326 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



until Lafayette time, when all the Coastal plain and part of the Pied- 

 mont district was brought beneath the sea by tilting, which elevated the 

 northern half of the plateau. The velocity of the streams was acceler- 

 ated so that they were able, for the first time since the Raritan epoch, 

 to carry to the sea coarse as well as fine gravel. It was at this time that 

 the formation known as the " Bryn Mawr gravel " was spread on the 

 plateau. The uplift which followed this tilting inaugurated a drainage 

 which could not have been very unlike the present. The lower courses 

 of the Schuylkill and the Delaware became established along the present 

 lines and the development of the present topography began. 



Following this uplift there was a subsidence of the extreme eastern 

 portion of the plateau. A stationary position of the shoreline is indi- 

 cated by the escarpment crossing the plateau diagonally from southwest 

 to northeast, mentioned on page 291. The 180-foot contour approxi- 

 mately outlines the escarpment which extends from Gordon heights 

 northeast to Somerton. It can be easily located by the present bounda- 

 ries of the Sunderland deposits, though erosion has removed this mate- 

 rial from the near neighborhood of the former shoreline. In the vicin- 

 ity of Somerton it is a well defined escarpment, from which the plain of 

 submarine erosion to the south may be overlooked. 



Only the extreme eastern portion of the plateau was affected by the 

 following crustal movements of uplift, erosion, and subsidence which are 

 recorded in the deposits of the Wicomico and Talbot formations, and 

 which affected the entire surface of the Coastal plain. 



Since Talbot time both the Piedmont plateau and the Coastal plain 

 have been continuously above water, and the drainage of the plateau 

 has assumed its present form. 



The Jurassic peneplain now stands at a height sufficient for the estab- 

 lishment of a drainage actively eroding its surface. 



The cover of Cretaceous, Tertiary, and Quaternary formations have 

 been so far removed as to lay bare large areas of the crystalline pene- 

 planed floor in which the streams are again cutting their valleys. The 

 peneplain has thus by elevation become a plateau and by the renewal 

 of erosion a dissected plateau. 



That the main streams maintain courses which are independent of 

 the lithologic character and structure of the underlying rock is due to 

 the presence of the cover of Cretaceous, Tertiary, and Quaternary mate- 

 rials which, existing at the time of the development of the drainage, 

 masked the pre-Paleozoic and Paleozoic series beneath it. The drainage 

 superimposed upon this cover became too well established in courses 

 consequent upon the slope of the plateau and independent of the con- 



