GROUND WATER IN NORTH-CENTRAL TENNESSEE

86 GROUND WATER IN NORTH-CENTRAL TENNESSEE
On the other hand, when a region underlain by cavernous limestone
is raised a considerable distance by rapid crustal movement, its-
streams are rejuvenated and much lower equilibrium profiles of erosion
and of underground channeling are established. The effect of the
uplift upon the ground-water conditions then depends upon the rela­
tive rates of surface erosion and subsurface channeling. Where the
rocks are extremely soluble and much fractured, a poorly drained
elevated plateau with a deep water table would result. If, however,
the cycle of stream erosion should proceed more rapidly than the cycle
of underground channeling, there would tend to be formed in such a
region two separate systems of subsurface conduits one formed in
association with and adjusted to the streams that formerly drained
the upland and another adjusted to the base level of the rejuvenated
streams. The conduits adjusted to the rejuvenated streams might
not extend themselves beneath the uplands, and the two systems
might remain virtually without underground connections, the channels,
associated with the uplands ultimately being completely destroyed by
erosion. Conditions of this sort prevail in north-central Tennessee,
where the Highland Rim plateau is drained in large part by under­
ground solution conduits that generally are not connected with the solu­
tion conduits associated with the Nashville Basin peneplain (pp. 16-21).
The conduits that drain the Highland Rim plateau have been
breached by tributaries of the rejuvenated Cumberland River; from,
them issue tubular springs, of which many discharge 100 gallons a
minute or more perennially (pp. 92-95). Along the Highland Rim
escarpment the limestones are generally not channeled to any great
extent, and there seems to be little or no ground-water percolation
much below the surficial mantle of weathered rocks. Such percolation
as does take place below the steep erosion slopes of the escarpment
occurs largely as a ground-water cascade in the weathered rocks.
For the most part this cascade is intermittent, but locally there is
continuous percolation that supplies perennial springs.
TXELATION TO UTILIZATION OF GROUND WATER
Not only do limestones in different regions differ greatly in water­
bearing properties, but there is also a great diversity in the yield of
wells and springs that issue from the same bed in a given locality.
Obviously, the yield of a well or spring depends upon the size and
transmission capacity of the joint or solution channel from which its.
water is derived. Hence, a considerable element of chance enters into
the search for a supply of ground water, for one well may enter a large
water-bearing channel and be virtually inexhaustible, whereas other
wells only a few feet away may be practically dry because they do not
find any water-bearing openings, or because they enter dry channela
or channels that are filled with clay.