94 The Lower Devoniax Deposits of Maryland 



Eowe was the first to explain the cause for this restricted leaching out 

 of the Oriskany fossils and their occurrence in " sand pockets." He states : 

 " Most of the fine collecting grounds for Oriskany fossils in this region 

 are within five or ten minutes walk from the hotels in Cumberland. The 

 disintegration of the sandstone has been carried on there much more com- 

 pletely than at any other place, and has been due, no doubt, to the cutting 

 of the Potomac Kiver and Wills Creek across Knobly Mountain and 

 Shriver Eidge. The constant downward percolation of the water from 

 the Potomac River and Wills Creek, when both flowed over this part of 

 Cumberland, has carried away the calcareous material of the Oriskany 

 arenaceous limestone and the fossils and has partially replaced the latter 

 with silica. As leaching continued, all the calcareous material was finally 

 removed and more or less large partially empty pockets or caves were 

 formed. These are filled with loose sand and in them the fossils are at 

 times found in considerable quantity."' 



The Ridgely fauna becomes abundant only in the upper 150 feet of the 

 member. 



Shkiver-Eidgelt BoruNDARY. — The line of division between these 

 members is not sharp. The amount of chert diminishes in the upper part 

 of the Shriver member and thin beds of sandstone appear, forming beds of 

 passage to the overlying sandstone. 



LIMITS OF THE ORISKANY 



Oriskany-Helderberg Boundary. — The lower limit of the Oriskany is 

 clearly indicated in the Cumberland area by the lithological difference 

 between the Shriver Chert and the New Scotland member of the Helder- 

 berg. The Shriver Chert is black when freshly exposed. Upon weathering 

 it yields small buff, or orange colored, sandy fragments which are thickly 

 strewn over the surface of the outcrop. The New Scotland chert on the 

 contrary is white and upon weathering yields large, irregular ragged 

 masses. The shale at the top of the New Scotland is usually not exposed 

 but its position is frequently indicated by a slight depression of the sur- 

 face between the chert beds. Because of their resistance to erosion the 

 New Scotland and Oriskany cherts usually occupy the crests of hills or 

 mountains of considerable altitude. 



