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