GROUND WATER IN NORTH-CENTRAL TENNESSEE

GROUND WATER JN NO^T^-^^EUAL TENNESSEE
of the Highland Rim plateau (pp. 16-18). Its. naajoF ridge crests,
remnants of the Highland Rim peneplain, increase in altitude from
about 800 feet above sea level in the northwestern part of th» county
to 1,020 feet in its southwest corner. None of these remnants are
extensive, however, for the peneplain has been dissected^ by the
closely spaced youthful valleys of a dendritic drainage pattern.
Hence the topography is rather rugged, especially along the. southern
boundary of the county, where the Harpeth and Duefc Rivers are
striving for drainage mastery. The eastern half of the county, which
constitutes a. part of the northern lobe of the Nashville Basin (p.
18), is drained northward by the Harpeth and West Harpeth Rivers.
In their lower reaches these s»fereams kave; planed laterally and have
cut valley floors half a mile to 3 miles wide at 620 to 670 feet above
sea level, a stage which is correlative with the much more extensive
planation by the Stone River in Rutherford County. Furthermore,
they have reduced the interstream tracts to groups of submature hills
and branching ridges, the bjghest of which, are in $he south-central
part of the county and range from 1,165 to 1,250 feet a^oye se^ level.
These hilly tracts are outliers of the Highland Rim plateau.
The rocks that crop out in WiUiamson County constitute a rather
full stratigraphic sequence from the St. Louis and Warsaw Hmestones
of Mississippian age, to. the, Lebanon Uinestone,, of Lower Ordovician
age (pp. 33-54). The youngest stratigraphic unit, which comprises
the St. I*ouis and Warsaw limestones, is made up of thick-bedded,
somewhat cherty limestone and craps out over an extensive tract
along the western boundary of the county. It also caps the Mgher
ridges a.a far eastward as the. Highland Rim escarpment but. is not
known at any point east of the West. Harpetk River. (See, pi. 4.)
However, visible exposures of the rock generally occur only qn the
slopes of youthful valleys, for on the remnants of the Highland Rim
peneplain it is overlain by residual clay and chert as much as 60 feet
thick. The Warsaw limestone is underlain everywhere by the Fort
Payne formation, which comprises thin beds of extremely cherty
limestone and shale, sandy limestone, calcareous shale, and clay shale.
These beds also 0ap outliers of the plateau in the north-central and
sotuthrcentiral pajts of the county as well as the highest summits oj t|ie
ridge that trends southeastward across the county between the
Harpeth and West Harpeth fivers. In a few places,, as oa the slopes
o$ Sugar- Riclge, in the south-central part, ol the county, thefije beds »?$
underlain by the New Providence shale, wflich consists of clay, sjiale,
ancj lenses of massive, crinoida) limest,one. The Fort Payne formation,
or the New Providence shale where that, formation is pyesen^ is under­
lain in all p&rts, of the county by the carbonaceous Cnattanqoga,
sh,aje. This alrftbigr^pjbic horizon marker crops out as a narrow bancj
in tine lowe? part of t&e Highland Rim escarpment and ifcg