482 GEORGE W. STOSE 



Nowhere on its surface is bed rock exposed, and only one or two out- 

 crops were found in stream cuttings on its borders, indicating a deep 

 deposit of stream wash. In the Hmestone area on the eastern side of the 

 valley there are numerous hilltops which approximate 740 to 760 feet 

 in altitude, but many rise higher, and there is no apparent uniformity 

 It is clear that the valley in this region was once a nearly level 

 plain, which has been deeply eroded, leaving remnants at approxi- 

 mately 700 to 750 feet elevation. Campbell^ has described a pene- 

 plain preserved throughout this region on the shale at approximately 

 this altitude. He has named it the Harrisburg peneplain from its 

 typical development at Harrisburg, Pa., and assigns to it an early 

 Tertiary age. It seems strange that the peneplain should be pre- 

 served in the Chambersburg plateau, which is exposed to such active 

 erosion from tributaries of two large creeks, whereas in m'ore favorable 

 areas only small remnants remain, but this is probably due to a soft 

 but tough sandstone which is locally interbedded and infolded in the 

 shale of this hill and has aided in its preservation. Nearly all the 

 other areas occurring at this altitude were protected by a covering of 

 stream gravels. At Black Gap the gravel- covered plain has a uniform 

 altitude of 840 feet. The capping of gravel and cobble, judging from 

 the nearest bed rock observed, is about 60 or 80 feet thick. East 

 Conococheague Creek, which issues from the mountains at this point, 

 is a large stream and drains a considerable portion of the moun- 

 tains to the east. On leaving its confined channel in the gap and 

 entering the open plain its velocity would be slackened, its transport- 

 ing power lessened, and a portion of its load of mountain rock would 

 be dropped. In this way a delta has been built up 60 or 80 feet 

 above the general level of the peneplain. The great extent and level 

 character of this delta bears evidence of a prolonged halt in the uplift 

 of the land and of active erosion on the headwaters of the stream. 

 The delta at Franklin Gap, which also stands at 800 to 840 feet ele- 

 vation, was similarly built up above the plain. Other local delta 

 gravels at about this height occur at smaller gaps along the mountain 

 front. In Path Valley two well-marked terraces on the shale at 820 

 feet may represent this stage but at a higher level in the narrow tribu- 

 tary valley. 



I Loc. cit., pp. 283-91. 



