GROUND WATER IN NORTH-CENTRAL TENNESSEE

18 GROUND WATER IN NORTH-CENTRAL TENNESSEE
and merges with Duck River Ridge in southwestern Williamson
County.
NASHVILLB BASIN
The Nashville Basin is an elliptical depression in the Highland Rim
plateau whose major diameter trends N. 30° E. through the approxi­
mate geographic center of the State. (See pi. 1.) The interstream
tracts of its floor constitute a very slightly undulating plain about 80
miles long and 45 miles wide, the northern segment of which slopes
northward and westward from an altitude of 700 feet above sea level
in southern Rutherford County to 650 feet in the vicinity of Franklin
and Columbia and 550 feet near Nashville. Hence the Nashville
Basin is 200 to 600 feet below the surrounding Highland Rim plateau.
The lower reaches of its drainageways the Cumberland and Duck
Rivers and their tributaries are intrenched about 100 to 150 feet, so
that the northwestern lobe of the plain is rolling and comprises many
rounded steep-sided hills. Moreover, numerous isolated hills or
monadnocks erosion remnants of the higher plateau are scattered
over-the floor of the basin along the Highland Rim escarpment.
Hence this physiographic unit displays some diversity in detail of land
forms.
The Nashville Basin is entirely surrounded by the Highland Rim
escarpment, which is breached only by four narrow water gaps. These
are the inlet and outlet of the Cumberland River, the one stream that
traverses the basin, and the outlets of the Duck and Elk Rivers, which
head upon the basin's floor. On the west, north, and east this bound­
ing escarpment is well defined, although intricately serrate in plan.
On the south, however, it is rather indefinite, and the open basin
pas^eg'into a rolling terrane which is a dissected plateau.
The outline of the Nashville Basin traced on Plate 1 incloses that
area in which stream erosion has proceeded well beyond the stage of
maturity and which, if isolated monadnocks are disregarded, is ap­
proaching complete planation. About 35 per cent of the area covered
by the investigation falls within the basin as thus defined.
PHYSIOOBAPHIC HISTOBY
CTJMBBBLAND CYCI/E
Hayes and Campbell 20 have concluded that the final stage in the
formation of the regional peneplain of which the present Cumberland
Plateau is a remnant was contemporaneous with the deposition of the
calcareous Selma formation (middle and lafce Upper Cretaceous).
They have also concluded that the peneplain was formed at or near sea
level and that it was warped and uplifted afc the beginning of the
Ripley^epoch .(late Upper Cretaceous). Its present remnantain cen-
M Hayes, O. W., and Campbell, M. R., Qeomorphology of the southern Appalachians: Nat. Geog. Mag.,
vol. 8, pp. 124-128,1894.