GROUND WATER IN NORTH-CENTRAL TENNESSEE

OCCURRENCE OF GROUND WATER IN LIMESTONE 75
nary domestic or industrial uses. However, some ground water of
meteoric origin may be trapped by beds of impermeable shale so that
its circulation is impeded or prevented, and it may then become so
highly concentrated as to be unfit for most purposes. The connate
waters of north-central Tennessee generally occur in rocks at great
depth, do not circulate, and do not receive water from the surface.
In marine sediments such as the calcareous formations of this region
the connate waters invariably contain a very large amount of dissolved
mineral matter and are unfit for practically all uses. The chemical
character of these two types of water is discussed on pages 120-123.
In any region that is underlain by thick bodies of massive limes tone,
such as north-central Tennessee, many areas between the perennial
rivers and creeks are wholly devoid of surface streams although they
receive their proportionate amount of rainfall and obviously contribute
water to maintain the flow of the perennial streams. The run-off
from such areas is diverted into joints and solution channels of the
limestone by sinks, or swallow holes, each of which is a rude funnel
gathering water from the surface. However, not all the depressions
that have no surface outlet divert water info the channels of the lime­
stone, for the floors of many are doubtless tightly puddled by clay or
other impermeable debris. The sinks range in size from open joints
and other small crevices to large pits formed by solution or by the
collapse of the roof above an underground channel or by a natural
well that reaches the surface. The walls of a large sink may gradually
be cut back by solution, by corrasion, or by collapse, so that its drain­
age area increases correspondingly. Some depressions of this sort in
north-central Tennessee drain several square miles by means of inter­
mittent or perennial creeks that discharge into natural wells or other
solution openings at the bottoms of the pits.
The water that enters the joints and solution openings of the lime­
stone from the sinks percolates downward to the zone of saturation,
the upper boundary of which in some districts is about as sharply
defined in limestone as in other rocks. Under such conditions the
network of joints and solution openings, both great and small, is filled
with water up to a certain level, which is the water table. Only those
openings that exist below the water table generally yield perennial
supplies to wells. In some places, however, a well that passes through
limestone will not strike a water-bearing opening until it has been
drilled a considerable distance below the water table. When the well
strikes the water-bearing opening the water will generally rise in it
about to the level of the water table. In other limestone districts
extensive networks of solution passages exist above th& regional water
table and yet are wholly or partly filled with water. This water may
be in transit downward to the zone of saturation or it may be trapped
above underground dams that obstruct major solution channels (see