darton.] MARYLAND. 125 
Formation. Characteristics. Age. 
. 6 r 
o fl i Paruunkey. Glauconitic marls and sands. Eocene. 
Severn. Black argillaceous, carbonaceous sands. Cretaceous 
Magotliy. White sands and brown sandstones. Cretaceous. 
Potomac. Clay, sands, and gravels. Early Cretaceous. 
THE POTOMAC FORMATION. 
This, the great basal member of the Coastal Plain series, lies clirectlv 
on the floor of crystalline rocks. In Maryland it consists mainly of 
clay with interbedded fine sands, but it also contains beds of water- 
bearing coarse sands and gravels, especially toward its base and lying 
on the crystalline rocks. Some features and relations of these basal 
beds are shown in PI. IX. The thickness of the formation is about 600 
feet east of Washington and of Baltimore. Its thickness and compo- 
nents out under the later Cretaceous and Tertiary deposits are not 
known, for it does not appear to have been pierced by any of the east- 
ern wells. The gravels and sands of the Potomac formation, notably 
those lying on or near the surface of the crystalline rocks, contain a 
large supply of water, and a number of wells in and near Baltimore 
and Washington draw from this horizon. I have great confidence in 
the extension of this water stratum eastward, but it has been explored 
by wells in only a relatively narrow belt. 
MAGOTHY FORMATION. 
This is a thin series of coarse white sands with sandstone streaks 
which overlie the Potomac formation in eastern Maryland. It is a 
water bearer. In PI. X are exhibited its relations to the Severn for- 
mation. 
SEVERN FORMATION. 
This overlies the Magothy formation in Maryland. It is the southern 
extension of the middle and lower beds of the great greensand marl 
series of New Jersey, and terminates at the surface near Washington, 
D. C. It consists of dark, carbonaceous, sandy clay below, merging 
into fine gray and brown sands above. The thickness in Maryland is 
150 feet on the eastern shore, 80 feet near Annapolis, and from 3 to 8 feet 
east of Washington. The formation does not appear to be a water 
bearer in Maryland. 
PAMUNKEY FORMATION. 
This overlies the Severn deposits in Maryland. It outcrops in a con- 
siderable area southeast of Baltimore and east of Washington. It 
consists mainly of glauconitic sands and marls, which are in greater 
part fine grained. It usually contains less water than the Chesapeake 
sands, but it furnishes a moderate supply to certain wells. The forma- 
tion, which attains a thickness of about 150 feet in surface outcrops, 
appears to thicken gradually to the east and south, but its precise thick- 
ness in those directions is not known. Some of its relations are shown 
in the two lower sections of PI. VII. 
