90 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



distance below City Point, while there are some exposures along the lower 

 course of the Appomattox Eiver. South of the Appomattox the only 

 exposures thus far known are in the vicinity of Boilings Bridge, on the 

 Nottuway Eiver. 



The Aquia Formation. 



Name.- — The formation receives its name from Aquia Creek, a stream 

 which empties into the Potomac Eiver in Stafford County. Along the 

 lower course of this stream as well as along the banks of the Potomac 

 Eiver near its mouth are found several fine exposures. The name was pro- 

 posed by W. B. Clarka in 1895, and further defined in 1896^. 



Stratigraphic relations. — The Aquia formation rests unconformably 

 upon the irregularly eroded surface of the Potomac formations. In the 

 northern portion of the State it overlies the Patapsco, while in the James 

 Eiver basin it rests upon the exceedingly uneven surface of the Patuxent 

 formation. In the latter locality the most marked unconformities thus far 

 recognized in the Virginia Coastal Plain are found. Thus along Shockoe 

 and Gillis creeks in Eichmond and along the tributaries of the Appomattox 

 Eiver a few miles north of Petersburg the Aquia is exposed only a little 

 above tide, while along the strike of these exposures and even to the east of 

 them at Drewry's Bluff, Howlett House Bluff, the high Ijhiff above the 

 Dutch Gap canal, on the James Eiver, and at Point of Eocks on the 

 Appomattox Eiver the Patuxent is found rising from 40 to 80 feet above 

 tide. At Point of Eocks the Patuxent would be expected to occur at from 

 25 to 30 feet below tide, as the Aquia deposits at Ashton Creek, two miles 

 directly west, are almost at tide, but instead of this the Patuxent rises about 

 80 feet above tide. No faulting has apparently occurred. Thus in the dis- 

 tance of about two miles there is an apparent unconformity amounting to 

 about 100 feet. The Aquia is conformably overlain by the Nanjemoy 

 formation or unconformably by the unconsolidated deposits of the Pleisto- 

 cene where the JSTanJeraoy is wanting. 



Lithologic character. — The Aquia formation consists primarily of green- 

 sands and greensand marls interbedded M'ith which are occasionally layers 

 composed almost entirely of shells. The latter often become thoroughly 

 indurated. Such beds are excellently exposed near the mouth of Aquia 

 Creek and near the tops of the divides east of Stafford. The greensand on 

 weathering frequently becomes reddish-gray, yellow, or reddish-brown in 



a Jolms Hopkins Univ. circ, vol. xv, 1895, pp. 3. 

 h Bull. U. S. Geol. Survey, No. 141, p. 39. 



