GROUND WATER IN NORTH-CENTRAL TENNESSEE

90 GROUND WATER IN NORTH-CENTRAL TENNESSEE
not primarily the result of fracturing. The distinctions between these
three classes are wholly arbitrary, and all classes grade into one
another.
SEEPAGE SPRINGS
Most of the springs in the region are seepage springs, the common
type being the contact spring, which issues from permeable material
just above the outcrop of some relatively impermeable material. In
springs of this type the impermeable material retards or prevents the
downward percolation of ground water and consequently deflects it to
the surface. These conditions are satisfied in four general cases, each
of which is associated with a characteristic type of spring performance.
First, the water-bearing material may be a rock, such as sandstone,
which is permeable in its unweathered state, and the underlying re­
taining bed a stratum of shale or other impermeable rock. The storage
capacity of such a permeable bed is relatively large, so that the dis­
charge of the spring is not likely to be highly variable, even though
it may not be large. Second, the water-bearing material may be a
stratum that has been rendered permeable by weathering, and the
underlying retaining bed a material that is not affected by weathering
to an appreciable degree. Third, the permeable material may be the
weathered portion of a massive stratum, and the retaining bed the
underlying fresh rock. In these two cases the volume of permeable
material that supplies each spring niLy be small, so that both the
storage capacity for ground water and the discharge of the spring
during the dry season may also be small. Finally, the water-bearing
material may be transported detritus, and the retaining bed the under­
lying solid rock. The storage capacity and permeability of the de­
tritus vary between wide limits, although under favorable conditions
both may be large; hence the discharge of such a spring may be large
and relatively invariable. The optimum condition favoring contact
springs exists in a terrane of steep slopes, and hence such springs are
especially abundant along the Highland Rim escarpment, which
bounds the Nashville Basin. They are somewhat less numerous on
the valley slopes throughout the region and are not common on the
Highland Rim plateau or the Nashville Basin peneplain.
Many contact springs issue from minute joints and bedding-plane
crevices in the uppermost part of the Chattanooga shale along the
Highland Rim escarpment, although their water is probably derived in
part from the weathered zone of the overlying limestone. Typical
examples, which are entered in the table of spring data, are Nos. 273
and 278 of Davidson County (pp. 138-139) and Nos. 358 and 395 of
Williamson County (pp. 218-219). Each of these springs has a small
area of influence and issues from material of low permeability, and
hence the discharge is generally very small. Other contact springs
issue from beds of permeable sandstone near the top of the Fort Payne