126 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



are exposed along almost every stream that lias cut through the superficial 

 Pleistocene capping. The most numerous exposures are along the channels 

 of the Potomac, Eappahannock, York, and James rivers. No outcrops of 

 Miocene beds appear on the Bay shore as in Maryland since the bluffs are 

 low and as far as known consist entirely of Quaternary deposits. 



The Calvert Formation. 



Name. — The basal beds of the Miocene constitute the Calvert formation 

 which receives its name from Calvert County, ]\Taryland, where in the 

 famous Calvert Clitfs are to be seen perhaps the best exposures of the strata 

 of this age anywhere in the Atlantic Coastal Plain. The name was first 

 applied to these deposits by G. B. Shattuck" in 1903. 



Stratigraphic relations. — The deposits of the Calvert rest unconform- 

 ably upon the Nanjemoy formation throughout most of the Coastal Plain. 

 In the James Eiver basin, however, this formation transgresses the Nanje- 

 moy and rests on the exposed margins of the Aquia formation while farther 

 south it also transgresses the Aquia and about Petersburg is in contact with 

 the Patuxent beds. Still farther to the south the Miocene at its western 

 margin rests upon the crystalline rocks of the Piedmont Plateau. 



The Calvert is unconformably overlain by the Choptank formation in 

 Maryland but whenever exposures of the contact have been found in Virginia 

 the St. Mary's formation succeeds the Calvert unconformably. It is pos- 

 sible, as stated later, that the Choptank formation may overlie the Calvert 

 for a short distance on the south side of the Potomac as on the north side 

 in Maryland but it seems more probable that this formation has already 

 thinned out and disappeared before the south side of the Potomac is reached. 

 It is impossible to definitely determine this point in the absence of an 

 exposed contact in this area due to the cover of Pleistocene deposits. 



Lithologic character. — The Calvert formation consists largely of sands, 

 clays, marls, and diatomaceous earth. Fine-grained, light-colored sands 

 predominate. The clays, which usually contain considerable sand, are dark 

 blue to black in color, weathering to a gray or sometimes to a dull white 

 on exposure. They grade into sands through an increase in the arenaceous 

 matter, or into diatomaceous earth due to an increasing number of diatom 

 tests. The blue clay almost invariably contains numerous casts of small 

 shells although at times the original shell substance still remains. Gypsum 

 crystals, sometimes single, at other times arranged in rosettes, are frequently 



"Science, Vol. xv, 1902, p. 906. 



