Mahtland Geological Survey 95 



The contact of the Oriskany and Helderberg is more obscure in the 

 eastern part of the Hancock area where the Shriver member is absent and 

 both formations are largely limestone. This is especially true where the 

 Coeymans contains beds of sandstone as it does in the North Mountain 

 area. The line of division is best distinguished under these circumstances 

 by the fact that the Becraft limestone is comparatively free from sand 

 grains, while the Oriskany limestone yields sandy or even conglomeratic 

 fragments upon weathering. 



Eomney-Oriskany Boundary. — The Lower Devonian was formerly 

 thought to be terminated by a marked hiatus. Unconformity by erosion 

 was reported by Darton ' between the Oriskany and Eomney of Virginia, 

 This view has been held by most subsequent observers. The belief was 

 based, in part, upon the supposed absence of the Onondaga fauna in Mary- 

 land and adjacent areas. It has recently been shown by Kindle,^ however, 

 that the Onondaga fauna is present in the basal beds of the Romney. 

 While the magnitude of any hiatus at the close of the Oriskany must there- 

 fore be small, the occurrence of at least a short erosional uncomformity at 

 that horizon is indicated by the following facts : the great abruptness of the 

 transition from the Oriskany to the Eomney, the apparent erosion of the 

 upper surface of the Oriskany at many places, its varying thickness, and 

 the local development of a basal conglomerate in the Eomney, the latter 

 being well shown at Warren Point, Fulton County, Pennsylvania, just 

 north of the Maryland-Pennsylvania line. 



composite section of the oriskany formation 



Heavy-bedded arenaceous limestone, gradually changing 

 downward into a black chert or siliceous shale. 



In the lowest beds are found Spirifer cum'berlandue, 

 S. concinnoideus, and Eatonia sinuata. Fossils, how- 

 ever, are rare until 100 feet above the base of this 

 division, where the characteristic Hipparionyx fauna 

 attains greater individual and specific representation, 

 0) ^ culminating in the upper 100 feet, which is the present 



source of nearly all the Cumberland Oriskany fossils. 



258 feet. 



' Amer. Geol., vol. x, 1892, p. 16. 

 •Bull. U. S. Geol. Survey No. 508, 1912. 



