GROUND WATER IN NORTH-CENTRAL TENNESSEE

OCCURRENCE OF GROUND WATER IN LIMESTONE 73
wells also form underground between solution channels at two different
levels.
. The collapse sink, as the name implies, is formed by foundering of
the roof of a subsurface channel. Its formation depends upon many
factors, of which the principal are the strength and thickness of the
roof strata, the orientation and spacing of joints in the roof beds, the
inclination of the strata, the depth of the channel below the surface,
the width, height, and shape of the channel, and the weakening of the
roof strata by solution. Obviously, there are many possible combina­
tions of circumstances that will cause collapse. Generally a collapse
sink flares upward, and its diameter at the surface is related to the
depth and to the span of the channel. Collapse sinks range from
shafts a few feet in diameter caused by subsidence of a single joint
block to depressions many hundred yards across caused by foundering
of the rocks above an extensive cavernous zone. Valleys several miles
long may be formed by gradual collapse of the roof above a major
underground stream.
The calcareous rocks differ appreciably with respect to their strength,
to resist fracture and their solubility. Dolomite and magnesian lime­
stone are less soluble than pure calcareous limestone, but when they
are subjected to weathering they may also become porous or even
cavernous by solution. Earthy limestone and calcareous shale are
intermediate in composition between limestone and shale; they are
also intermediate in water-yielding capacity. They are generally less
cavernous than limestone but somewhat more friable and brittle and
hence more jointed than shale. Several of the formations in north-
central Tennessee comprise alternating beds of limestone and thin
layers of shale. In such rocks a bed of shale may check the downward
percolation of ground water and localize the formation of solution
channels in the lower part of a limestone bed. Other beds of shale
may act as ground-water dams. Some limestone, especially certain
shaly beds, contains crystals and small masses of gypsum, the hy­
drous sulphate of calcium, which is dissolved readily by water without
the presence of natural acids. Such rocks may become very highly
cavernous when leached by circulating water.
Where a body of ground water in limestone has a free upper surface
or water table, the limestone is presumably dissolved most rapidly in
the zone between the highest and lowest positions occupied by the
water table in its seasonal fluctuations, for in that zone the ground
water percolates relatively rapidly and is most likely to contain natural
acids. The limestone is also presumably dissolved above the water
table and to a relatively shallow depth below the water table, for there
likewise the ground water circulates rather freely. However, the
ground water at considerable depth below the water table probably
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