GROUND WATER IN NORTH-CENTRAL TENNESSEE

200 GROUND WATER Mr NOBT&K5ENTRAL TENNESSEE
older rocks crowing out successively toward the south. The
rocks of the county, which crop out along the Cumberland River,
may belong to the uppermost |>art of the Stones River group, of
Lower OrdovMan age, although the stratigraphy of the Ordovician
rocks in that part of the county has not been traced. Thte general
character and stratigra^hic relations of the consolidated rocks tha't
are exposed in Suinner County are discussed on plages -33-^-58, an4 their
areal distribution is shown on Plate 4.
GROUND-WATER CONDITIONS
Of the rocks that lie within reach of the drill hi Sumner County,
only the sandy beds associated with the Pegram, Lobelvile, and
Laurel limestones are likely to be permeable over extensive areas.
These beds, which lie just below the Chattanooga shale, crop out
along the Highland Rim escarpment and are potential water bearers
only on the plateau farther north. The few wells that reach feheir
horizons, however, either find the beds to be impermeable or to yield
highly concentrated salt water with hydrogen sulphide. The over­
lying Chattanooga shale is also reported to contain salt watet at
many places, as in wells 117,118, and 120 (pi. 4 and pp. 204-205). In the
other rocks, all of which are limestones, the only water-bearing "open­
ings are bedding planes, joints, and solution channels, whose number
and transmission capacity are not at all uniform within any one stra­
tum but are rather related to physiographic environment (pp. 78-82).
On the Highland Rim plateau the residual clay that overlies the
St. Louis limestone in the northwestern part of the county contains
very little coarse material and in general is not a source 0f ground
water. Farther east, however, where the Fort Payne formation con­
stitutes the bedrock, the residual material cont/aias many chert frag­
ments, and some of it yields sufficient water to dug weUs and a few
drilled wells for household supplies and fot stock. The most per­
meable zone is usually just above the bedrock. Some of the 'shallower
dug Wells are inadequate in periods of extreme drought, so that Water
is drawn from cisterns. Comparatively few wells haVe been dflUed
into the limestone that underlies the residual material, alt&ougli
some of them find sufficient water for domestic purpose's between 50
and 100 feet below the surface. Others, however, are inadequate Jot
household use, and a fefr have failed to find water-bearing t»ed&.
The ground water from the residual clay and from the uppermost
beds of the underlying limestone is generally satisfactory Ifof any
ordinary purpose, containing a moderate amount of dissolved mineral
matter and being only moderately hard. Analyses 115 and 1-21 (pp.
112-113) are representative. However, someof the grottttd waiter tkat
occurs in the limestone less than i.00 feet fcelow the surface aftd mttck
of that at greater depth is high in n'0ncarbo»a*fc hardness *»M ra