GROUND WATER IN NORTH-CENTRAL TENNESSEE

OCCURRENCE OF GROUND WATER IN LIMESTONE 83
circulation downward. Consequently, a perched body of ground
water and a local equilibrium surface of solution may be created,
and the stratum just above the barrier may become extensively
channeled by solution. This condition exists in north-central Tennes­
see, where the impermeable Chattanooga shale (pp. 39-41) underlies
the Mississippian limestones. Much of the ground water that is
reported to occur in the uppermost part of the shale may circulate
in small solution openings that follow its upper contact. Also beds
of impermeable shale in some of the thin-bedded and shaly limeston®
formations may uphold bodies of ground water and induce channeling
considerably above the regional equilibrium profile of solution. A
local equilibrium profile of solution, if created by a bed that is slowly
soluble, may be only temporary. Once the restraining bed is breached,
further extensive channeling may take place at a lower equilibrium
profile, either local or regional, and the lower system of channels
may drain the upper system through natural wells.
Sandstone strata may accelerate the cycle of solution channeling
in limestones with which they are interbedded by facilitating deep
percolation of meteoric water.
During an erosion interval that follows an epoch of limestone
formation a system of solution openings may be formed, which may
not be filled during the subsequent epoch of sedimentation. Hence
an unconformity may be accompanied by openings of this sort, which
may be filled with fossil water of the date of submergence or may
serve as conduits for ground water of meteoric origin if the uncon­
formity crops out at the present time. In several places in north-
central Tennessee the Chattanooga shale, which rests unconforma-
bly upon limestones ranging in age from Middle Devonian to Ordo-
vician, fills unmistakable sink holes in the underlying surface, as has
been noted by Bassler 55 and Lusk.66 However, no water-bearing
openings associated with this erosion surface were noted. Some of
the highly concentrated connate waters (pp. 120-123) that occur in
the Ordovician limestones of this region may be trapped in solution
openings associated with unconformities.
Relation to structure. All features of geologic structure, both folds
and faults, may affect the ground-water conditions in limestone
profoundly. Thick-bedded pure limestones are rather brittle, so
that, where folded, they are generally broken by closely spaced
joints, which constitute water-bearing openings. Some of the
thinner-bedded formations in the folded areas and all the rocks in
the unfolded areas may be much less jointed, so that in them the
ground water circulates much less freely in the early stages of the
subsurface cycle. An impermeable or slightly soluble bed folded in
« Bassler, R. S., Sink-hole structure in central Tennessee [abstract]: Washington Acad. Sci. Jour., vol. 14,
p. 374, 1924.
M Lusk, R. G., A pre-Chattanooga sink hole: Science, new ser., vol. 65, pp. 579-580,1927.