GROUND WATER IN NORTH-CENTRAL TENNESSEE

STRATIGRAPHY 45
by Foerste,87 Pate and Bassler,88 and Miser.89 This section comprises
rocks of Cayuga, Niagara, and Albion age. The rocks of Cayuga
&ge constitute the Decatur limestone; those of Niagara age are divided
into the Lobelville, Bob, Beech River, Dixon, Lego, Waldron, Laurel,
and Osgood formations; and those of Albion age are the Brassfield
limestone. These formations, whose general characteristics are given
in the stratigraphic section (p. 27), constitute a somewhat variegated
stratigraphic unit, which is 233^ feet thick at Clifton, Wayne County.90
The Silurian rocks thin northward and eastward along the flank of
the Nashville dome and finally wedge out in Macon County, slightly
«ast of the area represented by Plate 4. According to Miser,91 this
thinning is due to post-Silurian erosion that truncated the section
at the top and is not primarily due to overlap, the only overlapping
unit in the Silurian being possibly the Lobelville formation. The
detailed stratigraphy of the Silurian rocks has been untangled at
only a few places in north-central Tennessee, so that full discussion
is not possible at this time.
As shown by Plate 4, the Silurian rocks crop out in north-central
Tennessee along the base of the Highland Rim escarpment and its
outliers in Williamson, Cheatham, Davidson, and Sunnier Counties.
According to Foerste 92 a rather full sequence of these rocks is also
exposed in the Wells Creek Basin of southeastern Stewart County.
ORDOVICIAN SYSTEM
GENERAL FEATURES
The Ordovician system of north-central Tennessee includes rocks
of Upper, Middle, and Lower Ordovician age, the top of the system
being placed at the top of the Richmond group. In this region the
Brassfield limestone, of Albion age, rests disconformably on the Fern-
vale formation (of early Richmond age, according to Ulrich), and the
underlying Arnheim limestone, which is the basal formation of the
typical Richmond group of Indiana, rests unconformably on the
Leipers limestone (of early Maysville age, according to Ulrich).
The stratigraphic break at the base of the Richmond group is con­
sidered by Ulrich and Bassler to be of considerable magnitude and
greater than the break at its top, the formations with which the
8' Foerste, A. F., Silurian and Devonian limestones of Tennessee and Kentucky: Qeol. Soc. America
Bull., vol. 12, pp. 395-444,1901; Silurian and Devonian limestones of western Tennessee: Jour. Geology,
vol. 11, pp. 554-583, 679-715,1903.
88 Pate, W. F., and Bassler, E. S., The late Niagaran strata of west Tennessee: U. S. Nat. Mas. Proc.,
vol. 34, pp. 407-432, 1908.
» Miser, H. D., Mineral resources of the Waynesboro quadrangle, Tenn.: Tennessee Geol. Survey Bull.
26, pp. 18-22, 1921.
'« Pate, W. F., and Bassler, R. S., op. cit., pp. 411-412.
" Miser, H. D., op. cit., pp. 18-22; also personal communication.
*> Foerste, A. F., Silurian and Devonian limestones of western Tennessee: Jour. Geology, vol. 11, pp.
690-694,1903.