300 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



90 degrees east and the dips vary from 35 to 85 degrees southeast. There 

 is a gradual change in the average strike and dips around the end of the 

 synclinal trough in the northeastern end of Chester valle} r . As in the 

 case of the quartzite on the limbs of the overturned isoclinal folds, strati- 

 fication and cleavage dip are coincident when the stratification dip is to 

 the southeast. The prevailing structure is isoclinal (see plate 50). 



In an abandoned quarry at Rennyson, 1£ miles northeast of Berwyn, 

 a compressed overturned syncline may be seen, with cleavage and bed- 

 ding coincident and dipping steeply southeast on the limb of the syn- 

 cline. An overturned isoclinal anticline is exposed in a rock cut on the 

 north bank of Valley creek one-half mile north of Howellville (see 

 plate 51). A similar overturned anticline is to be seen in the cut made 

 b}^ the Washington branch of the Pennsylvania railroad near Arlingham, 

 1? miles southeast of Fort Washington. These secondary folds illustrate 

 the type of the primary folding of the limestone. 



The limestone of Cream valley and the limestone appearing in scat- 

 tered exposures to the southwest, are brought to the surface by means of 

 erosion on the crest of low anticlines, or fill the troughs of overturned 

 synclines. 



Thickness. — With the interpretation of the structure given above, the 

 thickness of the formation must be much less than the width of its out- 

 crop. It is not perfectly determinable, but probably is not greater than 

 1,500 feet. 



Correlation and name. — Fossils of the Chazy, Calciferous, and Trenton 

 ages have been found in the limestone occurring to the west of Chester 

 valley and stratigraphically continuous with the Chester Valley lime- 

 stone. Fossils have also been found in Chester valley in somewhat 

 ambiguous material. This material is a geodiferous drusy quartzite, 

 which is found in place south of Bridgeport near the Trenton branch of 

 the Philadelphia railway, and which follows the contact of the mica- 

 schist and the limestone, showing as loose boulders on the surface of the 

 ground. It is interpreted as a secondary replacement of the uppermost 

 calcareous beds of the Chester Valley limestone. 



At-Henderson station loose material of this character lies on top of 

 the limestone, and in this material have been found gastropod and 

 cephalopod fossils. The following determinations have been made : 

 Raphistoma, two species: Maclurea, Lituites, and Cyrtoceras. These 

 are Ordovician forms, and indicate a horizon in the upper half of the 

 Canadian series. The limestone overlies conformably Cambrian quartz- 

 ite and is Cambro-Ordovician in age. It is correlated with the Stock- 

 bridge limestone of New England and New York, with the Cockeysville 

 limestone of Maryland, and the Knox dolomite of Virginia, 



