GROUND WATER IN NORTH-CENTRAL TENNESSEE

46 GROUND WATER IN NORTH-CENTRAL TENNESSEE
Leipers is in contact ranging in age from early Richmond (Arnheim
limestone) to late Devonian or Mississippian (Chattanooga shale).
Hence they include the rocks of Richmond age in the Silurian system.
The angular discordance is usually much too small to be discernible
in a small outcrop and is apparent only when diagnostic faunal hori­
zons are traced long distances.
The Ordovician system comprises many limestone and calcareous-
shale formations which were deposited in successive broad overlap­
ping belts from east or west across the Nashville dome. According
to Ulrich 93 and Bassler,94 the succession of overlaps was due to crus-
tal oscillation. Hence the Ordovician formations do not follow one
another in simple succession but interfinger in a rather complex
manner, which can be traced in the field only by precise classifica­
tion of the fauna.
UPPER ORDOVICIAN SERIES
FERNVALE FORMATION
The Silurian rocks are underlain disconformably at several places
in the western part of the region by the Fernvale formation, whose
type locality is at Fernvale,95 in the valley of South Harpeth Creek,
southwestern Williamson County. This formation is made up largely
of soft chocolate-colored and green shale with one or more beds
of coarsely crystalline flesh-colored or mottled limestone and, locally,
a basal member of highly ferruginous reddish or even vermilion
limestone from 5 to 6 feet thick. At some places the lower beds are
conglomeratic and highly phosphatic. Where the formation is thin it
may be composed wholly of shale. The Fernvale formation, which
ranges in thickness from 40 feet to the vanishing point, occurs only
in scattered small areas on the western flank of the Nashville dome.
Hayes and Ulrich have explained this discontinuity by the hypothesis
that the formation was deposited only in elongate embayments, but
this hypothesis is not generally accepted.
ARNHEIM LIMESTONE
The Fernvale formation is underlain at a few places in Williamson
County and possibly elsewhere by the Arnheim limestone, which is
made up of blue granular crystalline limestone and interbedded shale.
Usually the Arnheim limestone is less than 3 feet thick.
LEIPERS LIMESTONE
The Leipers limestone takes its name from Leipers Creek, a trib­
utary of the Duck River in extreme southwestern Williamson County
" Ulrich, E. O., Revision of the Paleozoic systems: Qeol. Soc. America Bull., vol. 22, pp. 416-419,
428-430, 1911.
M Bassler, R. S., Embayments and overlaps in central Tennessee [abstract]; Qeol. Soc. America Bull.,
vol. 34, p. 132, 1923.
w Hayes, O. W., and Ulrich, E. O., U. S. Geol. Survey Geol. Atlas, Columbia folio (No. 95), p. 2,1903.