GROUND WATER IN NORTH-CENTRAL TENNESSEE

OCCURRENCE OF GROUND WATER IN UMESTONE 71
Solution openings in limestone are of two kinds (a) intergranular
spaces produced by etching the faces of crystals or grains of rocks
that have some primary porosity, by selective leaching of the more
soluble constituents from such nonhomogeneous rocks as magnesian
limestone and gypsiferous limestone, or by leaching the cement
between the insoluble grains of a calcareous sandstone; (b) tubeHke
channels formed in pure massive limestone by enlargement of joints
or of primary openings along bedding planes. The tubular solution
openings are the type commonly found in the limestones of north-
central Tennessee.
Calcium carbonate, which makes up practically all of a pure lime­
stone, is almost insoluble in pure water. However, it is appreciably
soluble in rain water, which contains small quantities of carbon
dioxide absorbed from the air. Also, it is especially soluble in soil
water, which commonly contains an abundance of carbon dioxide
produced by decomposition of the unstable organic acids leached
from partly decomposed vegetation.42 Under favorable conditions
these natural solvents may percolate considerable distances before
they are neutralized. Rain water or soil water may enter joints or
bedding planes and convert tight fractures into relatively open
crevices * by etching the limestone walls, or it may enter primary pore
spaces and enlarge them by etching the faces of crystals or grains.
The rate at which the limestone is etched depends upon many factors,
of which the principal are the chemical composition and crystallinity
of the limestone, the permeability of the limestone, the rate of cir­
culation of the ground water, the amount and seasonal distribution of
rainfall, the concentration of carbon dioxide and natural acids in the
ground water, the thickness, composition, and texture of the soil,
and the type and density of vegetal cover. For the etching to be
continuous it is essential that the circulation be free, so that the
saturated water will be continuously displaced by unsaturated water
at the surface of the limestone.
As is pointed out by Fuller,44 the sheet form of solution passage is
the first stage in the enlargement of fractures or bedding planes in
dense limestone. The small solution openings first formed eventually
unite into an exceedingly narrow sheetlike opening which by differ­
ential solution may develop into a large irregular cavity. (See pi.
5, A) Under favorable conditions solution may progress until the
limestone is ramified by a network of caverns, some of which may
grow to great size. The smaller channels commonly follow joints or
bedding planes, but some of the larger channels and caverns do not
« Murray, A. N., and Love, W. W., Action of organic acids on limestone: Am. Assoc. Petroleum Geolo­
gists Bull., vol. 13, pp. 1467-1475, 1929.
« Spencer, A. C., U. 8. Qeol. Survey Press Bull. July 17,1922.
« Fuller, M. L., Summary of the controlling factors of artesian flows: U. S. Qeol. Survey Bull. 319,
p. 12, 1908.