GROUND WATER IN NORTH-CENTRAL TENNESSEE

22 GEOUND WATBK IN NOKTH-CENTRAL TENNESSEE
limestones. The gradients of the Duck and Elk seem to have been
flatter at the beginning of the peneplain stage, however, so that their
erosive power was less, and the planation they accomplished lagged
behind that of the more northerly streams. The culmination of this
stream work approached peneplanation and carved the Nashville
Basin virtually to its present dimensions and topography. Erosive
equilibrium seems to have been comparatively short-lived, however,
for the Nashville Basin peneplain does not carry a thick mantle of
residual soil, like the older Highland Rim plateau.
The Nashville Basin erosion cycle was by no means complete
when it was terminated by uplif t. The numerous monadnocks along
its borders were yet in the youthful or mature stage, and the bounding
escarpments were yet receding. Local deposits of gravelly detritus
at the bases of these upland remnants, where the stream gradients
flatten abruptly, attest the activity of the erosive agencies.
Hayes 34 correlates the Nashville Basin peneplain with the Coosa
peneplain of the region about Chattanooga, which he implies is
post-middle Tertiary, but does not date precisely. Shaw 8B suggests
that the Coosa peneplain may be correlative with the two or three
upland plains of northern Mississippi which lie above the Brook-
haven terrace. To the Brookhaven terrace Matson 86 and Berry ^
ascribe post-middle Pliocene age. If the physiographic correlations
by Hayes and Shaw and the stratigraphic correlations by Matson
and Berry are correct, the beginning of the Nashville Basin cycle does
not antedate the sub-Miocene unconformity of Mississippi, and the
peneplain stage of the cycle is older than middle Pliocene. On the
other hand, Galloway 88 states that the peneplain stage of the Nash­
ville Basin cycle began at the end of the Pliocene and was terminated
by further warping in middle Pleistocene time, but he does not give
the basis of his assignment. Proof of the age of this and other
physiographic features of the region can not be obtained in the
absence of accurate topographic maps.
RECENT CYCLE
During relatively late geologic time the upwarping of the Nash­
ville dome was resumed and the streams were again rejuvenated.
The Tennessee River, which bounds the region on the west, has since
deepened its channel at least 75 feet. The Cumberland River has
intrenched itself about 100 feet at Nashville and somewhat less
M Hayes, G. W., op. cit. (Nlnteenth Ann. Kept.), PP. 31,56,1890.
is Shaw, E. W., Pliocene history of northern and central Mississippi: IT. 8. QeoL Surrey Prof. Paper
108, pp. 139,163,1918.
» Matson, Q. 0., The Pliocene Citronelle formation of the Gulf Coastal Plain: TJ. S. Qeol. Survey Prof.
Paper 98, pp. 188-189,1917.
» Berry, E. W., The flora of the Citronelle formation: Idem, p. 195.
" Galloway, J. J.t op. cit., pp. 22-23.