92 The Lower Devonian Deposits of Maryland 



The thickness of the Shriver Chert member is variable. In the more 

 westerly localities it is about 100 feet thick. It thins eastward and is 

 absent in Washington County. 



Fauna. — The fauna of this member is meager. A list of the species 

 found in it is given in the table of distribution. 



Ridgehj Sandstone Member 



Character and Thickness. — The sandstone member of the Oriskany 

 formation was termed the Monterey sandstone by Darton '' in 1892. It was 

 later called the Upper Oriskany by Schuchert.' It is named from Eidgely, 

 West Virginia, at the north end of the Knobly Mountain, opposite Cum- 

 berland, at which place it is admirably exposed. 



This member is composed of a calcareous sandstone which passes in 

 places into an arenaceous limestone because of the great development of 

 calcareous cement. It also contains conglomeratic beds, one of which, situ- 

 ated near the top of the member in the vicinity of Cumberland, contains 

 pebbles resembling grains of wheat. When unaltered by exposure the rock 

 is bluish-gray, calcareous, very tough, and admirably adapted for use as 

 railroad ballast. Upon weathering the calcareous cement is dissolved out 

 by the surface waters and the rock disintegrates, forming sand and large 

 boulders of sandstone, which cover the mountain slopes and are often 

 carried far into the valleys. If the rock was originally very calcareous it 

 becomes friable upon weathering and yields a quartz sand which is 

 quarried extensively as a glass sand, particularly in the vicinity of 

 Berkeley Springs, West Virginia, where it is very free from iron. Less 

 pure beds are used as building sand. The character of the rock undergoes 

 a change eastward, becoming more calcareous until in the North Mountain 

 area it is a limestone sufficiently pure to be used as a source of lime. It 

 then contains numerous beds of chert and closely resembles the underlying 

 Becraft limestone. The thickness of the Eidgely member varies from 

 250 feet in the western exposures to 50 feet or less at the North Mountain. 



^ Amer. Geol., vol. x, 1892, p. 15; Staunton Folio, U. S. Geol. Survey, 1894. 

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



