510 STOSE AND JOXAS OEDOVICIAN IX PIEDMONT PROVINCE 



occur in places, but it was not until fossils of probable Chazy age were 

 found in slaty limestone beds interbedded with limestone conglomerate, 

 near York, that the age of the formation was determined. It has now 

 been established that this slaty limestone of probable CIislzy age, called 

 the Conestoga limestone, laps progressively southeastward over the forma- 

 tions below the Ledger dolomite down to the Harpers schist, on which 

 it rests in many places in southern Pennsylvania and in Maryland. The 

 extent of the formation from the Schuylkill southwestward into Mary- 

 land and its overlapping relation to other formations are shown on the 

 map (figure 1). 



Details of the Conestoga Oveelap 



The Conestoga limestone is restricted, so far as it has been observed 

 at present, to the southeastern part of the limestone valley in the Pied- 

 mont. At Lancaster, where it reaches about its northwesternmost posi- 

 tion in the valley, it rests on the Ledger dolomite. Its unconformable 

 relation to the dolomite is excellently shown in exposures in quarries and 

 stream banks, just northeast of the city. An old quarry near the Lan- 

 caster waterworks, on Conestoga Creek, excellently exposes the sharp 

 unconformable contact of coarse basal conglomerate, containing white 

 marble blocks as much as 5 feet across, on an eroded surface of well- 

 bedded Ledger dolomite (see figures 2 and 3), At the Pennsylvania 

 Eailroad bridge over Conestoga Creek at the Lancaster waterworks solu- 

 tion crevices 3 feet wide and 20 feet or more deep, in pure white marble, 

 are filled with limestone breccia containing smaller blocks of similar 

 white marble (see figures 4 and 5). The white marble, apparently a bed 

 in the Ledger dolomite, had been partly eroded prior to the deposition 

 of the Conestoga, and fragments of it were included in the conglomerate 

 that fills the solution crevices in the partly eroded masses of white 

 marble. The conglomerate rests on the floor of Ledger dolomite in adja- 

 cent areas where the marble bed had been entirely removed. These lime- 

 stone conglomerates are interbedded with dark-blue, slaty, argillaceous 

 limestone of Conestoga t3'pe, some of which weathers ribbed (see figure 

 9). The variable character of the sedimentation of the. Conestoga is 

 shown in a small quarry near Leaman Place, where thin-bedded siliceous 

 ribbed limestones pass abruptly into thick-bedded granular limestone and 

 lenticular beds of coarser conglomerate (see figures 6 and 7). 



Southeast of Lancaster the Conestoga overlaps onto lower and lower 

 beds of the Ledger until, near Leaman Place, it is nearly down on the 

 Kinzers formation. At the Bellemont quarries, 2 miles southeast of 



