GROUND WATER IN NORTH-CENTRAL TENNESSEE

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