GROUND WATER IN NORTH-CENTRAL TENNESSEE

STRATIGRAPHY 41
and sulphate (SO4), probably derived from oxidation of the pyrite;
generally it also carries a noticeable amount of hydrogen sulphide,
although the quantity of this gas is usually less than 5 parts per mil­
lion. Many drilled wells obtain water supplies large enough for
household use from the upper part of the shale close to its outcrop.
In general the Chattanooga shale is not likely to be notably water
bearing where it lies beneath deep cover and is unweathered, it being
not unlikely that the ground water which is supposedly encountered
in the shale in eastern Dickson County and elsewhere actually issues
from sandstone of Devonian age.
DEVONIAN SYSTEM
Although a rather full sequence of Middle and Lower Devonian
formations is exposed in the western valley of the Tennessee River,74
rocks of the Devonian system are known at very few localities in
north-central Tennessee. Those whose stratigraphy is well known
are of Middle Devonian age, but Foerste 76 has identified Lower
Devonian beds in the Wells Creek Basin of Stewart County. If the
Chattanooga shale is wholly of Mississippian age, the Upper Devonian
series is absent in north-central Tennessee.
MIDDLE DEVONIAN SERIES
PEGEAM LIMESTONE
The type locality of the Pegram limestone is at Pegram, Cheatham
County. The formation has been defined and its occurrence in
central Tennessee described by Foerste.76 In the type section it is a
relatively pure heavy-bedded light-gray limestone that attains a
maximum thickness of 12 feet at its westernmost exposure in the
quarry north of the Nashville, Chattanooga & St. Louis Railway at the
bridge across the Harpeth River. Eastward from that locality the
member thins, and 3 miles to the southeast, at Newsom, in south­
western Davidson County, it is only 3 feet thick. At Newsom the
formation contains the diagnostic blastid Nucleocrinus verneuili, as
well as Stropheodonta demissa, S. perplana, Rhipidometta penelope,
and Nucleospira concinna. The only other known occurrence of the
Pegram limestone in north-central Tennessee is at the whirl on the
Buffalo River, which is 2K miles north of Bakerville, Humphreys
County, and 46 miles west and somewhat south of the type locality.
At that place the formation is a massive bed 3 feet thick, which is
74 Dunbar, C. O., Stratigraphy and correlation of the Devonian of western Tennessee: Tennessee Geol.
Survey Bull. 21,127 pp., 1019.
75 Foerste, A. F., Silurian and Devonian limestones of western Tennessee: Jour. Geology, vol. 11, p. 682,
1903.
79 Foerste, A. F., Silurian and Devonian limestones of Tennessee and Kentucky: Geol. Soc. America
Bull., vol. 12, pp. 425-426, 1901.
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