86 The Lower Devonian Deposits of Maryland 



of the underlying Tonoloway. The uppermost strata of the Tonoloway are 

 commonly hard and resistant. The beds a little below the top of that forma- 

 tion on the contrary are usually very shaly and are often concealed, 

 weathering to soil. At many places a massive stratum 18 inches to 3 feet 

 thick lies immediately below the nodular limestone of the Keyser. This 

 stratum is not nodular and diagnostic fossils have not been found in it. 

 It is referred to the Tonoloway. 



The Coeymans Member 



Character and Thickness. — The Coeymans was originally known to 

 geologists as the Pentamerus limestone, later as the Lower Pentamerus 

 limestone, in the lower part of which is the Stromatopora limestone. In 

 1899 Clarke and Schuchert changed this biologic term to the geographic 

 name Coeymans limestone, after the village of Coeymans in Albany 

 County, New York, through which the Helderberg escarpment extends, 

 affording a fine exposure of this formation. 



The Coeymans member of the Helderberg of Maryland consists of 

 massive blue, crystalline limestone, commonly crinoidal and containing a 

 small amount of chert. Thin bands of sandstone develop locally near its 

 base in the Cumberland area, being especially well seen at Dawson, where 

 they appear as raised brown bands upon the weathered surface of the rock. 

 East of Hancock the Coeymans is at places a sandstone, which is espe- 

 cially prominent on Elbow Ridge. The strata of tliis member are very 

 resistant to weathering and hence frequently form projecting ledges, a 

 feature strikingly shown at Corriganville where it forms the " backbone " 

 of the cliff termed the Devil's Backbone. 



The Coeymans is quite thin, varying from S to 13 feet in thickness. It 

 may be absent locally although this fact is not well established. 



Fauna. — The Coeymans is highly fossiliferous, abounding especially in 

 Gypidula coeymanensis which is its guide fossil. A list of the species con- 

 tained in it is given in the table showing the distribution of the species. 



Coeymans-Keyser Boundary. — The Coeymans-Keyser boundary is 

 sharply defined, both lithologically and faunally. The upper beds of the 

 Keyser are thin-bedded and many strata appear banded, consisting of thin 



