Maryland Geological Survey 31 



feet, and they show an average dip of 12| feet in the mile to the 

 southeast. 



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.^ These formations are 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 southward 

 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 f aunal affinities rftore 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 Eairhaven member, and numerous marl beds packed with mol- 

 luscan shell remains in the upper or Plum Point member. The Chop- 

 tank formation 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 

 quantities of fossils. 



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

 most common group being the mollusca. 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 and 500 feet, 

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



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

 Group in Virginia and North Carolina. 



