64 THE EOCENE DEPOSITS OF MARYLAND 



Zone S. — The highly characteristic greensands and greensand marls 

 of the preceding zones are succeeded in the Potomac sections by a zone, 

 some 30 feet in thickness, in which the grains of glauconite have been 

 extensively weathered when exposed, giving the strata, where outcrops 

 are found, a greenish-gray appearance, which changes to a reddish- 

 brown in the upper layers. Several irregular bands packed with Turri- 

 tella mortoni' are present in both the Aquia Creek "and Potomac Creek 

 sections, while associated with that species at both localities are Turritella 

 humerosa, CucuUaea gigantea, CrassateJIites alaeformis, Ostrea compressi- 

 rostra, and many other forms. The upper portions of this bed have 

 ■ afforded most of the species obtained from the Potomac Creek bluff. 



Zone 9. — The thick-bedded limestone layers which compose this zone 

 are almost exclusively made up of the shells of Turritella mortoni, form- 

 ing a Turritella rock. (Plate III, Fig. 2.) Between the indurated layers 

 are interstratified layers of unconsolidated and much weathered green- 

 sand, Avhich contain few fossils of any description. Great masses of this 

 Turritella rock strew the shore at the base of both the Aquia Creek and 

 Potomac Creek bluffs. The bed is about 10 feet thick in the Aquia 

 Creek bluff, but reaches 17 feet at Potomac Creek. At the latter locality 

 it consists of five feet of limestone at the base, followed by five feet of 

 greensand, two feet of limestone, two feet of greensand, two feet of 

 limestone and one foot of yellowish greensand. The fauna of Zone 9 is 

 evidently identical with that of Zone 8. 



The Nanjemoy Formation or Stage. 



The Nanjemoy formation, so-called from Nanjemoy Creek, which 

 enters the Potomac river from the Maryland side in Charles county, just 

 below Maryland Point, is composed of greensand, often highly argil- 

 laceous, and less frequently calcareous than the lower beds, and with 

 here and there layers containing abundant crj'stals and crystalline 

 masses of gypsum. The thickness of the deposits is about 125 feet. 



The Nanjemoy stage comprises Zones 10 to IT. The following fossils 

 are foimd in both substages: 



Meretrix ovata var. ovata (Rogers). Leda improcera (Conrad). 



Lucina dartoni Clark. Leda po^oHirtcew.s/.s Clark and Martin. 



Lucina whitei Clark. Lcda tysoni Clark and Martin. 



Venericardia potapacoensis Clark & Martin. Nncala potontacensis Clark and Martin. 

 Ostrea sellaeformis Conrad. 



