84 THE GEOLOGY OF CALVERT COUNTY 



to Drum Point. Other exposures are found on both banks of the. 

 Patuxent River and along St. Johns Creek and ]\Iill Creek in St. Mary's 

 County. 



Strike, Dip and Thichtiess. 



The strike of the St. Mary's formation, like that of the two preceding 

 ones, is from northeast to southwest. On the Western Shore, because 

 of the great diversity in the topography, the outcrop is extremely irregu- 

 lar and departs very widely from the direction of the strike. The 

 St. Mary's formation rests conformably on the underlying Choptank 

 and is overlain unconformably by younger materials. The dip averages 

 about 10 feet to the mile toward the southeast. 



The thickness of the St. Mary's formation varies from a few to about 

 280 feet. In the hilltops south of Prince Frederick, where the dip 

 carries the formation up to an elevation of 100 feet or more, the 

 thickness thins down gradually to extinction; while in the well boring 

 at Crisfield it occupies a thickness of about 280 feet, although it is pos- 

 sible that the upper portion of this may be Pliocene. The average 

 thickness of the St. Mary's formation in Calvert County appears to be 

 about 50 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 quantities 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. 



Stratigraphic Relations. 



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

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

 belonging to various members of the Columbia Group. 



