78 THE GEOLOGY OF CALVERT COUNTY 



down the dip from 5 feet where it is exposed in the bluffs at Chesa- 

 peake Beach to 13 feet 1^ miles south of Plum Point Wharf, where it 

 approaches tide level. It is unfossiliferous or carries a few imperfect 

 fossil casts. 



Zone 12. — When typically developed, this zone consists of a brownish 

 sandy clay, although at times it changes to a bluish color. In many of 

 its exposures only imperfect fossil casts can be distinguished, but in 

 other places it is found to carry Ecphora quadricostata var. umhilicata, 

 Venus mercenaria, Cytherea stamitiea, etc. It varies in thickness from 

 two to four feet and corresponds to " Zone c " of Harris.^ 



Zone 13. — The materials of this zone consist of a bluish sandy clay 

 more or less changed in sections to a yellowirli or bro\^Tiish color. It 

 carries imperfect fossil casts and varies in thickness from 32 feet at 

 Chesapeake Beach to 10 feet at a point one mile south of Parker CreeJv, 

 thus gradually thinning as it passes down the dip. 



Zone 14. — The materials which make up this stratum consist of a 

 brownish to yellowish sandy clay abundantly supplied with Isocardia 

 fraterna. It varies in thickness from 2 to 7 feet and corresponds to 

 "Zone d" of Harris.* 



Zone 15. — This zone is the uppermost member of the Calvert forma- 

 tion and consequently has been considerably eroded so that its true 

 thickness is not definitely known. It consists of a yellowish sandy clay 

 grading down locally into yellowish sand in its lower portions. At a 

 point one mile south of Plum Point Wharf this zone shows a greater 

 thickness than anywhere else along the Calvert Cliffs; at that place it 

 measures 48-| feet. Sections north and soutli of this point have either 

 been in great part replaced by Pleistocene sand or have suffered by the 

 unconformable overlapping of the Choptank formation. 



THE CHOPTANK FORMATION. 



The Choptank Eiver has suggested the name for this formation be- 

 cause of its great development on the northern bank of that estuary a 

 short distance below Dover Bridge in Talbot County. In this locality 



^ Loc. cit. " Loc. cit. 



