GROUND WATER IN NORTH-CENTRAL TENNESSEE

80 GROUND WATEE IN NORTH-CENTRAL TENNESSEE
transverse profile the lower parts of the valley slopes become concave
upward. A process of integration goes on by which the branches
that have the greatest erosive power drain more and more of the total
drainage area and become major tributaries of the trunk stream.
They also erode their beds downward to a local profile of equilibrium,
below which they can not cut effectively, and subsequently they
aggrade their lower reaches. Provided the rocks are equally resistant
to erosion and the erosion cycle is not interrupted by crustal move­
ment or other cause, the pattern of the surface drains remains den­
dritic or branching. The mature stage ends when the principal
divides begin to disintegrate into groups of isolated hills.
In the analogous stage of the underground cycle integration of the
solution channels goes on, those which gather the largest amounts of
surface water and those which discharge at the lowest points increas­
ing in size most rapidly and becoming major ground-water conduits.
These conduits may converge toward a common point of discharge or
they may diverge toward several points of discharge after the manner
of distributaries on the delta of a surface stream. Also, they are
likely to be connected by small looped channels or to be looped them­
selves. At the same time the divides between the underground
drainage systems are lowered as new channels are etched at lower
altitudes and the hydraulic gradient that induces circulation of the
ground water is reduced. Collapse sinks may reach the surface in
this stage of the underground cycle, but if the limestone is thick bed­
ded, strong, and not closely jointed and if the large solution channels
are not close to the surface such sinks may not be numerous. If the
limestone is highly permeable and particularly soluble, the cycle of
underground channeling may well reach maturity while the surface
streams are still extremely young. As the surface streams into which
the ground water discharges lower their beds, the ground-water con­
duits seek lower points of discharge until a condition of equilibrium is
established and then extend themselves laterally. The profile of the
ground-water conduits after such equilibrium has been established
defines a surface that may be designated the equilibrium surface of
solution channeling. Below this surface the ground water does not
circulate effectively and hence probably does not etch the limestone
appreciably except where deep artesian circulation takes place (pp.
"96-98). As circulation is controlled by points of discharge into the
surface streams, the equilibrium profile of solution channeling is
generally adjusted to the local equilibrium profile of erosion, and the
ground-water conduits tend to adjust themselves to the surface
streams where the rocks are equally jointed and equally soluble
throughout. However, where the rocks are unequally soluble a local
equilibrium profile of solution may be established above the surface
streams by a bed that is impermeable or more or less insoluble. Such