GROUND WATER IN NORTH-CENTRAL TENNESSEE

GEOLOGIC STRUCTURE 67
rounding the lowland and abutting against the bounding scarp. The
central hill is composed of limestone and dolomite of Beekmantowra
(?) age, which are faulted into blocks of all sizes and every conceiv­
able orientation and locally are reduced to a breccia of blocks not more
than 2 feet in diameter. These rocks are at least 1,000 feet higher
than their normal altitude outside the uplift. The ring-shaped low­
land is underlain by poorly exposed pre-Trenton post-Beekman-
town limestones, which are likewise complexly faulted, though much
less brecciated than the rocks of Beekmantown (?) age. The fault
blocks of this unit also are in every conceivable orientation, and
vertical beds occur almost a mile north of the center of the uplift.
Radially outward from the center of uplift the faulting becomes more
orderly and the strike of the rocks tends to become parallel to the
outer margin of the lowland. Faulting is least complex in the belt
of foothills.
The area of structural deformation, however, is about 8 miles in
diameter and extends well beyond the topographic basin. From the
erosion scarp that bounds the topographic basin the Mississippian
rocks dip radially outward into a ring-shaped syncline, which is rudely
concentric about the center of the uplift and whose axis is about 2J£
miles from it. In this syncline the rocks are commonly faulted and
brecciated more complexly than at the outer margin of the topo­
graphic basin. In its deepest part they are depressed about 300 feet
below their normal altitude in the adjacent areas and about 1,300 feet
below their projected position at the center of the uplift. Surrounding
this ring-shaped syncline is a zone about 2 miles wide in which the
rocks rise to their normal altitude, though broken by many normal
faults. These faults are in part tangential to the trend of the zone
and in part radiate about the center of the disturbance, but the tan­
gential faults are the more common. Several can be traced for more
than a mile along the strike, and one is more than 4 miles long; the
vertical components of their displacements are several hundred feet.
In addition to these major faults there are many secondary fractures,
which divide the rocks into blocks of all sizes. Abrupt changes in
strike and dip of the blocks are common, and in many places the beds
are vertical.
The zone of maximum uplift and brecciation at the center of the
disturbance is believed by Bucher to be the result of a violent shock
or explosion and the marginal zone of depression to be due to collapse
of crustal material as if into a void. A hypothesis to account for the
forces and for the transfer of subcrustal material is yet in the foramla-
tive stage.
FAULTS AND JOINTS
Faults, which are fractures along which the rock strata have suffered
relative displacement, are comparatively rare in north-central Tennes-