MAKYLAXD GEOLOGICAL SURVEY 103 



The Piscataway member was named from Piscatawaj Creek, 

 Maryland, where it is typically developed. The member is charac- 

 terized by two well-marked and rather persistent layers of indurated 

 marls. Its thickness somewhat exceeds 50 feet. It is further char- 

 acterized by a fossil fauna among which are the following forms: 



Thecachampsa sericodon (?) Cope. 

 Synechodus clarkii Eastman. 

 Odontaspis elegans (Agassiz). 

 Otodiis obliquus (Agassiz). 

 Pholadomya marylandica Conrad. 

 Gryphsea vesicularis Lamarck. 

 Textularia subangulata D'Orbigny. 



The Paspotansa member was named from Paspotansa Creek, Vir- 

 ginia. It consists of a bed of greensand and greensand marl some- 

 what less than 50 feet thick. Among the characteristic fossils of 

 this member are the following: 



Bythocypris subsequata Ulrich. 

 Pleurotoma harrisi Clark. 

 Cancellaria graciloides Aldrich var. 

 Trophon sublevis Harris. 

 Chrysodomus engonatus (Heilprin). 

 Calyptraphorus jacksoni Clark. 

 Discosparsa varians Ulrich. 

 Membranipora angusta Ulrich. 

 Textularia gramen D'Orbigny. 

 Anomalina ammonoides (Reuss). 



The Nanjemoy Formation. 



The formation receives its name from ]S^anjemoy Creek, one of 

 the tributaries of Potomac River in Maryland, in whose valley 

 deposits belonging to this horizon are characteristically developed. 

 This name was proposed by Clark and Martin in 1001.^ 



Areal Distribution.- — The Nanjemoy formation is much less exten- 

 sively exposed in this county than the Aquia. It extends across the 

 county from Hardesty to the valley of Piscataway Creek in a very 

 circuitous and broken outcrop. In its larger relations it extends 

 from Virginia northwestward through Maryland as far as Chesa- 



^Eocene, Maryland Geol. Survey, p. 64. 



