GROUND WATER IN NORTH-CENTRAL TENNESSEE

84 GROUND WATER IN NORTH-CENTRAL TENNESSEE
a synclinal trough may uphold a large body of perched ground water
far above the regional water table. As a limb of a fold, a bed of
the same sort may constitute a ground-water dam that inhibits
horizontal extension of the subsurface drainage system. Where the
dip is greater than the inclination of the topographic surface, a sol­
uble limestone that is inclosed by less soluble or by impermeable beds
may be channeled to considerable depth, so that it retains water
under artesian head. Where the dip is less than the inclination of
the topographic slope and in the same direction inclosed water-bear­
ing beds are likely to discharge as hillside springs above the level of
the perennial streams. On the other hand, where the limestones are
thick and are more or less equally soluble, folding might impede
but probably would not prevent development of the normal sub­
surface drainage system. However, the channels would presumably
be looped in a more complex pattern than in horizontal beds, for
solution would take place largely along the inclined bedding planes
and along joints transverse to the beds.
Faults and the breccia zones that commonly accompany them in
rocks as brittle as limestone may constitute ground-water conduits
extending to great depth. Furthermore, the walls of a fault are
likely to be jointed for considerable distances from the principal frac­
ture. Hence a fault and the accompanying secondary fractures may
promote ground-water circulation and subsurface channeling.
Relation to physiographic history and land forms. If limestone that
has been rendered cavernous by solution were depressed somewhat by
subsidence of the crust, the water table would rise with relation to
the equilibrium profile of solution, so that channels that were formerly
above the water table would be filled with water and might become
ground-water conduits of very large transmission capacity. Under
favorable conditions water might also be retained under artesian pres­
sure. Obviously this sequence of events is ideal for producing the
maximum water-yielding capacity hi a water-bearing limestone.
Conditions that are analogous with those just outlined exist in some
parts of the north-central United States, where the deposition of ex­
tensive sheets of glacial debris caused the water table to rise and to
submerge cavernous portions of the Galena and Niagara limestones.
The occurrence of ground water under these conditions is described
by Meinzer B7 as follows:
Before the glacial epoch these limestones [Galena and Niagara] lay at the surface
over wide areas and were subjected to extensive weathering. Then they were
overridden by successive ice sheets and became covered with glacial drift. To-day
the water table in most places passes through the drift mantle, leaving the under­
lying cavernous limestone within the zone of saturation. In these areas limestone
* Meinzer, O. E., The occurrence of ground water in the United States: U. S. Geol. Survey Water-Supply
Paper 489, p. 132,1923.