MARYLAND GEOLOGICAL SURVEY IxXV 



at intervals from the cliffs below Lower Marlboro southward to 

 Ben Creek, in Calvert county. On the west bank of the river they may 

 be occasionally seen from a point opposite Lower Marlboro down the 

 stream to 1^ miles below Forest Wharf. 



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 cliffs at the mouth of the Chaptico Bay and on 

 the Virginia side a considerable thickness of the marls is exposed the 

 entire length of the Nomini Cliffs. 



When fresh, the Plum Point marls and the Fairhaven diatomaceous 

 earth do not differ much in appearance from each other. The thickness 

 of the Plum Point marls increases constantly down the dip and it is 

 probable that the greater portion of the 310 feet of the Crisfield well 

 section, w^hich has been assigned to the Calvert formation, is to be 

 referred to this member. 



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

 has been found possible to subdivide the 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 sand}^ 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 



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

 1893, pp. 21-31. 



