THE TALBOT FORMATION. 187 



between the drab clay and the gravel band above is a very sharp one and rep- 

 resents a decided change in deposition. It is believed that the swamp accu- 

 mulation was formed in the lower course of a tributary stream of the Eappa- 

 hannock which had been dammed by the formation of a bar across its mouth. 

 Many examples of such ponded streams are now to be observed all along the 

 tidal estuaries. Partially separated from the waters of the Eappahannock, 

 SAvamp vegetation, probably similar to that now growing in the Dismal 

 Swamp^ flourished until an extensive bed of vegetable debris was formed. 

 A further submergence of the area, however, depressed the protecting sand- 

 bar, permitting the Eappahannock Eiver waters to flood the area, finally 

 killing the swamp vegetation. During this greater submergence the upper 

 three beds of the described section were probably deposited. 



Along the Dismal Swamp canal between Deep Creek and Lilly, X. C, 

 and also along the feeder which connects Lake Drumniond and the canal, 

 strata containing marine fossil shells are exposed. The dredge has brought 

 up quantities of these shells in several different places. As stated on a pre- 

 vious page these fossils were formerly believed to belong to the Pliocene but 

 are now regarded as of Talbot age. The matrix is a bluish-green argillaceous 

 sand and shells are in excellent state of preservation. 



The following section passed through in sinking a well just to the east 

 of Norfolk is typical of the normal development of the Talbot formation. 



Well Section near NorfolTc. 



Fet't 



Pleistocene. Talbot Fine white quartz sand IS 



Blue claj' 13 



Sandy clay 3 1 2 



Loose sand S 



Gravel with water-bearing horizon ininiedi- 



atelv overlvinff shell marl Tti; 



Total 50 



Strike, (lip. and tJriclncss. — The Talbot strata, as in the case of the 

 Wicomico and Sunderland formations, occupy a practically horizontal posi- 

 tion with but a slight inclination toward the main waterways of the Atlantic 

 Ocean, the Chesapeake Bay and the tributary estuaries. 



The average thickness of the formation over the greater portion of the 

 Virginia Coastal Plain is somewhat less than 30 feet. It thickens to the 

 eastward, however, and in the vicinity of Norfork and Portsmouth seems 

 to be as much as 50 to 60 feet in thickness. In Accomac County on the 

 Eastern Shore the Talbot attains its greatest thickness and as shown bv 



