Maryland Geological Survey 31 



Miocene 



The Miocene deposits of Maryland are represented by the Chesapeake 

 Group, which is made up of the Calvert, Choptank, and St. Mary's 

 formations. 1 These formations are chiefly of marine origin. They attain 

 a very extensive development in the drainage basin of Chesapeake Bay, 

 both in Maryland and Virginia, from which area they can be traced south- 

 ward into North Carolina and northward into Delaware and New Jersey. 

 To the south of the Hatteras axis the conditions change materially, and 

 other formations presenting faunal affinities more or less close are found. 



The several formations comprising the Miocene are apparently slightly 

 unconformable to each other, although this unconformity is oftentimes 

 not apparent, the Choptank in some areas being apparently conformable 

 to the Calvert, while the St. Mary's seemingly presents the same rela- 

 tions to the Choptank. The deposits of the Chesapeake Group consist 

 largely of sands, clays, and marls. The Calvert is in part sandy and 

 in part clayey, with extensive deposits of diatomaceous earth in the lower 

 or Fairhaven member, and numerous marl beds packed with molluscan 

 shell remains in the upper or Plum Point member. The Choptank forma- 

 tion is essentially sandy, although clays and marls also occur. The 

 St. Mary's formation is decidedly clayey with sands or sandy clays, the 

 latter typically greenish-blue in color and often containing large quan- 

 tities of fossils. 



The organic remains consist largely of fossil invertebrates, by far the 

 most common being molluscs. Diatoms are very common, and remains 

 of land plants are not rare in the basal strata, while corals, bryozoans, 

 and echinoderms are not infrequent. Many cetacean forms have been 

 found at some localities. 



The thickness of the Miocene deposits is between 450 feet and 500 feet, 

 and the strata have an average dip of 10 feet in the mile to the southeast. 



1 Another formation, the Yorktown, occurs at the summit of the Chesapeake 

 Group in Virginia and North Carolina. 



