Ixxxii GEOLOGICAL AXD PALEONTOLOGICAL RELATIONS 



Zone 18. — This zone is for the most part unfossiliferous, although 

 in places it carries some imperfect fossils and fossil casts. The mate- 

 rial of which it is composed is for the most part yellowish sand above 

 bnt grades down into bluish clay below and at times the entire stratum 

 is composed of bluish clay. In thickness it varies from 18 to 22 feet 

 along the Calvert Cliffs and is continuously exposed along the Calvert 

 Cliffs 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 8 

 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 12 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 within this zone: Balanus concavus, 

 Corhula idonea, Macrocallista marylandica, Dosinia acetabulum, Cardium 

 laqueatmn, Phacoides anodonta, Crassatellites ino/rylandicus, Astarte tliis- 

 pliila, Ostrea carolinensis, Pecten madisoniiis. 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 overlain 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 forma- 

 tion is found exposed in numerous places along the St. Mary's river, in 

 the vicinity of St. Mary's City as well as along the southern bank of 

 the Patuxent river, 



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 



^ Loc. cit. 



