78 THE GEOLOGY OF ST. MART's COUNTY 



eroded entirely away and disappears in the vicinity of Hollywood. 

 Further to the southeast, in the well-boring at Crisfield, it attains a 

 thickness of about 280 feet, although it is possible that the upper portion 

 of this may be Pliocene. The average thickness of the St. Mary's forma- 

 tion in this county is about 140 feet. 



Character of Materials. 

 The materials composing the St. Mary's formation are clay, sand, 

 and sandy clay. As exposed in this county, it is typically a greenish- 

 blue sandy clay bearing large qviantities of fossils and resembling very 

 closely the sandy clay of the Calvert formation described above. Locally, 

 the beds have been indurated by the deposition of iron. 



Stratigrapliic Relations. 

 The St. Mary's formation lies conformably on the Choptank forma- 

 tion. It is overlain unconformably by clays, loams, sands and gravels 

 belong-iug to various members of the Columbia Group. 



Sub-Division.s. 



There are certain faunal differences which separate it from the Chop- 

 tank formation. It has been subdivided into the following zones : 



Zone 21. — This zone lies at the base of the St. Mary's formation and 

 conformably on the Choptank formation. It consists of a drab clay 

 carrying sandy bands of about the same color and appears to be devoid 

 of fossils. It may best be seen along the cliffs south of Flag Pond, 

 where it has a thickness of about 15 feet. 



Zone 22. — Lying immediately above the last mentioned stratum is 

 another band of drab clay in which thin beds of fossils are developed. 

 These first made their appearance in the cliffs south of Flag Pond, and 

 although the continuity of this bed is interrupted along the Bay shore 

 by talus slopes and overgrowth of woodland, still it is believed to be 

 continuous with the fossil-bearing beds at the base of the cliff at 

 Cove Point. The following are some of the more important fossils 

 found in this zone: Balanus concavus, Terebra inornata, Mangilia 

 parvaj Nassa peralta, Columbella communis, Ecphora quadricostata, Tur- 



