158 I'lIYSIOGnAPHY AND GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



A few hundred yards below tlie above locality there is a better exposure 

 of the marl. The material is less weathered and the matrix consists of the 

 usual dark green argillaceous sand. 



y]. Section on the Melicrriii Eiver a fev hinuJrod yards, heloir preceding 



section. 



Feet 

 Pleistocene lleddisli yello\\' sandy clay and sand, lam- 

 inated about 20 



Light pray or drab sandy clay mottled red 



with iron 10 



Miocene. St. Mary's Dark green sandy clay yellow in places, due 



to weathering, full of MuUnia covgesta 

 ^^itll a few other forms. Pecten, Crepidula, 



etc about 36 



Similar material with many fossils. To 

 water's eda^ 5 



Total 71 



The Yorktown Formation. 



Name. — The Yorktown formation has been so named because of the 

 excellent exposure of the strata of this age in the prominent cliffs at York- 

 town. The name was first employed in a formational sense by Clark and 

 Miller" in 1906. 



Stratigrapliic relations. — The Yorktown formation overlies the St. 

 Mary's formation conformably, although more careful work may reveal 

 the presence of a slight unconformity. Such a break would help to 

 explain the striking changes in physical environment between the two epochs 

 of sedimentation as shown in the lithologic character of the strata. The 

 Yorktown is for the most part concealed from view over the divides as well 

 as in many of the larger valleys by uncouformably overlying deposits of 

 Pleistocene age. 



Lithologic character. — The most characteristic materials of the Yorktown 

 formation are beds of comminuted shells in which are contained numerous 

 shells still entire but often more or less worn. These fragmental beds are 

 well exposed along the York Eiver below Yorktown at Fergussons Wharf, at 

 Smithfield, at Bonn's Church, at Suffolk, and near Eeid's Ferry. They are 

 usually quite firmly indurated by calcium carbonate obtained from the same 

 or overlying marl beds. The appearance of the rock is not unlike the coquina 



aClay Deposits of the Virginia Coastal Plain. Geol. Series, Bull. Xo. 11, Geol. 

 Survey of Virginia, 1906, p. 19. 



