GROUND WATER IN NORTH-CENTRAL TENNESSEE

OCCURRENCE OF GROUND WATER IN LIMESTONE 79
have been narrowed to linear divides but have not been reduced in
altitude.
In the analogous stage of the underground cycle solution channels
are first developed beneath the slopes of the deeper surface valleys
and extend themselves headward beneath the uplands. The stage
may be considered to end when the entire region is underlain by a
network of channels and when linear divides first appear between the
underground systems. As in a dense massive limestone the channels
are generally formed by etching the faces of blocks bounded by joints
and bedding planes, both lateral and vertical connections will be made
from crevice to crevice as the process of solution goes on. Ultimately
the small solution openings become braided or looped in both hori­
zontal and vertical projections. Such a looped pattern without large
trunk channels may be considered characteristic of this first stage.
The divides between the underground drainage basins may or may
not coincide with the surface divides, and they may even shift laterally
with seasonal or annual variations in the distribution of rainfall.
The joints in massive dense limestone may be so tight that the
ground water percolates slowly below a new upland, even though the
surface streams occupy deep trenches and the hydraulic gradient is
steep in consequence. Under such conditions the solution channels
may extend themselves headward very slowly, especially if the lime­
stone is impure or is interbedded with less soluble layers, and the
surface streams may progress beyond the first stage of their erosion
cycle before the underground system diverts any considerable part of
the surface run-off. On the other hand, if the joints are open or the
limestone has considerable primary continuous porosity and is espe­
cially soluble the underground drainage system may develop very
rapidly and may pass through the first stage of its cycle before the
upland is completely drained by surface streams. Under these con­
ditions extensive upland tracts might be thoroughly drained into
underground channels before surface drainage became established.
In north-central Tennessee, where the limestones are nearly hori­
zontal and not excessively jointed and where the rainfall is moderately
high, the surface streams seem to pass through their youthful stage
before the system of underground solution channels is well estab­
lished. Indeed, in areas of youthful topography in this region the
solution channels are small and discontinuous and do not extend to
great depths. Generally the limestone becomes tight and not water
bearing at 75 feet or less below the surface. Locally small quantities
of ground water drain from the upland and pass through the imperfect
system of underground channels as perennial or intermittent cascades,
but most of the drainage is carried by surface streams.
In the second or mature stage of the surface erosion cycle the inter-
stream divides are lowered gradually, the valleys are widened, and in