GROUND WATER IN NORTH-CENTRAL TENNESSEE

68 GROUND WATER IN NORTH-CENTRAL TENNESSEE
see except in the Wells Creek Basin. Galloway 38 has noted two faults
of small displacement in the Ridley limestone in Rutherford County.
One of these crosses the Stone Kiver at Jefferson, in the north-central
part of the county, and can be traced about a quarter of a mile from
the river in each direction. Its strike is about N. 25° E., its dip about
£0° E., and its vertical displacement about 20 feet. The other fault
-occurs 2 miles south of Christiana, in the south-central part of the
county. Its trace, which can be followed on the surface for a little
less than half a mile, strikes about N. 40° E. The vertical component
of the displacement is about 50 feet at the center of the fault but
diminishes rapidly in both directions; the southern block is down-
thrown.
A third minor fault cuts the Fort Payne formation in the north
bluff of the Duck River about a tenth of a mile upstream from Paint
Rock Bluff, in southwestern Humphreys County. This fracture
strikes N. 65° W. and is approximately vertical; the northern block
is downthrown an unknown distance. It is well exposed in the river
bluff and in the cut bank of the Memphis-Bristol highway.
These faults are clearly younger than the Nashville dome and its
associated secondary folds, but their exact age is unknown. All are
of such a magnitude that they may be due to slumping of the roofs of
large solution caverns formed at a comparatively recent time.
At many places in north-central Tennessee the limestones and other
compact rocks are much jointed, especially along and near axes of
relatively intense secondary folding. Where these joints are well
developed they are generally between 2 and 10 feet apart. As a rule
they are closer together in the thin-bedded and platy limestones and
farther apart in the thick-bedded, dense, and brittle limestones,
although these relations are by no means invariably true. Commonly
the joints of this region are approximately vertical and those at any
one place fall into two sets, which divide the rock into rhomboidal
blocks. Most common directions of jointing in north-central Ten­
nessee are N. 55°-65° W. and N. 25°-45° E., and the joints of the
northwesterly set are generally the more persistent and cut across
those of the northeasterly set. These directions are approximately
but in most places not precisely normal and parallel, respectively,
to the axis of the Nashville dome. The other joints, generally less
persistent along their strike than those of the dominant two sets,
commonly fall in the acute angle between N. 65° E. and S. 70° E.
As joints and bedding planes are the most common seats of solution
channels, it follows that the water-bearing properties of limestone
are closely related to the number and direction of its joints. These
relations are discussed on pages 150-155.
18 Galloway, J. J., Geology and natural resources of Butherford County, Tenn.: Tennessee Geol. Survey
Bull. 22, pp. 62-63,1919,