GROUND WATER IN NORTH-CENTRAL TENNESSEE

STRATIGRAPHY 51
thick, which is a medium-bedded phosphatic sandy limestone made
up in large measure of fragments of shells and corals. Its individual
laminae are characteristically cross-bedded (pi. 7, B), as is well shown
in the masonry of the State capitol at Nashville, the rock for which
was quarried from this member.
Still farther east,5 in Trousdale and Smith Counties, beyond the area
represented by Plate 4, the Bigby limestone is 120 to 150 feet thick, of
which fully half consists of very compact light bluish-gray or tan
limestone. The formation as a whole contains much more subcrys-
talline matter than the Bigby limestone of the type locality. More­
over, the fauna! differences are even more striking than these litho-
logic differences, nearly all the beds in Trousdale and Smith Counties
being profusely fossiliferous and containing a large and varied fauna.
In this fauna the Mollusca predominate and the Bryozoa and Brach-
iopoda that characterize the Bigby limestone of the type locality are
rare or absent altogether.
In Rutherford County, according to Galloway, 6 the Bigby lime­
stone is variable in lithology and is at most 30 feet thick. In the
central-western part of the county, near Almaville, it is a gray massive
granular laminated limestone, of which some beds are sandy and
others are shaly and highly fossiliferous. The most abundant species
of the locality are Hebertella frankfortensis, Rhynchotrema increbes-
cens, Hallopora multitabulata var., Platystrophia colbiensis, Tetradium
minus, and several undetermined species. The characteristic fossils
of the type locality are not present. North of Almaville the forma­
tion is a granular gray or brown laminated and cross-bedded lime­
stone that contains no fossils.
The Bigby limestone is relatively persistent at its proper horizon
along the west and north sides of the central basin but is thin or absent
at most places on the east side. 7 Its fauna is of Middle Ordovician
(middle Trenton) age.
HERMITAGE FORMATION
The Bigby limestone is underlain disconformably, in all parts of the
Nashville Basin, by the Hermitage formation (" Orthis bed " of Safford),
whose type section is at Hermitage station on the Tennessee Central
Railroad, in eastern Davidson County. In the Columbia quadrangle
Hayes and Ulrich 8 subdivided the Hermitage formation into two
portions, the upper of which, about 40 to 50 feet thick, is composed
of medium-bedded sandy and phosphatic subgranular limestone that
is accompanied locally by a small amount of shale. Many of these
beds are crowded with the silicified shells of Dalmanella testudinaria
* Hayes, C. W., and Ulrich, E. O., op. cit, p. 2.
* Galloway, J. J., Geology and natural resources of Rutherford County, Tenn.: Tennessee Geol. Survey
Bull. 22, pp. 51-52,1919.
7 Ulrich, E. 0., Revision of the Paleozoic systems: Geol. Soc. America Bull., vol. 22, p. 416,1911.
8 Hayes, C. W., and Ulrich, E. O., op. cit., pp. 1-2.