GROUND WATER IN NORTH-CENTRAL TENNESSEE

STEATIGEAPHY 33
The Tuscaloosa formation crops out only on upland tracts which
are thinly populated and in which there has been little or no ground-
water development. Hence, its water-bearing properties are not
known, although it may be inferred from the lithology that the per­
meability is high. However, the formation is probably drained over
extensive areas and therefore may not contain much water.
CARBONIFEROUS SYSTEM
MISSISSEPPIAJSr SERIES
ST. LOUIS LIMESTONE
The St. Louis limestone, the youngest of the marine sediments of
north-central Tennessee, is generally a massive or medium-bedded
fine-grained bluish-gray limestone, which locally contains a great deal
of nodular and cellular chert (see pi. 6, A) and some beds of shale and
sandstone, particularly in the lower part. So far as is known, the
top of the formation has not been observed in the region covered by
this survey, so that its total thickness before the region was peneplaned
during the Highland Rim cycle is not known. However, in northern
Overton County,45 northeast of this region, it is 110 to 140 feet thick.
Safford 4a has estimated its maximum thickness in central Tennessee
as 250 feet, although he probably included in this estimate the under­
lying Warsaw formation. The St. Louis limestone is identified by
the massive colonial corals Lithostrotion basaltiforme, which occurs at
all horizons through the area, and Lithostrotion proliferum, which
occurs locally in the lower part of the formation and which is dis­
tinguished from the much more abundant L. basaltiforme by having
cylindrical rather than polygonal corallites. According to Butts,47
Archeocidaris and Melonites are abundant and Lithostrotion proliferum
occurs sparsely immediately above a bed of earthy limestone that is
about 10 feet above the base of the formation, and this sequence is
diagnostic throughout northern Overton County. Hayes and Ulrich **
also note the presence of Melonites just above the base of the forma­
tion in the Columbia quadrangle, so that this fossil may prove to be
a trustworthy stratigraphic guide throughout the region.
The St. Louis limestone is the topmost formation over the greater
part of the Highland Rim plateau from the central part of Sumner
County westward to the Tennessee River (see pi. 4), although out­
crops of the unweathered rock are seen only in the stream beds. All
the upland tracts which it underlies are covered by a thick mantle of
bright-red soil and clay that contains many rounded fragments of
chert and silicified colonies and fragments of the characteristic fossil
« Butts, Charles, Geology and oil possibilities of the northern part of Overton County, Tenn., and of
adjoining parts of Clay, Pickett, and Fentress Counties: Tennessee Geol. Survey Bull. 24, p. 19,1919.
« Safford, J. M., Geology of Tennessee, p. 339,1869.
« Butts, Charles, op. cit., p. 18.
« Hayes, C. W., and Ulrich, E. 0., op. cit., p. 3.