GROUND WATER IN NORTH-CENTRAL TENNESSEE

154 GROUND WATER IN NORTH-CENTRAL TENNESSEE
at the "whirl" on the Buffalo Biver and probably also at Hurricane
Eock Spring, on the Duck Eiver (No. 181, pp. 161-162). It is likely
that this formation also crops out at other places along the lower
reaches of the Duck and Buffalo Kivers, but the limits of its outcrop
have not been traced in detail. Some of the limestone that underlies
the Chattanooga shale in this same area resembles beds of Silurian
age in Davidson County, but its exact position in the stratigraphic
section is not known. These formations are described on pages 33-45;
their areal distribution is shown on Plate 4.
The youngest unconsolidated rocks of the county are the clay, sand,
and gravel that form the present flood plains of the major streams
plains which are nearly continuous and locally extensive along the
Tennessee River and the lower reaches of its tributary, the Duck
Eiver. Similar stream deposits also form extensive terraces above the
flood plains on the east bank of the Tennessee Eiver and locally along
the Duck and Buffalo Eivers on the convex banks of meanders. The
oldest of the unconsolidated rocks is the Tuscaloosa formation, an
earthy gravel deposit of coastal-plain origin, of which an erosion
remnant covers a relatively extensive area along the divide between
the Cumberland and Tennessee Eivers in the eastern part of the
county. These unconsolidated deposits are described on pages 30-33,
and the more extensive areas covered by them are shown on Plate 4.
GROUND-WATER CONDITIONS
In Humphreys County, as in other parts of north-central Tennessee,
the water-bearing properties of any particular limestone bed are
not related to its stratigraphic position alone but rather are dependent
upon its solubility, the number and persistence of joints and the
position of the bed with relation to present and past equilibrium pro­
files of solution channeling (pp. 78-82). Hence, the ground-water
conditions in any stratum are likely to vary greatly from place to place
but may be relatively uniform in any one physiographic district.
None of the field relations suggest that there exists in Humphreys
County any large body of limestone that has been depressed with rela­
tion to the water table after it had been rendered permeable by
channeling. Hence the principal channeled zones generally do not
contain water under hydrostatic pressure, and their water-yielding
capacity is correspondingly limited.
In the present stage of economic development in Humphreys
County most of the water used for domestic purposes is derived from
dug wells or springs. On the high interstream tracts the wells range
in depth from 12 to 65 feet and derive their supply from cherty zones
in the residual clay and soil that overlie the limestone or from beds of
sand and gravel in the Tuscaloosa formation. In some places an
adequate supply can not be obtained without digging to great depth,