GROUND WATER IN NORTH-CENTRAL TENNESSEE
GROUND WATER IN NORTH-CENTRAL TENNESSEE
GROUND WATER IN NORTH-CENTRAL TENNESSEE
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OCCURRENCE OF <strong>GROUND</strong> <strong>WATER</strong> <strong>IN</strong> LIMESTONE 81<br />
a local equilibrium surface of solution may be either temporary or<br />
permanent, depending upon the character of the restraining bed.<br />
In their final or old-age stage the surface streams widen their valleys<br />
by lateral planation at the equilibrium profile of erosion, and the di<br />
vides are gradually worn down by the tributaries. As planation<br />
progresses the grades of the streams decline, and their capacity to<br />
transport the mechanical products of erosion diminishes and finally<br />
becomes essentially zero. Ultimately the divides are essentially<br />
destroyed and the entire region is reduced nearly to a plane surface,<br />
the peneplain, which is the final product of the erosion cycle.<br />
In a corresponding manner, once the ground-water conduits have<br />
been established at the equilibrium profile of solution channeling they<br />
tend to extend themselves along this profile within the limits imposed<br />
by the stratigraphy and structure. Ultimately there tends to be<br />
produced at this level a network of large solution channels in which<br />
most of the ground-water circulation takes place. Such a network<br />
may be considered as defining a peneplain of solution.64 Collapse<br />
sinks should be most numerous in this stage of the underground cycle,<br />
and under favorable circumstances much of the drainage may be<br />
returned to the surface by the formation of collapse valleys.<br />
The ultimate product of the underground cycle would be the same<br />
as that of the surface cycle a peneplain drained in large part by sub<br />
surface channels of very low gradient. This condition probably<br />
existed on the Highland Rim peneplain of north-central Tennessee<br />
(pp. 1 19-20), which in its final form seems to have been drained largely<br />
by a network of solution channels about 100 feet below the surface.<br />
These channels are those which are entered by numerous wells and<br />
from which issue many large tubular springs (pp. 92-95). Indeed,<br />
the final steps in degradation of the peneplain may have been effected<br />
wholly by solution and by ground-water circulation, for in some placea<br />
the mantle of insoluble rock waste that overlies the bedrock extends<br />
downward practically to a zone of extensive solution channeling.<br />
(See pi. 6, 5.)<br />
Although the cycles of surface erosion and of underground solution<br />
start simultaneously whenever an area is raised above regional base-<br />
level, and although the two processes tend to a common ultimate<br />
product, the peneplain, the analogous intermediate stages of the two<br />
cycles may not be even roughly contemporaneous. If the limestone<br />
is readily soluble and is permeable even before being attacked by the<br />
natural solvents, and if the area is raised only a moderate amount<br />
above base-level, underground planation by solution may be far<br />
advanced while the surface streams are yet very immature. These<br />
conditions would presumably be the optimum for the formation of<br />
large solution and collapse sinks and for sculpturing the surface by<br />
« Meinzer, O. E., Geology of large springs: Qeol. Soc. America Bull., vol. 38, p. 215,1927.