MAKYLA^^D GEOLOGICAL SUEVEY 107 



a subdivision of the formation into two members, which are described 

 below. 



In a great many places, the Calvert is separated from the Eoceni:; 

 by a siliceous sandstone 6 to 12 inches in thickness. This ledge itself 

 has been found to contain Miocene fossils and in places it is separated 

 from the Eocene greensand by as much as a foot of loose sand. In 

 Prince George's County this ledge is exposed in many places in the 

 southeastern portion of the County, and is every^vhere distinguish- 

 able from any other material present in the Miocene or Eocene. 

 One of the best exposures of it is about Sl/o miles northwest of Not- 

 tingham along Mataponi Creek, where the following section has been 

 taken. 



Section along Mataponi Creek. 

 Calvert: Ft. In. 



Diatomaceous earth, gray in color, mixed with con- 

 siderable sand in the lower portion but passing 

 upward into a very pure variety 20 



Siliceous ledge with sand grains visible on roughened 

 weathered surface but indistinguishable in interior; 

 vitreous cleavage; outside gray, interior black 6 



Iron-stained sand with a considerable admixture of 

 clay, arranged in thin layers suggesting shales 10 



Nan JEMOY : 



Coarse glauconitic sand, dark at base becoming lighter 

 in color and more micaceous near Miocene contact. 

 Exposed 6 



27 4 



Paleontologic Character. — The diatomaceous earth and the dark- 

 colored clays represented in the Calvert of this county contain abun- 

 dant casts of marine mollusks, almost invariably of small size. These 

 beds also contain leaf remains. 



Stril'c, Dip, and Thichness. — The strike of the Calvert formation 

 is from northeast to southwest, and it dips toward the southeast at 

 the rate of about 11 feet to the mile. Thei full thickness of the 

 Calvert formation has been nowhere actually observed along the line 

 of outcrop. The formation has been diagonally truncated by the 

 Choptank, so that in this region it shows a maximum thickness of 

 only about 100 feet. The Choptank and younger formations lie 



