76 THE GEOLOGY OF ST. MARY's COUNTY 



feet along the Calvert Cliffs, where it is continuously exposed from 

 Parker Creek to a point a few miles south of Flag Pond. Where this 

 zone is exposed at Drum Cliff it is thinned down to about S feet in 

 thickness. 



Zone 19. — This constitutes the upper of the two great fossiliferous 

 zones of the Choptank formation. Like Zone 17 it is composed almost 

 entirely of fossils with the interstices filled with reddish and yellow 

 sand. It varies in thickness from 13 to 15 feet along the Calvert Cliffs 

 and is continuously exposed from Parker Creek southward to near Cove 

 Point, where the stratum dips beneath the beach. The following is 

 a partial list of fossils found in this zone : Balanus concavus, CorMiIa 

 idonea, Macrocallista marylandica, Dosinia acetahulum, Cardium laquea- 

 tum, Pliacoides anodonta, Crassatellites niarylandicus, Astarte thispliila, 

 Ostrea carolinetisis, Pecten madisonius, Area staminea, etc. This zone 

 corresponds to " Zone f " of Harris." 



Zone 20. — This zone lies at the top of the Choptank formation. It 

 consists of greenish sand. which is frequently oxidized to a red color, 

 and at times it carries bands of clay. It seems to be devoid of fossils 

 and is 15 feet thick, although it has frequently suffered by erosion. It 

 may be best seen near Flag Pond, where it is overlaid by the St. Mary's 

 formation. 



THE ST. mart's FORMATION. 



The name of this formation has been suggested by St. Mary's County 

 on account of its great development within that region. The formation 

 is found exposed in ninnerous places along the St. Mary's Eiver in the 

 vicinity of St. Mary's City. In Calvert County it is best seen along 

 the Calvert Cliffs from Point of Eocks southward to Drum Point. 



Areal Distribution. 

 The St. Mary's formation, like the Calvert and the Choptank forma- 

 tions, crosses the state from northeast to southwest. On the Eastern 

 Shore, it is present, if at all, in Caroline, Talbot, Wicomico and Dor- 

 chester counties. 



On the Western Shore the St. Mary's formation is found developed 

 "Loc. cit. 



