:makyla^'d geologicai, si.kvey 105 



StriJce, Dip, and Thicl-ness. — The formation lias a northeast-south- 

 west strike and dips toward the southeast at an average rate of about 

 121/2 feet to the mile. The Nanjemoy is about 100 feet thick in 

 Prince George's County and thickens gradually toward the east. 



Strailgrapliic Relations. — The Xanjemoy overlies the Aquia con- 

 formably, but is overlain unconformably by the ]\Iiocene, and in 

 some places along the line of outcrop by deposits belonging to the 

 Lafayette and the Pleistocene. 



Subdivisions. — This formation, like the Aquia, is subdivided into 

 two members or substages, known as the Potapaco and Woodstock. 



The Potapaco member is so called from the early name of Port 

 Tobacco (a corruption of the word Potapaco) Creek, one of the 

 Maryland tributaries to Potomac Kiver. It is characteristically 

 clayey, especially in its lower portions. It is about 60 to 65 feet 

 thick and carries the following characteristic fossils: 



Cypra?a smithl Aldrich. 



Solen lisbonensis Aldrich. 



(?) Lucina astartiformis Aldrich. 



Ceiropora micropora Goldfuss. 



This member is further subdivided into six zones which, together 

 with their characteristic fossils, are fully discussed by Clark and 

 ^lartin in the report already cited. ^ 



The Woodstock member has been named from Woodstock, an old 

 estate situated a short distance from Mathias Point on the Virginia 

 side of the Potomac. This member is characterized by fine homo- 

 geneous greensands and greensaml marls which are less argillaceous 

 than the underlying Potapaco beds. It ranges in thickness from 

 60 to 65 feet and contains certain characteristic fossils, a few of 

 which are the following: 



Pyrula penita Conrad var. 

 Meretrix lenis (Conrad). 

 Leda parva (Rogers). 

 Spiroplecta clarki Bagg. 

 Nonionina afflnis Reuss. 



lEocene, Maryland Geol. Survey, 1901, pp. 65-66. 



