Maryland Geological Survey 91 



Oriskanian. This embraces the Camden, Port Ewen, Esopus, Oriskany, 

 and Decewville foimations. 



The Oriskany formation of Maryland consists of lower black cherty 

 shale, overlain by calcareous sandstone or arenaceous limestone. The 

 sandstone, because of its resistance to weathering, unites with the New 

 Scotland cherty limestone to form a series of high ridges or mountains 

 wherever it is exposed. 



The thickness of the Oriskany is variable. At 21st Bridge it is 350 

 feet, and at Tonoloway 417 feet thick, while but 14 miles northeast of the 

 latter place it is only 50 feet thick. These facts suggest an unconfonnity 

 by erosion of its upper surface. 



SUBDIVISIONS 



The Oriskany formation is subdivided into two members which differ 

 both lithologically and faunally. They are, in ascending order, the 

 Shriver Chert and the Ridgely sandstone members. 



Shriver Chert Memher 



CHARiVCTER AND THICKNESS.— The name Shriver Chert is proposed for 

 this member from Shriver Eidge at Cumberland, Maryland, upon which it 

 is extensively exposed. It was called the Lower Oriskany by Schuchert.' 

 It consists of a dark siliceous shale containing large quantities of 

 black impure chert in the form of nodules or layers of nodules. 

 Upon weathering both shale and chert become buff or yellowish in 

 color and break into sandy fragments, the chert in some cases be- 

 coming spongy. The rock has the property of cementing upon ex- 

 posure and is much used as a surfacing material for roads for which 

 purpose it is admirably adapted. Where it outcrops the surface of the 

 ground becomes covered with a thick mantle of gravel-like fragments, the 

 resulting soil being well adapted to the cultivation of orchards and berries. 

 This member occupies extensive areas upon the slopes of the mountains 

 upon which the Oriskany outcrops in Allegany County. 



' Proc. U. S. Nat. Mus., vol. xxvi, 1903, p. 422. 



