292 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



Northwest of the escarpment a more rugged topography prevails. The 

 upland rises more rapidly, and 10 to 14 miles northwest of the Delaware 

 elevations of 400, 460, and 500 feet are reached. These elevations mark 

 the level tops of hills, which trend northeast and southwest, and which 

 constitute a more or less well defined topographic feature known as Buck 

 ridge. 



In the central portion of the belt Buck ridge is separated by a shallow 

 valley ("Cream valley") from well. defined hills rising to a height, at 

 Paoli and Devon, of 520 feet. These hills are known as the south Ches- 

 ter Valley hills. 



Beyond them lie Chester valley, with a width of 2 miles, and the 

 north Chester Valley hills, also trending northeast and southwest and 

 rising to a height of 600 feet. The contours of these hills and of Chester 

 valley are controlled by the underlying Paleozoic formations. 



To the northward these materials pass under a cover of Triassic shales. 



In the northeast Buck ridge and the south Chester Valley hills are 

 merged ; Chester valley is first reduced in width and then disappears 

 and Buck ridge itself passes under the Triassic cover. 



With the presence of the Trias there appears a marked change in to- 

 pography; the marked northeast-southwest trend of ridge and valley 

 disappears ; well defined ridges do not exist and the range of elevation 

 decreases. The more open, level character of the district underlain by 

 Triassic formations is conspicuous from the summit of the north Chester 

 Valley hills, which separate the country of the Paleozoics from that of 

 the Triassic formation and command both districts. 



In the southwest the belt of pre-Paleozoic and Paleozoic materials 

 widens, passing into Delaware and Maryland with a width of more than 

 80 miles and preserving the southwest trend of ridge and valley. 



The south Chester Valley hills and Buck ridge again merge. Chester 

 valley maintains its integrity until within 8 miles of the Susquehanna 

 river, where the topography abruptly alters by reason of underlying 

 harder formations coming to the surface. 



Geology of the Piedmont 



general geology 



In the Piedmont of western New England, Pumpelly, Dale, and Wolff* 

 have determined the following succession : 

 Silurian : Berkshire schist. 

 Cambro-Silurian : Stockbridge limestone. 

 Cambrian : Vermont quartzite. 

 Pre-Cambrian : Stamford gneiss. 



* Monograph xxiii, U. S. Geol. Survey, p. 13. 



