fort Loudon, which 

 was covered by mountain wash from Bear Valley when its outlet was 

 at this altitude. Another hill covered with water- worn gravel occurs 

 at Franklin Gap in North Mountain, the outlet of Wilson Run. 

 This, however, has an altitude of 800 to 820 feet. The east branch 

 of Little Antietam Creek, which occupies a re-entrant valley in South 

 Mountain in the southeast corner of the area, has bowlder- covered 

 terraces on both sides of the stream at 760 feet elevation. A similar 

 but more extensive level tract covered by quartzite bowlders, occurs 

 at Black Gap, in South Mountain, where the East Conococheague 

 leaves the mountains and enters the valley. Here a very level plain 

 extends for four miles along the creek at an elevation of 840 feet. 



