GENEEAL RELATIONS 509 



tion at Littlestown is due to the overlap of the limestone by the Triassic 

 beds. The Frederick Valley of Maryland is the sonthwestward con- 

 tinuation of the York-Hanover Valley and is due to the emergence of 

 the limestone from beneath the Triassic cover. 



The limestones exposed in the Hanover-York-Lancaster-Chester Valley 

 were until recently treated as a unit by workers in this area and were 

 called the Shenandoah limestone. Detailed study by the writers, espe- 

 cially in the vicinity of Lancaster, has resulted in the distinguishing of 

 seven limestone formations and one shale and the determination of their 

 age and relations. Seven of these formations are found in normal undis- 

 turbed sequence northeast of Lancaster, as follows : 



Cocalico shale 1 



Beekmantown limestone ( 



Conococlieague limestone Upper Cambrian. 



Elbrook limestone Middle Cambrian. 



Ledger dolomite ^ 



Kinzers formation L Lower Cambrian. 



Vintage dolomite I 



These correspond in part with the limestone formations that occur in 

 the Cumberland-Lebanon Valley to the north. The three lower forma- 

 tions are divisions of the Tomstown dolomite of that area, and the upper- 

 most formation, the Cocalico shale, is the equivalent of post-Beekman- 

 town limestones of the Cumberland Valley section. 



Walcott, in his study of the limestones of the York-Lancaster Valley,^ 

 called attention to coarse limestone conglomerates which he described as 

 intraformational and stated that these conglomerates and all the asso- 

 ciated limestones of this part of the valley were probably not younger 

 than Lower Cambrian. 



In the detailed study by the present writers it was found that the con- 

 glomerates occur at no definite stratigraphic position, but are associated 

 with different beds at different places. They are also generally inter- 

 bedded with thin-bedded, dark-blue, slaty, argillaceous limestones similar 

 to some in the Kinzers formation, but of much greater thickness. Fur- 

 thermore, in the southern part of the valley, especially around Lancaster, 

 similar thin-bedded, dark-blue, argillaceous limestones, interbedded with 

 distinctive blue granular limestone beds and dark graphitic slates,, 

 predominate. The relation of these beds to formations whose sequence 

 was known led the writers to the conclusion that they belong to an over- 

 lapping younger formation at the base of which limestone conglomerates 



2 C. D, Walcott : U. S. Geol. Survey Bull. 134, 1896, p. 29. 



