76 THE GEOLOGY OF CALVERT COUNTY 



increases constantly down tlie dip and it is probable that the greater 

 portion of the 310 feet assigned to the Calvert formation in the Crisfield 

 well is to be referred to this member. The actual thickness of the Plum 

 Point marls within Calvert County is nowhere directly visible, but one 

 may gain a good idea of its development within the region by comparing 

 the various sections. 



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

 has been found possible to subdivide Plum Point marls into 12 zones. 

 They are characterized as follows: 



Zone 4. — At the base of the Plum Point marls and lying conformably 

 on Zone 3, the uppermost member of the Fairhaven diatomaceous earth 

 is a six-inch deposit of greenish sandy clay carrying Ostrea percrassa. 

 This zone first makes its appearance along the Calvert Cliffs at Chesa- 

 peake Beach and continues on down the shore for about 2^ miles, when 

 it can be no longer distinguished. Throughout this distance, the zone 

 does not dip toward the southeast in harmony with the other zones, 

 which are visible above it, but actually appears to rise slightly against 

 the dip until it finally vanishes at the point indicated. The erratic 

 behavior of this zone would seem to indicate a local migration and 

 temporary occupation of this particular area by Ostrea percrassa. This 



zone corresponds to " Zone a " of Harris.* 



Zone 5. — This zone is developed immediately above Zone 4 and at 



Chesapeake Beach has a thickness of 7 feet; as it is followed southward, 

 however, along the Calvert Cliffs, it is found to thin rapidly until at 

 a distance of about 2^ miles south of Chesapeake Beach it has a thick- 

 ness of only 2 feet and 6 inches. At this point the base actually lies 

 higher than at Chesapeake Beach, although on account of the thinning 

 the top lies lower. From this point southward it dips away in har- 

 mony with the dip of the other beds of the Calvert formation. The 

 materials making up this zone consist of a greenish sandy clay, which 

 carries scattered bands of Corhula elevata. 



Zone 6.- — This zone consists of a greenish sandy clay carrying large 



' Tertiary Geology of Calvert Cliffs, Maryland. Amer. Jour. Sci., vol. xlv. 

 1893, pp. 21-31. 



