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