104 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



Division. — The Nanjemoy formation has been divided into two mem- 

 bers known as the Potopaco clay member and Woodstoclc greensand marl 

 member, the former having a thickness of from 60 to 65 feet and the 

 latter from 50 to 60 feet. The Potopaco member is composed of greensand, 

 often very argillaceous and at times gypseous. The clayey character of 

 this member especially in its lower portions is in marked contrast to the 

 more highly glauconitic beds of the Aquia formation below. The Wood- 

 stock member is characterized by fine homogeneous greensands and green- 

 sand marls that are much less argillaceous than the underlying Potopaco 

 beds. The Potopaco member in the Potomac region is subdivided into six 

 zones and the Woodstock member into two zones. The two members show 

 paleontological differences to which reference will be made again later. 



Paleontologic character. — The fossils of the ISTanjemoy are not quite so 

 plentiful as in the Aquia formation yet locally they are very abundant. All 

 of the chief groups of the invertebrates are represented, as well as fishes, and 

 a few plant remains. The latter consist of some small lignitized nuts found 

 along the Potomac Eiver near Popes Creek, Maryland. In 1850 a fossil 

 nut the size of a large hickory nut was described by Kuffin, from the Eocene 

 of the Pamunkey, probably from the Nanjemoy. As in the case of the 

 Aquia, the molluscs predominate. The most widely distributed and abun- 

 dant form is Venericardia potapacoensis, it being found in almost every 

 exposure of ISTanjemoy greensand. 



Among the forms which are limited to the ISTanjemoy formation but 

 found in both members are : 



Meretrix ovata var. ovata (Rogers) Ostrca sellaeformis Conrad 



Lncina dartoni Clark Leda improcera (Conrad) 



Lucina whitei Clark Leda potomaeensis Clark & Martin 



Venericardia potapocoensis Clark & Leda tysoni Clark & Martin 



Martin Niicula potoniacensis Clark & IMartin 



The following species have been found only in the Potopaco member : 



Cypraea snnthi Aldrich Periploma sp. 



Solen lishonensis Aldrich Geriopora inicropora Goldfuss 



( ?) Lucina astartiformis Aldrich 



The following species among others are found only in the Woodstock 

 member : 



Galeocerdo latidens Agassiz Venericardia marylandica Clark & Mar- 



Levifusus trabeatus ( ?) Conrad tin 



Pyrula penita Conrad var. Modiolus marylandicus Clark & Martin 



Turritella potomacensis Clark & Martin Leda parva (Rogers) 



Dentalium minutistriatum Gabb Carpolithus marylandicus Hollick 



Meretrix lenis (Conrad) 



