GROUND WATER IN NORTH-CENTRAL TENNESSEE

34 GEOTJND WATER IN NORTH-CENTRAL TENNESSEE
Lithostrotion basaltiforme. This mantle, which is locally at least
100 feet thick, is the insoluble residuum from the weathering of the
rock, the calcareous matter having been dissolved by soil water
percolating downward through joints and entering the underground
drainage system. Naturally, the depth of weathering is not the same
at all places, so that the surface of separation between the mantle
of residuum and the unweathered rock is extremely uneven. (See
pi. 6, #.) The St. Louis limestone seems to be more soluble than the
other Mississippian formations, so that in it such features as solution
channels, sink holes, and caves have developed more extensively than
in the other rocks under similar conditions of topography and geomor-
phologic history. (See pp. 78-86.) Indeed, the presence of numerous
sink holes and other features of solution has been invoked as a means
of identifying the St. Louis limestone wherever the unweathered rock
does not crop out. However, this criterion should be employed with
caution.
The St. Louis limestone yields a large quantity of ground water to
tubular springs (see pp. 92-95), the discharge of which is the under­
ground run-off from large upland tracts of the Highland Rim plateau.
The coarser phases of the cherty residuum from the weathered rock
yield moderate supplies to drilled wells, especially at and just above
the base of the weathered zone. The unweathered rock, however,
yields water only in wells that encounter a water-bearing crevice or
solution channel.
WARSAW FORMATION
Beneath the St. Louis limestone and probably separated from it by
a slight stratigraphic break 49 is the Warsaw formation, which is not
differentiated from the St. Louis limestone on Plate 4. Butts M also
states that the formation is relatively heterogeneous and in Overton
County comprises equal parts of calcareous sandstone, shale, and
limestone. The upper third of the formation in that area is mostly
sandstone, some of the layers of which are highly calcareous and
resemble limestone in the unweathered condition but weather by
solution of the calcareous matter into a loose aggregate of quartz
grains. In many localities the very top of the formation is composed
of layers between 2 and 4 inches thick of clastic, ripple-marked sand­
stone. The middle third is compact, thick-bedded limestone that
contains many fragmental fossils, and the bottom third usually com­
prises alternating beds of shale and limestone together with several
sandy layers that weather to resemble coarse yellow sandstone.
Toward the south and west the limestone beds contain an abundance
of dark chert, and the sandy facies of the formation seem to be less
well developed or to be truncated by an unconformity between the
« Butts, Charles, op. cit., p. 18.
M Idem, pp. 16-17.