PALEOZOIC FORMATIONS OF MARYLAND 



417 



the zigzag Helderberg area; then, in order named, that crossing 

 the county along the eastern side of Shriver Ridge ; the narrow 

 band extending across the county to the west of the Helderberg 

 area west of Wills Mountain ; to the southwest the band on the 

 western side of Fort Hill, and, lastly, the area extending from 

 the Twenty-first Bridge to Monster Rock at Keyser. 



Fig. I. — Devil's Backbone, near Cumberland, showing Helderberg limestone in 

 the steeper part and Oriskany sandstone in the farther railroad cut. 



The lower part of the formation is mainly a bluish-black 

 cherty limestone, 75 to lOO feet in thickness, the chert in 

 nodules and layers, with some dark gray arenaceous shales ; and 

 the remainder of the formation is mostly a sandstone, fre- 

 quently calcareous, and varying in color from gray to white, 

 about 250 feet in thickness. Toward the top there are a few 

 bands varying from grit to conglomerate. The sandstone, on 

 account of its calcareous cement, weathers readily to a friable 

 brownish or buff, porous rock, which, when protected from 

 erosion eventually forms beds of sand. Its thickness varies 



