Chapter IV. 



The Faults of Perry county. 



The country lying immediately south of Mahanoy ridge 

 is one of the best collecting grounds in the county for the 

 fossils of the Hamilton and Chemung groups. The upper 

 Hamilton shales are there exposed better than elsewhere, 

 and the Chemung, especially the lower part of the group, 

 may also be examined in many small wayside cuts and field 

 exposures. 



This ground has been hitherto supposed and was repre- 

 sented on the preliminary map of the county to be a syn- 

 cline between Mahanoy Ridge and Nick's hill, bounded by 

 outcropping edges of Hamilton sandstone, the middle of 

 which was occupied by a sheet of Chemung rocks. But a 

 very short examination sufficed to show that the Hamilton 

 upper shale extended much farther out into the valley from 

 Mahanoy ridge than the bounding line drawn on the map. 



Hamilton fossils were found farther and farther out from 

 the ridge in the ground represented as Chemung, until it 

 became evident that in the western part of the so-called 

 basin or trough the Hamilton upper shales were repeated 

 by the extension to the eastward of one of the anticlines 

 represented at its west end. {Crawley Hill is a mass of 

 Hamilton sandstone rising immediately to the south of the 

 turnpike road running to Little Germany at a point not 

 more than three miles from Bloomfield.) The influence of 

 this anticline is to bring up the Hamilton upper shales again 

 to the surface, so that the lower beds crop out at or near 

 the school-house on the branch road to the south. Along 

 this branch road the Hamilton shales still occupy an im- 

 mense space, far more than their thickness, and yet they 

 dip steeply. 



6 F\ ( 81 **0 



