GROUND WATER IN NORTH-CENTRAL TENNESSEE

70 GROUND WATER IN NORTH-CENTRAL TENNESSEE
Secondary porosity in limestone is due to (1) fractures, including
(a) joints caused by contraction of the sediment during consolidation,
(6) joints and faults resulting from crustal movement, (c) joints
due to mineralogic changes; (2) solution openings related to present
or former erosion surfaces; and (3) intercrystalline voids produced by
mineralogic change. This classification is essentially that given by
Howard. Secondary pore space in limestone is in large part con­
tinuous and therefore renders the rock permeable. That caused by
joints and solution openings is by far the most common and most
efficacious in imparting large water-yielding capacity to the rock.
Joints are fractures along which there has not been appreciable
displacement of the rocks. They are produced by internal stresses
such as those induced by shrinkage or by external stresses that
^accompany deformation of the earth's crust. Commonly they are
nearly plane and approximately vertical (dip 75°-90°); some extend
for long distances and to considerable depth, whereas others are
very short in both horizontal and vertical extent. In sedimentary
rocks such as limestone flat or nearly horizontal joints generally do
not form unless the rock is very thick bedded, the stresses that tend
to form such joints in crystalline rocks being dissipated by sliding
of one bed upon another. Furthermore, the vertical joints may extend
across one stratum or several strata. At any particular locality the
joints generally occur in one or more sets, each set comprising several
or many fractures that are approximately parallel; these sets of
joints intersect at various angles and divide the rock in rhomboidal
Mocks. Many joints are tight and not water bearing, but the walls
of others stand apart so that they constitute ground-water conduits
with large transmission capacity. In some places joints are close
together and in others far apart, their position depending upon dif­
ferences in the competence of the strata to resist fracture and upon
differences in the intensity of external stresses causing crustal deforma­
tion. In some regions where there has been intense deformation the
rocks have been displaced along the fractures, producing faults and
breccia zones that extend far below the surface and may yield unusual
amounts of water. However, faults are not common in north-central
Tennessee except in the Wells Creek Basin (pp. 65-67). Not only
do joints generally become tighter with depth but they also become
farther apart, so that the chances of striking a water-bearing opening
of this sort in drilling a well become less as the depth increases.
The intersections of joints with one another are especially likely to
be open and to permit circulation of ground water. Although any
joint may intersect others, the circulation is easiest where the joints
of the principal sets intersect and where, in addition to the vertical
joints, horizontal fractures or open bedding planes occur.