GKOGRAPHIC RELATIONS 291 



east and southeast, with shallow valleys and scattered residual emi- 

 nences. South of the New England states these eminences do not rise 

 above a height of 1,600 feet, while the plateau varies from 200 feet in 

 the east to 1,200 feet on its western edge. The level lines of the plateau 

 are unrelated to the underlying rock structure. The larger streams, 

 which have cut into the plateau, converting it into a diversified upland, 

 have maintained courses which are independent of the structure and 

 character of the rock floor. The tributary or subsequent streams, on the 

 other hand, are adjusted to the rock floor, and by means of them the 

 heterogeneity of rock constitution and complexity of rock structure is 

 finding expression. 



The upland is covered by a thick mantle of fertile soil, comparatively 

 free from stones and exposing few rock ledges. 



LOCAL RELATIONS 



The Piedmont district of Pennsylvania, forming with a width of 65 

 miles the southeast portion of the state, is an important part of the 

 Atlantic Piedmont. In geology and physiography it is an epitome of 

 the larger district. 



Geologically the Piedmont district is a complex of highly metamor- 

 phosed sedimentary and intrusive igneous materials of pre-Paleozoic 

 and Paleozoic age largely concealed beneath a cover of unmetamor- 

 phosed sandstones and shales and unconsolidated gravels, clays, sands, 

 and marls of Mesozoic and Cenozoic age. The pre-Paleozoic and Paleo- 

 zoic crystallines are exposed on the southeastern and northwestern bor- 

 ders of the Piedmont district and are covered in the central portion of 

 the region by the Triassic sandstone and shale series. 



It is the purpose of this paper to discuss the formations and structures 

 of the southeastern exposure of crystallines. 



This belt trends northeast and southwest, with a width, in the region 

 of Philadelphia, of 20 miles. It widens south westward and pinches out 

 northeastward, where it disappears under a cover of Triassic, Cretaceous, 

 Tertiary, and Quaternary material and emerges again at the surface in 

 northeastern New Jersey, southeastern New York, and in New England. 



The valley of the Delaware is the southeast boundary of this belt. 

 From this valley, which is less than 20 feet above sealevel, the upland 

 gradually rises, reaching, at a height of 160 to 180 feet, the base of a well 

 defined escarpment. This escarpment, which is marked by the 180 and 

 200 foot contour lines, extends from Somerton on the northeast south- 

 westward to Gordon heights. Fox Chase summit, Green Lane reservoir, 

 Swarthmore College buildings, and the Chester reservoir are located on 

 its crest, from which the relatively flat slope to the Delaware can be 

 surveyed. 



