520 stose and joxas ordoviciax ix piedmoxt provixce 



Fossils axd Coerelatiox 



Before fossils were found in the Conestoga limestone its overlapping 

 relation to the underlying formations had been observed and worked out 

 in the vicinity of Lancaster. The Conestoga was known to be younger 

 than the Ledger dolomite, on which it rests, with a marked imconformity 

 near Lancaster. Within 5 miles northeast of Lancaster, the Ledger 

 dolomite is overlain by the Elbrook, Gonococheague, and Beekmantown 

 limestones, in normal sequence. i\.s none of these formations in any 

 way resemble the Conestoga in lithology, it is reasonable to assume that 

 the Conestoga is younger than they are — that is, post-Beekmantown. 

 The first fossils found in the limestone were small segments of crinoid 

 stems collected in the vicinitv of York, and thev determined the ao-e of 

 the rocks to be not older than Ordovician. Later, brachiopods and 

 crinoid plates were collected from slaty impure limestones interbedded 

 with thick conglomerate beds at the York Lime & Stone Co.'s quarry 6 

 miles northeast of York (see figure 11). The only species of shell that 

 is determinable is Strophomena stosei, a form not previously kno^vn else- 

 where except in the Frederick limestone of Mar}dand, which has been 

 described by Bassler.* The crinoid plates are not determinable, but are 

 apparently of Ordovician types. The Frederick limestone has yielded, 

 besides Stropliomena stosei, other shells and trilobites identified by 

 Bassler as Acidaspis ulrichij Triartlirus sp., and undetermined species 

 of Reteocrinus?, Cameroceras, and Isotelus. These forms are not known 

 to occur in any other formation, but are regarded by Bassler as probably 

 Chazy. 



^ear Oregon, 7 miles northeast of Lancaster, the Beekmantown is 

 overlain by the Cocalico shale in which graptolites of Normanskill type 

 have been found and which is therefore also probably of Chazy age. 

 Similar shales of Chazy age have been discovered in the Lebanon Valley, 

 where Xormanskill graptolites were collected by the senior author near 

 Highspire, southeast of Harrisburg, and the shale belt was traced east- 

 ward as far as Heading. The shales outcrop on opposite sides of the 

 Triassic basin and probably are parts of a shale mass that underlies the 

 cover of Triassic sediments. 



The Conestoga limestone is characteristically a dark-blue crystalline 

 limestone and dark argillaceous limestone with graphitic slate partings 

 and shale beds, some of which are of considerable thickness. Even the 

 pure marble conglomerate beds at the base are interbedded with slate and 



* R. S. Bassler : The Cambrian and Ordovician deposits of Marj'land, Maryland Geol. 

 Survey, Cambrian and Ordovician, 1919, p. 117. 



