72 THE GEOLOGY OF ST. JIARY's COUNTY 



greenish-brown and Iniff. Fossil remains, although abundant, through 

 the entire member, are usually found massed in beds. In Calvert 

 County there are two of these, from 30 to 35 feet apart, and with a 

 thickness Tarying from 4^ to 13 feet. In St. Mary's County, however, 

 these fossil beds are not conspicuous, but the Plum Point marls, in 

 which they are imbedded, may be occasionally seen in the area of the 

 Calvert formation, as explained above. On the Potomac River, the 

 banks are usually very low and composed of Columbia sand and gravel. 

 In consequence of this, the Plum Point marls are seldom met with. 

 On the Maryland side of the river they may be seen in the low eliifs 

 at the mouth of Choptank Bay, and on the Virginia side a considerable 

 thickness of the marls is exposed the entire length of the Nomini Cliffs. 



The actual thickness of the Plum Point marls within St. Mary's County 

 is nowhere open to observation and cannot be accurately determined, 

 but it is probable that at least 150 feet of the average thickness of tho 

 Calvert formation should be assigned to this sub-division. 



From a detailed study of exposures along the Calvert Cliffs, in Calvert 

 County, it has been found possible to sub-divide the Plum Point marls 

 into twelve zones. In St. Mary's Coimty it is certain that only the 

 higher zones are present, but the lack of exposures has made it impossible 

 to determine which or how many of these occur in this county. The 

 fact that the Choptank formation lies unconformably above the Calvert 

 introduces another element of uncertainty, for some of the higher zones 

 found in Calvert County may be absent in this region or the Plum 

 Point marls, in St. Mary's County, may contain higher members than 

 are to be found in the same division in Calvert County. 



THE CHOPTANK FORMATION. 



The Choptank Eiver has suggested the name for this formation be- 

 cause of its great development on the northern bank of that estuary a 

 short distance below Dover Bridge in Talbot County. In this locality 

 the Choptank formation is very fossiliferous, and may be seen at the 

 base of a low cliff which borders the stream for some distance. 



