FOSSILS AND CORRELATION 



521 



hard slaty ribbed limestones (see figures 6 to 9). These impurities are 

 chiefly argillaceous matter similar to that which forms the Cocalico shale. 

 It is believed, therefore, that the Cocalico shale and Conestoga limestone 

 are approximately of the same age and were deposited in the same sea, 

 the argillaceous matter predominating where the Cocalico shale was de- 

 posited, and a mixture of lime silt and argillaceous mud being deposited 

 in the area of Conestoga limestone. 



Summary 



The history of Cambrian sedimentation and deformation and the 

 Conestoga overlap in early Ordovician time may be summed up as fol- 

 lows: 



During early Cambrian time a shallow arm of the sea occupied the 

 western. part of the Piedmont area of Maryland and Pennsylvania and 

 the adjacent part of the Appalachian trough, in w^hich sediments were 

 deposited. First, sandy and argillaceous detrital material, eroded from 

 an uplifted land area farther southeast in the Piedmont Province, was 

 washed into the sea and deposited, forming the siliceous Lower Cambrian 

 sediments. This deposition was followed by a period in which the eleva- 

 tion of the Piedmont land area was insufficient to cause large quantities 

 of detrital material to be washed into the sea, and lime silt was deposited. 

 Small amounts of siliceous and argillaceous silt w^ere included as impuri- 

 ties in some of the beds, giving rise to shaly and impure limestone beds 

 which occur chiefly in the Kinzers, Elbrook, and Conococheague forma- 

 tions. Lime silt was deposited apparently continuously throughout the 

 rest of Cambrian time and the early part of Ordovician time, although 

 there may have been epochs when deposition ceased and the sea-bottom 

 may even have been temporarily raised above sealevel. Such breaks in 

 sedimentation were most likely to have occurred along the axis of the 

 Mine Eidge uplift, for the sediments observable on its flanks are some- 

 what thinner than those in the trough to the northwest, but the beds 

 that w^ere originally on its crest are now entirely eroded so that their 

 former extent can not be determined. The formations in Chester Valley, 

 southeast of the axis of uplift, are also thinner than those in the Lan- 

 caster Valley. In the diagrammatic section (figure 14), which shows 

 the writers' conclusions as to the deposition of sediments in the area, the 

 deposits are represented as thinner over the rising axis. 



At the end of Beekmantown sedimentation further uplift along the 

 Mine Eidge axis brought the newly deposited sediments above sealevel 

 and subjected them to erosion. All the softer lime-silt formations, which 



