GROUND WATER IN NORTH-CENTRAL TENNESSEE

82 GROUND WATER IN NORTH-CENTRAL TENNESSEE
the secondary effects of underground solution rather than by surface
erosion. On the other hand, if the limestones were relatively insolu­
ble or not equally soluble and not highly permeable, and if the area
were raised far above base-level, the land might be sculptured almost
entirely through corrasion by surface streams. Indeed, it seems that
diversion of the drainage into solution or collapse sinks might begin
at any stage of the surface erosion cycle, or, on the other hand, that
surface streams might breach the underground channels and disrupt
the subsurface drainage until a relatively late stage of the under­
ground cycle. Furthermore, both the surface and subsurface cycles
are likely to be interrupted by crustal movement or other cause before
the peneplain stage is attained, and both the stratigraphy and the
structure may cause profound modifications of the ideal cycles that
have been outlined. Hence, many complex patterns of subsurface
drainage channels may exist.
In a given area, the cycle of channeling by solution proceeds most
rapidly where the rocks receive the largest or most constant inflow
of water from the surface. Generally this condition exists near the
perennial surface streams, provided their channels are somewhat
above the profile of equilibrium and have not been rendered imperme­
able by natural puddling. In north-central Tennessee the most
cavernous limestone and the largest solution channels generally
occur within the meander belts of the major streams, or in the acute
segments between converging tributaries in the vicinity of their
confluence. Many of the largest subsurface openings are evidently
by-pass channels that convey a part of the perennial stream flow
across meanders or from one tributary to another by a course that
is shorter than that of the surface stream. From channels of this
sort issue some of the largest perennial springs of the region, including
Hurricane Kock Spring (No. 181, pp. 161-162), whose discharge on
September 10, 1927, was about 60 cubic feet a second (27,000 gallons
a minute). Generally the limestone underlying the floors of all those
valleys in north-central Tennessee that are occupied by perennial
streams is somewhat channeled, the equilibrium profile of active
ground-water circulation seeming to be 50 to 75 feet below the mean
low-water surface of the Cumberland Kiver.
RELATIONS OF WATER-BEARING OPENINGS TO GEOLOGIC AND
PHYSIOGRAPHIC HISTORY
Relation to stratigraphy. Even relatively pure limestone differs
somewhat in solubility; furthermore, limestone formations may be
separated by impermeable rocks such as shale or by permeable rocks
such as sandstone. Hence many conditions of ground-water cir­
culation may result. A bed of impermeable shale or a stratum of
limestone that is not readily soluble or is not jointed may prevent