Many hurry to and through and away from Mammoth Cave; but let us go in a
more leisurely manner. Suppose we begin by a stroll amid the rounded
hills that environ Cincinnati. We find their flanks full of corals,
shells, crinoids, and other marine objects by myriads. These are
fossils, yet perfect as if freshly cast up from the sea. But we observe
that the limestone lies in thin, level layers, with no signs of volcanic
or earthquake action. They were gently cut down by an undermining
process that left no caverns, because the strata are so thin that they
can not hold together. This is the same Lower Silurian formation that
elsewhere made the famous “bluegrass region,” causing Central Kentucky
to be the fairest bit of the globe’s surface known.
Go by steamboat down the Ohio to Madison, Indiana, and the scenery
changes with the geology. Near the river are still seen the thin blue
limestone strata that we saw at Cincinnati, but capped by the marble
heights of the Upper Silurian. Cascades from the cliffs wash out the
thinner, softer material, making wide, shallow grottoes, each being, as
a rule, at the head of a ravine, which is a cave in ruins.
At the charming city of Louisville we encounter another geological
change, and meet a striking proof that the region was once flooded by
the ocean, namely, the grand old coral reef over which tumble the Falls
of the Ohio. It used to bristle with branching corals like stag-horns
and was strewn with tens of thousands of more delicate varieties,
car-loads of which have since been carried away; but enough remain to
show that all this country was uplifted by continental forces from a
primeval sea. Probably its altitude was once above the present level, to
which it has been reduced by causes some of which are still at work.
Rambling through the valleys and examining their rocky beds, we find
fissures no doubt caused by that continental uplifting to which we have
referred. These cracks, or “joints,” are visible over large areas,
wherever the country rock is exposed. Usually they run at nearly right
angles with one another, north and south lines crossing those from east
to west. The joint-walls may closely fit, or have been parted to make
channels by which falling rain might be drained.
You have noticed that soda-water roughens and eats away the marble slab
on which the soda-fountain rests. On asking the reason you are told that
it is due to the carbonic acid gas (carbon dioxide) with which the water
is charged. In nature this same gas is formed by the decay of animal and
vegetable matter. Rainwater absorbs it from the atmosphere and while
sinking through the loam and soil. It also takes up humous acids, which
aid in the work of corrasion effected on reaching the limestone.
Mechanical energy assists chemical action in slowly dissolving and
removing the limestone particles.
All limestone caves, great and small, were carved by this slow yet
irresistible process. The downward flow follows the joints till a
lateral “bedding-plane,” or something else, turns the stream
horizontally, when there results a widening of the passageway. Should
the roof collapse there would be “a tumble-down” within and perhaps a
“sink-hole” without. Should the cave cut through from one bedding-plane
to another, a series of galleries would result; the upper ones dry as
tinder and the lower ones wet with water that finally reaches the
drainage level, whence it emerges into some open valley.
Occasionally the whirling water bores straight down through all
galleries, making what is termed a pit, or a dome, according to the
point of view. Standing pools deposit nitrous earth and various other
mineral substances. Water trickling through the roof evaporates, each
drop laying down its load of the bicarbonate of lime to create a
stalactite; or a stalagmite if it first falls on the floor. A general
and convenient term is “dripstone,” masses of which are found at almost
any crossing of the joint-planes. Should “fixed air” (carbon dioxide),
which is fifteen times as heavy as the atmosphere, settle into the lower
parts of any cave, it would make visiting dangerous or fatal. But air
currents and other causes make every part of Mammoth Cave free from any
except the sweetest, purest air ever inhaled.