GROUND WATER IN NORTH-CENTRAL TENNESSEE

56 GROUND WATER IN NORTH-CENTRAL TENNESSEE
crystalline beds may occur at any horizon in the section; further­
more, their composite thickness is generally between one-third and
two-thirds the thickness of the formation. The platy layers are
separated by very thin seams of calcareous shale, which weathers
rapidly and allows the rock to disintegrate into a mass of loose slabs.
In general the Pierce limestone is lithologically very similar to the
Lebanon limestone, although it is much thinner; it also resembles
platy facies of the Murfreesboro and Kidley limestones.
The fossil fauna of the Pierce limestone is rich in the number of
species and of specimens alike. The bryozoans are especially abun­
dant and valuable as stratigraphic guides. The most common and
characteristic species are Nicholsonella pulchra, N. frondvfera, Anolo-
tichia explanata, Stictoporella cribttina, Pianodema stonensis, and
Batostoma, sp.
The Pierce limestone, which is from 23 to 28 feet thick, crops out in
narrow peripheral bands surrounding the minor structural domes that
expose the Murfreesboro limestone in central Rutherford County.
(See pi. 4.) Even though the outcrops are narrow usually less than
a quarter of a mile wide complete sections are exposed for study at
only a few localities.
MURFREESBOBO LIMESTONE
The Pierce limestone is underlain by the Murfreesboro limestone,
the oldest formation to crop out at the apex of the Nashville dome,
whose type locality 20 is the vicinity of the city of Murfreesboro
Rutherford County. The contact between the two seems to be
strictly conformable except at a point half a mile west of Lofton,
Rutherford County, where the upper 10 feet of the Murfreesboro is
missing. The Murfreesboro limestone is generally a thick-bedded
dense, brittle, dark bluish-gray or drab limestone, which emits a bitu­
minous odor when freshly broken and contains much disseminated chert.
The individual beds are from 6 inches to 4 feet thick and in some sec­
tions are separated by thin partings of shale or sand. This facies of
the formation is lithologically almost identical with the Ridley lime­
stone, which lies above it. On Bradleys Creek at Lascassas, however,
a shore phase of the formation is exposed, the lower 15 feet of the 27
feet of beds that crop out being sandy, laminated, sun-cracked, and
ripple-marked limestone that contains laminated chert nodules.
The Murfreesboro limestone contains few fossils other than fucoids
(?), but silicified forms are abundant at some places in the chert
debris that remains after advanced weathering. The most common
and diagnostic species are Salterella billingsi, Lophospira perangulata,
Liospira abrupta, Helicotoma tennesseensis, H. declivis, and Leperditia
fabulites.
» Safford, J. M., and Killebrew, J. B., op. cit., p. 125.