THE AQUIA FORMATION. 91 



color. Argillaceous beds occur but there is much less clay in the Aquia 

 than in the Nanjemoy formation. The greater portion of the strata con- 

 stituting the Aquia formation seems to have been accumulated in quiet and 

 probably relatively deep water, as the bedding planes are very regular and 

 the materials are largely fine sand. While the basal beds of the Aquia were 

 being formed the water Avas probably shallower and the shore-line closer, as 

 we find in these beds :^ome small pebble bands which are absent in the 

 higher beds. The general character of the formation is shown in the de- 

 tailed sections given on a subsequent page. 



Strike, dip, and thickness. — The strike of the Aquia formation is almost 

 due north and south and the beds dip to the east at the rate of from 12 to 15 

 feet to the mile. This dip has been determined from surface exposures in 

 the region of outcrop in Stafford County and probably continues at about 

 the same rate after it passes at tide beneath the overlying formations as the 

 base of the Eocene is found at a depth of 840 feet in the new government 

 well at Fortress Monroe. 



The thickness of the Aquia formation in Stafford County is about 100 

 feet, and this thickness is probably rather constant throughout the Coastal 

 Plain. In the Fortress Monroe well Just mentioned the total thickness of 

 the Eocene is only a little in excess of 200 feet and if half of that thickness 

 represents the Aquia, as seems probable, the formation will be seen to 

 maintain about the same thickness even down the dip. 



Divisions. — The Aquia formation has been divided into two members 

 known as the Piscataway indurated marl and Paspotansa greensand marl, 

 each having a thickness of about 50 feet. The Piscataway member 

 is characterized by greensands and greensand marls, the lower beds often 

 very argillaceous. The well-marked and rather persistent layers of 

 indurated marl characterizes the upper beds in the Potomac region. The 

 Paspotansa member is composed of a thick bed of greensand, overlain by 

 thick-bedded, indurated layers of greensand marl. The Piscataway mem- 

 ber in the Potomac region is again divided into seven zones and the Paspo- 

 tansa member into two zones. The two members show paleontological 

 differences to which reference will be made later. 



PahonioJof/ica] character. — The Aquia formation has yielded a great 

 many fossils belonging to most of the gi'oups of the invertebrates as well as 

 to the fishes and reptiles among the vertebrates. The moUuska, especially 

 the Peleeypoda and Gastropoda, are represented by countless individuals and 

 by many species. All are marine forms. Many of the Aquia species are 

 limited to this formation although a considerable number survived during 

 tlie deposition of the l^anjemoy. 



