GROUND WATER IN NORTH-CENTRAL TENNESSEE

48 GROUND WATER IN NORTH-CENTRAL TENNESSEE
similar in its general aspects to and is actually a recurrence of the
fauna from the underlying Catheys formation of Middle Ordovician
(upper Trenton) age. Moreover, the Leipers and Catheys formations
are very similar in lithology, so that extreme care must be used if
they are to be discriminated accurately in the field.
On the west side of the Nashville dome the Leipers formation
reaches a maximum thickness of 100 feet but locally has been com­
pletely removed by erosion that began in Upper Ordovician (Lorraine)
time. Toward the east, however, the formation thins and becomes
less persistent, and over extensive areas on the east side of the basin
it is entirely absent. These stratigraphic relations are due in small
part to post-Leipers erosion but more especially to the fact that the
Leipers formation overlaps or transgresses the flank of the Nashville
dome from the west and was not deposited uniformly over the entire
region. Northwest of the type area, however, in the Wells Creek
Basin of Stewart County, the formation has not been found, although
the stratigraphic relations have not been traced in detail, and it is
not clear whether the omission is due to erosion or to overlap. At
most localities the formation lies immediately beneath the Chatta­
nooga shale, although at some it is separated from the Chattanooga
shale by Silurian and Devonian rocks.
MIDDLE OKDOVTCIAN SERIES
CATHEYS LIMESTONE
On the north and west sides of the Nashville Basin the Leipers
formation is underlain by the Catheys limestone, whose type area is
the basin of Catheys Creek,98 a tributary of the Duck River in Lewis
and Maury Counties. The two formations are separated by a slight
erosional unconformity. The Catheys formation, as deposited, was
not less than 50 feet and in some places was at least 100 feet thick,
but the subsequent erosion removed much of the accumulated ma­
terial. The upper half of the formation is as a rule made up of com­
pact impure blue limestone in layers from a few inches to 4 feet thick,
separated by thin layers of calcareous shale. Northward from the
type locality more and more beds of fine-grained impure limestone
appear in this part of the formation. The fossil fauna, which is re­
stricted to the fine-grained beds and with them increases in abundance
toward the north, includes Brachiopoda, Mollusca, Crustacea, and,
locally, Bryozoa. Several large ostracodes, of the genera Leperditia
and Isochilina, are especially characteristic of these beds. The lower
half of the formation is made up of highly variable beds of fossiliferous
knotty and fine-grained earthy limestone and shale, many of which
are identical in lithology with the overlying Leipers formation. In
these beds Cyclonema varicosum is probably the most diagnostic fossil.
" Hayes, O. W., and Ulricb, E. O., op. cit., p. 2.