GROUND WATER IN NORTH-CENTRAL TENNESSEE

DAVIDSON COUNTY 133
have the same water-bearing properties wherever they occur. For
example, the outcrop of the Chattanooga shale is everywhere accom­
panied by perennial sulphur springs of rather small discharge, although
the same stratum may not be water bearing beneath deep continuous
cover. The Leipers and Catheys formations, which are for the most
part very impure earthy limestone and calcareous shale, are generally
poor water bearers throughout the county.
On the Highland Kim plateau, in the northwestern part of the
county, the mantle of residual cherty soil supplies dug wells of house­
hold magnitude, though many such wells prove inadequate during
long dry periods. Drilled wells 75 to 160 feet deep encounter water in
the coarse cherty debris at the bottom of the residual soil mantle or
in the weathered and channeled limestone just below, 700 to 750 feet
above sea level. On the slopes of the escarpment that bounds the
plateau on the southeast many tubular springs occur at the same
altitude. This water-bearing zone is probably more nearly constant
in water-yielding capacity than any other within the county.
On the Highland Kim escarpment and among the near-by outlying
hills, which constitute the more rugged parts of the county, erosion
has been comparatively rapid, and there has been no fixed equilib­
rium profile to which solution channels in the limestone could adjust
themselves. Hence in this area water-bearing channeled zones are
very erratic in altitude and extent. Under such conditions drilled
wells in the limestone are very uncertain as sources of water, and the
likelihood of finding water decreases greatly as the depth exceeds 50
or 100 feet, as in well 315 (pp. 135,137). Also, dry holes are not
uncommon, such as well 320. Wells that reach the Chattanooga shale
in the vicinity of its outcrop, however, generally obtain enough sul­
phur water to supply individual dwellings. In terranes of this sort
springs are a relatively certain source of water, particularly the peren­
nial tubular springs (pp. 92-95), which drain large areas of channeled
limestone. The pure subcrystalline Silurian limestones supply many
of the larger tubular springs along the base of the Highland Kim
escarpment. In most of the rugged tracts enough such springs exist
to meet the present and probable future requirements for water.
In the part of the county that lies in the Nashville Basin, drilled
wells from 60 to 200 feet deep generally find ample water for household
purposes in channeled limestone. The water-bearing zones of most
of these wells fall into one of two groups one from 300 to 350 feet
above sea level and the other from 400 to 450 feet above sea level.
The tested capacities of these wells range from a fraction of 1 gallon
to 228 gallons a minute, although most of the wells have not been
proved for drafts exceeding 10 gallons a minute. Dry holes have
been drilled in several localities, as in the blocks bounded by deeply
cut meanders of the Cumberland River and in the earthy beds of the