darton.] MARYLAND DISTRICT OF COLUMBIA. 155 
portion. No prediction can be made as to the extent of these waters 
or the possible southward extension of higher Chesapeake waters on 
account of entire lack of definite data. There is a fair chance that 
the Crisfield waters underlie, the county at depths near 1,200 feet, but 
this can be surely determined only by a boring. 
DISTRICT OF COLUMBIA. 
GEOLOGIC RELATIONS. 
The District of Columbia extends across the zone in which the crys- 
talline rocks emerge from beneath the Coastal Plain deposits and rise 
into the Piedmont Plateau to the west. The contact line crosses the 
Potomac River at Washington, passes through the western portion of 
the city, and to the northward extends along the east side of Rock 
Creek valley. The formation that lies on the crystalline rocks is the 
Potomac, which consists mainly of water-bearing sands and gravels 
below and of clay and fine sands above. Some features and relations 
of the basal beds are shown in Pis. XI and XII. The formation has 
in all a thickness of about 700 feet, and outcrops in a belt from 7 to 8 
miles wide, which extends to the eastward high up the slopes on the 
east side of Anacostia River. In these slopes it is surmounted by 
younger formations, consisting of a succession of the thin western edges 
of the dark sandy clays of the Severn formation, the impure marl of 
the Pamuukey, the gray clays of the Chesapeake, and the gravels 
of the Lafayette. Washington is situated on a series of broad, low 
terraces cut in the Potomac sands and clays and across the edge of 
this formation on the crystalline rocks. 
This series of terraces, and its extension along all the lower land of 
the District, is capped by from 20 to 35 feet of gravelly sands and 
loams of the Columbia formations. In the three sections in PI. XIV a 
fairly clear representation is given of the structure of the Washington 
region, and in the map, PI. XIII, is shown the relative distribution of 
the crystalline rocks and the Potomac formation. It is in Good Hope 
Hill that the Potomac formation passes under the Severn, Pamunkey, 
Chesapeake, and Lafayette formations, as shown in section 2, but there 
are also small outlying areas of the Chesapeake and Lafayette forma- 
tions on the high lands of Soldiers 7 Home Park and the ridge which 
extends to Tenley. 
The Potomac formation is the only member in the District that con- 
tains underground waters of any importance, for the water in the 
Columbia and Lafayette cappings are of local surface origin. 
