i'>24 STOSE AND JONAS ORDOVICIAN IN PIEDMONT PROVINCE 



had been more or less consolidated into limestone by pressure, were 

 (jroded on the flanks of the uplift and the older and harder siliceous sedi- 

 ments were laid bare and also partly eroded. The beveled edges of the 

 limestone formations thus appeared one after another on the flanks of 

 ilie bowed-up siliceous sediments, as shown in the diagrammatic section, 

 figure 15. The reinvasion of the sea later in Ordovician time, probably 

 in the Chazy epoch, spread a mantle of sediment unconformably over the 

 beveled edges of the older formations, as shown in figure 16. Fragments 

 of limestone from the waste of the eroded underlying or adjacent forma- 

 tions were included in the initial sediments, giving rise to thick lime- 

 stone conglomerates. Fine black argillaceous silt that was carried into 

 this sea was apparently not washed from the Piedmont land area to the 

 southeast, but was probably brought from some more distant source at 

 the head of the bay to the northeast. This argillaceous silt formed the 

 Cocalico shale in the open part of the basin, where currents supplied it 

 freely, and at approximately the same time it probably contributed the 

 argillaceous matter to the impure slaty Conestoga limestone on the south- 

 east side of the basin, where the water was shallower owing to the Mine 

 Ridge uplift, and the currents were thus deflected or retarded, so that a 

 larger proportion of lime silt was there deposited. Later, the further 

 upbowing of the Mine Ridge anticline and of minor anticlines on its 

 flanks, accompanied by thrust-faulting, resulted in the emergence of the 

 land above the sea and the erosion of the folded strata, as shown', in 

 figure 17, which presents conditions as they are today. 



