PHYSIOGRAPHIC STUDIES IN PENNSYLVANIA 483 



The larger streams have very meandering, tortuous courses, not 

 due to present aggraded conditions, since they occupy deep gorges 

 in the broad shale areas and their grade is about 100 feet in 6 to 10 

 miles air line. These crooked streams originated on a graded plain, 

 an additional evidence of the existence of a peneplain at about the 

 Harrisburg level, and this mature drainage was rejuvenated by uplift 

 and cut deep, sinuous valleys. The spurs between the bends of the 

 streams in the shale areas are terraced at various levels, ranging in 

 altitude from 580 to 680 feet. Many of the terraces and slopes along 

 East and West Conococheague creeks are covered with quartzite 

 bowlders transported from the mountains by these streams during 

 the cutting of the gorges, but no such deposits have been found on 

 the surface of the Chambersburg plateau. 



As to the presence in this area of a lower peneplain of later Tertiary 

 age equivalent to the Somerville peneplain of Davis, as suggested by 

 Campbell,^ the evidence is not so clear. Along both Back Creek 

 and East Conococheague Creek terraces at 680 feet are very con- 

 spicuous, and the shale plateau near St. Thomas also attains this alti- 

 tude. To the south the level tops of the central shale belt are all at 

 600 to 620 feet, and in many cases this altitude is maintained to the 

 ends of the spurs between the creek bends. Other spurs have been 

 lowered to 580 and 560 feet. At Upton the 600-foot plain is very pro- 

 nounced and extends several miles on to the limestone area to the 

 northwest. In the western shale belt the upland is at 600 to 620 feet 

 elevation, with a few higher tables previously mentioned nearer the 

 mountains. The limestone tracts adjoining the shale have in general 

 been reduced to a rolhng lowland about 550 to 560 feet in altitude, 

 which probably indicates a more recent epoch of erosion affecting 

 these soluble rocks. The limestone area along East Conococheague 

 Creek from Chambersburg south, however, stands at 600 to 620 feet 

 elevation, forming a very level tract covered largely by stream 

 gravel. 



Of these later erosion features the most marked is the 600-foot 

 plain which forms plateaus of relatively wide extent and level charac- 

 ter. This, if any, represents the lower peneplain of late Tertiary age 

 which Campbell correlates with the Somerville plain of Davis. The 



I Loc. cit,, p. 287. 



