MARYLAND GEOLOGICAL SURVEY 71 



in St. Mary's County. The formation appears to thicken rapidly down 

 the dip until, at Crisfield, in Somerset County, it shows a thickness 

 of 310 feet in an artesian well. As the base of the Calvert formation 

 is nowhere visible within St. Mary's County, the thickness of the forma- 

 tion must be determined, as was the dip, by studying regions beyond these 

 borders. From various calculations it appears that the average thickness 

 of the Calvert formation in this county is about 185 feet. 



Character of Materials. 

 The character of the materials of the Calvert formation are, as a 

 whole, quite uniform. They consist of clay, marl, and diatomaceous 

 earth. In St. Mary's County only the two former are typically developed. 

 The diatomaceous earth is not present as a well defined member. 

 Throughout the entire extent the formation is abundantly supplied with 

 fossils. These crumble readily on exposure to the atmosphere and are 

 seldom discernible except where active erosion constantly exposes fresh 

 surfaces. The clay and marl are dark brown to bluish-green when 

 fresh and change to various tints of buff on exposure to the weather. 



Stratigraphic Relations. 

 The Calvert formation in St. Mary's County is believed to lie, as 

 explained above, unconformably on the eroded edges of the Eocene. 

 This unconformity is in the nature of an overlap, but is nowhere visible 

 in this region. Above, the Calvert formation lies unconformably be- 

 neath the Choptank. 



Suh-Divisio7is. 

 Beyond the borders of this county the Calvert formation falls into two 

 divisions which are known as the Fairhaven diatomaceous earth and 

 the Plum Point marls. Only one of these sub-divisions, the Plum Point 

 marls, is found in this county. These marls consist of a series of sandy 

 clays and marls in which are imbedded large numbers of organic remains 

 including some diatoms. The color of the material is bluish-green to 



