GROUND WATER IN NORTH-CENTRAL TENNESSEE

132 GROUND WATER IN NORTH-CENTRAL TENNESSEE
Davidson County lies on the northwest flank of the Nashville dome
(see pp. 62-63), so that at any particular altitude successively older
strata appear toward the southeast. The exposed rocks constitute
nearly a full section from the St. Louis limestone, of Mississippian
age, down to the Kidley limestone, of Lower Ordovician age. (See
pp. 33-55.) The massive subcrystalline St. Louis limestone crops
out only in the Highland Kirn plateau northwestward from Joelton,
where it is overlain by a thick mantle of cherty residual soil. The
underlying Fort Payne formation, which in this county is generally
thin-bedded earthy cherty limestone, crops out in all the upper slopes
of the Highland Rim escarpment and forms the divide between the
Cumberland and Harpeth Rivers. The New Providence and Ridge-
top shales, which are stratigraphically beneath the Fort Payne
formation, crop out in an area several miles across northwest of
Nashville but are not widespread elsewhere. The Chattanooga shale
crops out as a narrow band following the lower slope of the Highland
Rim escarpment and inclosing erosion outliers of the plateau as far
east as Nashville. Heavy-bedded dense and crystalline limestones
of Devonian and Silurian age crop out along the base of the escarp­
ment and of the outliers of the plateau and in the valley of the Harpeth
River below Belleview. The stratigraphic relations of the formations
of these two systems, which lap eastward over the underlying strata,
have not been traced in detail. The earthy and dense pure limestones
of Upper and Middle Ordovician age cover most of the rolling terrane
of the Nashville Basin north and west of Nashville. The massive
compact Carters limestone member of the Lowville limestone is exposed
by the Cumberland River in the southeastern part of Nashville and
crops out extensively farther southeast. The Lebanon and Ridley
limestones of the Stones River group appear successively in the lower
parts of the county and cover larger and larger areas toward the
southeast. The areas within which the several stratigraphic units
crop out are shown on Plate 4.
GROUND-WATER CONDITIONS
The water-bearing properties of the rocks in Davidson County dif­
fer greatly from place to place and for the most part can not be fore­
told from stratigraphic position alone. Rather, the ability of any
particular stratum to transmit water is generally limited by the number
and size of solution channels and hence is dependent upon the solu­
bility of the rock, the number and persistence of joints, and the posi­
tion of the stratum with relation to present and past equilibrium
profiles of underground drainage. (See pp. 78-82.) Thus in several
parts of the county a water-bearing zone occurs in the limestone at
about the same depth below the surface even though at different
stratigraphic horizons. On the other hand, a few of the formations