MARYLAXD GEOLOGICAL SURVEY 75 



ing are the most important : Ecphora trxcostata, Panopea whitfieldi, P. 

 aniericana, Corhula elevata. Phacoides {Ludnoma) contractus, Veneri- 

 cardia granulata, Astarte cuneiformis, A. thomasi, Thracia conradi, Pec- 

 ten madisonius. P. humplireydi, Chione latilirata, Cytherea staminea. 



Zone 3. — This stratum when freshly exposed consists of a greenish 

 colored diatornaceons earth which, on weathering, bleaches to a white 

 or l)iiff-colored deposit breaking with a columnar parting and pre- 

 senting perpendicular surfaces. It is very rich in diatomaceous matter, 

 tlie mechanical analyses of specimens yielding more than 50 per cent 

 of diatoms. The thickness of this bed varies from place to place, but 

 where it is penetrated at Chesapeake Beach by an artesian well it has 

 a thickness of about 5.5 feet. At Fairhaven, where 'it is well exposed, 

 it carries large numbers of Phacoides (Lucinoma) contractus. This 

 zone is best exposed at Popes Creek, Lyons Creek, Fairhaven, and in 

 stream gullies lying along the northern margin of the Miocene beds. 



Plum Point Marls. — The Plum Point marls occupy the remainder 

 of the Calvert formation above the Fairhaven diatomaceous earth. 

 Plum Point in Calvert County, where the beds are typically developed, 

 has suggested the name for this member. Thase marls consist of a 

 series of sandy-clays and marls in which are imbedded large numbers 

 of organic remains including diatoms. (Plate II.) The color of the 

 material is bluish-orocn to grayish-brown and buff. Fossil remains al- 

 though abundant through the entire deposit are particularly numerous in 

 two prominent beds from 30 to 35 feet apart. These beds vary in thick- 

 ness from 4^ to 13 feet. They may bo easily traced along the Calvert 

 Cliffs from Chesapeake Beach to a point 2 miles below Governor Pun. 

 At Chesapeake Beach they lie high up in the cliffs, and pass gradually 

 downward beneath the surface of the water as the formation is followed 

 southward. Along the Patuxent Eiver the Plum Point marls are not 

 exposed so extensively as in the Calvert Cliffs but they are visible 

 at intervals from the cliffs below Lower Marlboro southward to Ben 

 Creek. 



When fresh the Plum Point marls and the Fairhaven diatomaceous 

 earth do not differ much in a])pearance. The thickness of the former 



