WATER RESOURCES. 343

lowing their more porous layers, and as rapidly as their porosity per-
mitted would be drained away. Under the conditions that exist, how-
ever, the mass of the Coastal Plain materials remains constantly sat-
urated with relatively motionless water to within a relatively short
distance of the surface, the exact distance depending upon local condi-
tions. The upper limit of the saturated portion is known as the water
table.

In the geologic section to which reference was made above it will be
observed that a great mass of materials underlying the region adjacent
to the coast are of marine origin. When these were deposited they
must have been saturated with sea water to the extent permitted by
their porosity. It is believed that these beds have never been above
sea level, and that the salty waters originally imprisoned in them have
never had opportunity to escape from their confinement. An attempted
representation of the manner in which this imprisonment has been
effected is given graphically in Fig. 17, p. 344. Either the pressure of
the sea water, whose specific gravity is greater than that of fresh water
(1.027), upon the submerged beds of the Coastal Plain, or the imper-
vious character of the strata, or both together, assisted by the retard-
ing effect of friction, have been sufficient to overcome the hydrostatic
pressure from the landward direction. In places at least it is believed
that not only has the pressure of the ocean water been sufficient to pre-
vent movement in the direction of the ocean, but that it has actually
foreed the salty sea water backward into materials which have origi-
nated in less brackish or nonbrackish water. The dotted portion of
the line in the geologic section, which indicates the upper limit of the
salty, noncirculating waters, has been drawn hypothetically, for the
reason that no well data are obtainable which indicate its exact posi-
tion in this portion of the Coastal Plain. The lack of this information
is due to the fortunate circumstance that where the data are wanting
an abundant supply of good water can be obtained in almost all cases

at shallower depths than the postulated upper limit of salty waters.
This great body of noncirculating, salty water serves as an effectual

barrier to the passage downward along the inclined beds beyond the

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limit indicated (see Fig. 17, p. 344) of the waters entering them from the
’ surface. It is believed that above this limit the rs, having been
prevented from passing farther down the beds, are able, on account of

   
 

 

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the pressure from the accumulated mass of water behind them, to pass

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very slowly oceanward in an approximately — yrizontal direction
through not only the porous sand beds, but likewise the less pervious,

compact sand and clay beds, and to enter and mieie with the ocean

 

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