162 
ARTESIAN WELLS ON THE ATLANTIC COAST. 
[BULL. 138. 
VIRGINIA. 
GEOLOGIC RELATIONS. 
The Coastal Plain region of Virginia lies east of a line which passes 
through Alexandria, Fredericksburg, Bothwell, Richmond, Petersburg, 
and Emporia, comprising an area of about 9,500 square miles. This 
district is traversed from northwest to southeast by a series of great 
rivers, which divide it into a succession of long ridges. These are 
remnants of a plateau surface which slopes from an altitude of from 
300 to 200 feet on the west to about 100 feet on the east. The larger 
rivers are tidal estuaries occupying low valleys and meandering through 
wide terrace plains. Chesapeake Bay is widely bordered by these 
terraces, and the eastern shore of Virginia, which lies east of the bay, 
is a low plain, as is also the wide area which lies between Dismal 
Swamp and the Atlantic Ocean. 
The geologic structure is in general similar to that of eastern 
Maryland, but there are certain differences in the stratigraphy. The 
Potomac formation becomes predominantly sandy, and the Severn 
and Magothy sands are absent in the outcrop zone, so that the Pamun- 
key marls lie directly on the surface of the Potomac beds. Par to 
the eastward, under the bay and Eastern Shore, there is probably 
presented the same sequence that is found in Maryland. The Chesa- 
peake formation extends farther to the west in southern Virginia and 
often overlaps on the crystalline rocks. In the sections, PI. XVI, 
there are shown the general structural relations in Virginia so far as 
they are known. The Coastal Plain formations which outcrop in the 
State are as follows : 
Formation. 
Characteristics. 
Age. 
Chesapeake 
Early Cretaceous. 
POTOMAC FORMATION. 
This formation underlies the other Coastal Plain formations through- 
out, and is exposed in certain areas where those formations have been 
eroded. These areas are short narrow belts in the Nottaway, Appo- 
mattox, James, Pamunkey, and Rappahannock depressions near the 
western border of the Coastal Plain region, and a district of consider- 
able width extending from Fredericksburg to the Potomac River and 
thence on both sides of the river to Washington. The formation con- 
sists of sand, coarse below and in part finer above, with many beds of 
