GROUND WATER IN NORTH-CENTRAL TENNESSEE

72, GROUND WATER IN NORTH-CENTRAL TENNESSEE
seem to be related to any preexisting openings of that sort. A single
passage in the Mammoth Cave of Kentucky is more than 8 miles long.
Many such passages are 20 feet high, a few as much as 75 feet high,
and some as much as 50 to 150 feet wide. The great vertical wells of
the Mammoth and other caves have diameters of 10 feet or more and
depths of more than 200 feet. At many places in north-central Ten­
nessee there are similar solution caverns, such as Dunbar Cave, near
Clarksville (No. 53, pi. 4), whose passages are 10 to 20 feet wide and
are reported to sum up to 7 miles in length, and Ruskin Cave, in Dick-
son County (No. 211, pi. 4), which is about 20 feet high and 75 feet
wide at its largest section.
Although the large channels in limestone are formed principally by
solution along preexisting bedding planes or joints, mechanical erosion
doubtless plays a part in the formation of those that transmit turbid
water. For example, the channel of Ruskin Cave maintains a slight
and uniform northward gradient across thick beds of dense cherty
limestone that dip about 5° S. The blanket of alluvial sand and clay
that covers the floor of this and many other large channels is adequate
evidence of the erosive power of the large underground streams that
formerly existed in north-central Tennessee.
Sinks or sink holes are natural openings that extend from the land
surface down to a cavernous zone in the limestone. They are of two
general types solution sinks and collapse sinks and their modes of
origin are described in the following paragraphs.
The solution sink commonly originates at a vertical joint or at the
intersection of two joints, the upper part of the crevice being enlarged
by the solvent action of water that is descending from the land surface
to the zone of saturation. At first the descending water is largely de­
pleted in solvent power before it percolates far below the surface, sa
the deeper part of the joint is not likely to be enlarged appreciably.
The result is a conical depression in the limestone, the base of the cone
being at the land surface and its apex pointing downward. As this
depression increases in diameter and depth, the insoluble soil subsides
appreciably. This subsidence is commonly the first surface indication
of the solution sink; it is very generally mistaken for incipient founder­
ing of the roof of an underground channel. In course of time the walls
of the sink are cut back by solution and possibly by corrasion, so
that a very large surface depression, with or without a functional
swallow hole at its center, may be produced. The diameters and
depths of such sinks in any area afford clues to their relative ages and
to the depths of the channels into which they discharge. The natural
wells, which are rudely cylindrical,, are commonly formed in a similar
manner where etching goels on at about ^tibe same rate from top to
bottom of the original crevice. Both funnel-shaped sinks and natural