GROUND WATER IN NORTH-CENTRAL TENNESSEE

STRATIGRAPHY 55
near Jefferson, northwestern Rutherford County. The Ridley lime­
stone is for the most part a massive, dense light-blue, dove-colored, or
light-brown limestone, which at some places contains abundant
bluish-black or white chert. Locally, as at Sulphur Spring, 1% miles
north of Jefferson, it contains thin-bedded or platy members. The
color of the formation is due largely to the presence of bituminous
matter, the odor of which is usually noticeable when the rock is
freshly broken; at many places it is streaked with granular fucoid
markings of lighter color. The weathered rock is light gray and has a
finely granular appearance, but the weathering does not extend more
than 1 or 2 inches into the rock.
The Ridley limestone can not be discriminated from the older
Murfreesboro limestone on the basis of lithology alone. These two
formations are alike in color, in hardness, and in brittleness; they
contain about the same amount of chert and bituminous matter;
each is platy at a few places; their changes on weathering and their
influence upon topography and soil are identical. Hence the Ridley
limestone can be identified only where it is fossiliferous or where its
contact with the overlying Lebanon limestone or the underlying Pierce
limestone is exposed.
The Ridley limestone is not highly fossiliferous, although Stroma-
tocerium is locally abundant. The most common and diagnostic
species are Stromatocerium rugosum, Camaretta volborthi, Hebertetta
bettarugosa, Gonioceras anceps, Orbignyetta sublamettosa, IAOSJWTQ,
convexa, Rafinesquina minnesotensis, and Protorhyncha ridleyana.
The Ridley limestone is from 95 to 120 feet thick, although most of
the measured sections are between 100 and 105 feet. It crops out
over the greater part of the Nashville Basin peneplain in Rutherford
County and is also exposed in Davidson, Williamson, and Wilson
Counties. (See pi. 4.) In spite of its rather general distribution,
however, complete sections of the formation are exposed at few places
on account of the low relief of the area of outcrop.
PIERCE LIMESTONE
The Ridley limestone is underlain by the Pierce limestone. The
two formations seem to be conformable except at Jefferson, Ruther­
ford County, where the contact surface between them is undulating
with respect to the bedding planes. The Pierce limestone takes its
name from Pierce's mill, 19 half a mile south of Walter Hill, Ruther­
ford County. It is rather variable in lithology and comprises many
layers of dense blue or gray unfossiliferous limestone between half an
inch and 2 inches thick and one or more massive beds of coarsely
crystalline bluish-gray or brown fossiliferous limestone. The coarsely
w Safford, J. M., op. cit., p. 281.