208 
ARTESIAN WELLS ON THE ATLANTIC COAST. 
[bull. 138. 
Wateree Eiver. From a study of this problem in the field, I find that 
the so-called basal members of the Eocene are not Eocene at all, but 
are representatives of the Potomac formation, and they extend north- 
ward beneath the marine Cretaceous formations (marls), the edge of 
which emerges from beneath the Eocene north of the Wateree. Thus 
it was found that, as suggested by Mr. McGee, there is a continuous 
sheet of Potomac formation lying on the crystalline rocks throughout 
South Carolina, which is overlain by Eocene to the south and marine 
Cretaceous to the north. The Potomac formation extends out under 
the Coastal Plain to the eastward with gradual increase in depth, and 
is probably the source of the water in the Charleston, Orangeburg, 
Darlington, and some other deep wells. The following are the principal 
Coastal Plain formations in South Carolina : 
Formations. 
Characteristics. 
Gray sand, etc. 
Orange loams. 
Sands and marls. 
Buhrstono below, marls above. 
Marls and sands. 
Sands, sandstone, and clay. 
Miocene 
The general structural relations of these formations are shown in the 
sections on PL XVIII. 
POTOMAC FORMATION. 
This formation consists of a series of sands, sandstones, and clays, 
which lies on the crystalline or bed rock to a thickness of several hun- 
dred feet. It outcrops in a belt 4 or 5 miles in width, which extends 
from Augusta, Ga., through Aiken, south of Lexington, and through 
Columbia to Camden and Cheraw. The basal beds are mainly coarse 
sands with pebbles, which to the southward are sometimes consolidated 
to a soft sandrock. Finer sands clays and occur higher up in the forma- 
tion, and these are overlain by the Eocene buhrstone to the southward 
and by the marine Cretaceous marls to the north of the Wateree. The 
formation is, however, very irregular in character, and clays. occur at 
low horizons at some points, while near Congaree Creek, south of Lex- 
ington, I observed the Eocene beds lying on cross-bedded sandstones 
which merged into a kaolinic arkose on the one hand and into white 
clays on the other. On the Pedee River I found coarse sands with 
intercalated beds of gray sandy clays which are overlain un conform ably 
by the Cretaceous marls. The sands and sandstones in the bottom of 
the Charleston wells are thought to be the upper portion of the Potomac 
beds, but the identification is not established. The upper beds of the 
Potomac formation may finally merge into marine deposits as far to 
the eastward as Charleston. 
