GROUND WATER IN NORTH-CENTRAL TENNESSEE

WILL1AMSON COUNTY 211
perennial springs, most of which are seepage springs issuing from the
weathered rocks. Such springs are especially numerous along the
outcrops of some beds of earthy and sandy limestone in the Fort
Payne formation, many of which are rendered permeable when the
calcareous matrix is leached out during weathering, although they may
be quite impermeable if unweathered. In fact, the outcrops of some
of the more sandy limestone beds resemble a coarse and somewhat
friable sandstone, such as that from which Stillhouse Spring (No. 366, pi.
4; also pp. 218-219) issues near the head of Dobbins Branch of Leipers
Fork, as well as springs 380 and 384, on other tributaries of that
stream. Although they are generally variable and some are inter­
mittent, the minimum annual discharge of the perennial seepage
springs ranges from a few gallons to about 30 gallons a minute. The
water from these leached outcrops is generally slightly or moderately
concentrated in calcium bicarbonate (analysis 366, pp. 116-117), and is
satisfactory for most uses. As elsewhere, many springs, such as Nos. 358
and 395, issue from the uppermost part of the Chattanooga shale along
its outcrop, although generally the minimum annual discharge is less
than 1 gallon a minute, and the water contains so much iron and hydro­
gen sulphide that it is unsatisfactory for many purposes. Analysis 395
is representative. None of these strata that are permeable in the out­
crop zone are known to be water-bearing where they are far below the
surface and unweathered. Other perennial springs in the dissected
tracts issue from jointed beds in the St. Louis and Warsaw limestones
or in the massive limestone facies of the Fort Payne formation.
Although some of these issue from small solution channels that follow
joints or bedding planes, all are rather variable and presumably derive
their water largely from the zone of weathering. The discharge from
any one spring is generally less than 25 gallons a minute, as in Nos.
379, 381, 382, 383, and 385. However, the aggregate discharge from
several springs of this sort may constitute a reliable supply of con­
siderable magnitude, such as the municipal supply of Franklin
(pp. 213-214). The massive crystalline limestones of the Silurian sys­
tem, which crop out locally just below the Chattanooga shale, seem to
be relatively much more soluble than the overlying Mississippian lime­
stones, so that under favorable conditions they may be rendered
cavernous by solution. Cold Spring (No. 359), which issues from a
solution channel in these rocks, is the largest known spring in the
west half of the county. Its water contains only a moderate quantity
of dissolved mineral matter and has moderate carbonate hardness, as
is shown by analysis 359 (pp. 116-117); hence it is wholly satisfactory
for most purposes, especially if softened.
In the east half of Williamson County both the quantity and the
chemical character of the ground water differ greatly from place to
place, especially in the hilly areas. In a few places the limestone is