DAETON.] VIRGINIA. 163 
clay of various sizes and degrees of purity. The sandstones are a 
local feature. The basal beds are mainly coarse, cross-bedded sands, 
with scattered pebbles and bowlders and local pebble and bowlder 
beds, which are filled with water. How far east these coarse materials 
extend is not known, for the deepest wells do not appear to have 
reached them. The thickness of the formation in the outcrops in the 
Fredericksburg region and northward is not over 350 feet. The basal 
Potomac beds lie mainly on an irregular floor of crystalline rocks con- 
sisting of pranites, gneiss, and slate. In the vicinity of Bothwell they 
lie on red and gray sandstones and shales of the Newark formation, 
which there occur in a local area among the crystalline rocks. 
PAMUNKEY FORMATION. 
This member underlies nearly all of the Coastal Plain region south 
of Stafford County, but it is exposed only in the valleys where the 
Lafayette and Chesapeake formations are eroded. Its upper surface 
passes beneath tide-level along a line extending nearly due south from 
the eastern boundary of King George County to a point on James Eiver 
about 7 miles below City Point. South of the James it does not appear 
to be exposed unless possibly in the Nottoway Valley. The Chesa- 
peake-Pamunkey contact dips almost due east 5° south at the rate of 
from 10 to 12 feet per mile. 
3 .e n ^terials of the formation are marls or dark sand and sandy 
cla s containing a large amount of a greensand" or glauconite, and fos- 
sil onells. Some thin beds of purer clays and sands occur, and also 
local thin beds of very hard rock. The thickness of the formation rap- 
idly increases from a thin edge westward to about 200 feet eastward. 
The formation lies in an irregular, steeply east- dipping floor of Potomac 
formation, and for a long distance eastward the basal beds are mainly 
coarse sands with gravel and bowlders, containing a large amount of 
water. 
CHESAPEAKE FORMATION. 
H s formation underlies all the Coastal Plain area south of Fred- 
ericksburg except when it has been cut through for a greater or less 
distance by the Rappahannock, Mattapony, Pamunkey, James, and 
(possibly) Nottoway rivers. It consists mainly of clay, fullers' earth, 
shell marl, and very fine sand. It lies on the east- sloping floor of 
the Pamunkey formation and thickens rapidly to the eastward under 
its thin mantle of Lafayette orange sands. 
The deeper wells along the bay shore near the mouth of the Eappa- 
hannock penetrate a thickness of 400 feet of the formation, and in 
the well at Fort Monroe considerably more was found. Along the 
western margin of the region the formation thins, as is shown in sec- 
tions 1, 2, and 4 on PI. XVI. There are many thin local beds of coarser 
san- s among the clays and fine sands, and at the base of the formation 
