GROUND WATER IN NORTH-CENTRAL TENNESSEE

74 GROUND WATER IN NORTH-CENTRAL TENNESSEE
circulates slowly and before it moves a great distance becomes satu­
rated in calcium bicarbonate and thereby depleted in solvent power, as
its natural acid is neutralized by reaction with the limestone. Conse­
quently, it is commonly held, as by Swinnerton, 46 that below the water
table the limestone does not dissolve readily and continuous systems
of large solution passages do not form. This hypothesis seems to be
compatible with the relation between the principal systems of solution
channels and the surface streams (pp. 23-24) in north-central
Tennessee. On the other hand, Davis 46 contends that limestone
caverns are formed in part by solution and in part by corrasion and
that they may be formed at any depth below the water table.
Where a stratum of limestone has primary permeability or is
thoroughly jointed and is overlain and underlain by impermeable
rocks, ground water may circulate along that stratum under hydro­
static pressure, as through a conduit. Under such conditions, if the
geologic structure is favorable, circulation may take place to con­
siderable depths below the water table. If the circulation is relatively
rapid, so that ground water passes entirely through the limestone
conduit before it becomes saturated, then the original openings in the
limestone may be enlarged by solution from one end of the conduit to
the other, regardless of its depth below the water table. Such con­
ditions are not known to have existed in north-central Tennessee.
Systems of solution passages may be formed in limestone during
successive erosion epochs and then buried beneath younger sediments
after submergence.
In some regions the limestone formations can be discriminated more
or less sharply by the abundance and size of the solution passages and
other water-bearing openings. This is true to some extent in parts of
north-central Tennessee, but generally the effect of differential sol­
ubility is a secondary factor in determining the water-bearing proper­
ties of the limestone of this region.
SOURCES AND CmCTJLATION OF GROUND WATER
In general the ground water that occurs in the limestone of north-
central Tennessee is derived from two sources meteoric water, or
that which falls as rain and percolates to the water table, and connate
or fossil water, or that which was trapped in the sediments by the
deposition of overlying beds and has not since been flushed from the
rocks. The waters of meteoric origin are those which circulate freely
through joints and solution openings and are discharged from most
springs and wells. They generally contain only small or moderate
amounts of dissolved mineral matter and are suitable for most ordi-
48 Swinnerton, A. C., Changes in base-level indicated by caves in Kentucky and Bermuda [abstract]:
Geol. Soc. America Bull., vol. 40, p. 194,1929.
« Davis, W. M., Origin of limestone caverns: Geol. Soc. America Bull., vol. 41, No. 3, pp. 475-628,
1930; The origin of limestone caverns: Science, new ser., vol. 73, No. 1891, pp. 329-330,1931.