GROUND WATER IN NORTH-CENTRAL TENNESSEE

164 GROUND WATER IN NORTH-CENTRAL TENNESSEE
above sea level. The south hall of the county is traversed by the
somewhat meandering youthful valley of the Cumberland River, which
at Clarksville is about 350 feet above sea level. Hence the total re­
lief within the county is about 350 feet.
The massive crystalline beds of the St. Louis limestone and Warsaw
formation, of Mississippian age, cover the greater part of the county,
although there are few visible exposures of these rocks except in the
youthful stream valleys. In the interstream areas bedrock is mantled
by as much as 125 feet of clay and soil, which are residual from the
chemical weathering of the limestone. Montgomery County is un­
like areas in which the cherty Fort Payne formation underlies the
surface in that this residual clay contains comparatively few chert
fragments. The Fort Payne formation, which crops out in the val­
leys of the Cumberland River and of Barton and Little Barton Creeks,
in the southeast corner of the county, consists of thin beds of cherty
and earthy limestone. The geologic map (pi. 4) shows the outcrop
areas of these formations; the discussion of stratigraphy (pp. 33-37)
treats of their lithologic character.
GROUND-WATER CONDITIONS
Most of the residents of the upland areas rely upon cisterns as a
source of water, inasmuch as the residuum from the weathering of the
limestone is generally not water bearing and dug wells are generally
unsuccessful. Water for stock is generally impounded in natural or
artificial ponds. Comparatively few tubular wells have been drilled,
although they generally obtain sufficient water for domestic purposes.
Most of the drilled wells are between 100 and 160 feet deep, and all
derive their water from channeled zones in the limestone. (See pp.
69-89.) A few wells, such as Nos. 49, 50, and 55 (pp. 166-167), have
been drilled more than 200 feet deep in search of water, and several
relatively deep holes have been drilled in search of oil or gas. So far as
is known, however, the deepest water-bearing stratum disclosed in any
of the upland tracts by these wells is 187 feet below the surface, If the
experience of drilling in adjacent parts of the Highland Rim plateau
is a sound basis for judgment, it seems likely that potable water will
not be found more than 200 feet below the upland areas of Montgom­
ery County. In the more deeply dissected parts of the county ground-
water conditions are relatively erratic, although many wells from 50
to 100 feet deep yield sufficient water for domestic supply. Several
wells in and near Clarksville, such as No. 52, are reported to find
water about 300 feet below the surface in a sandstone or sandy
limestone that occurs approximately at the horizon of the sandstone
member of the Pegram limestone (pp. 41-43). However, there is no
sound basis for making a definite correlation. This stratum is not
water bearing in some wells that reach its horizon, as in well No. 55,