OHIO CAVERNS
The largest known cave in
Ohio is Ohio Caverns, just east-southeast of the town of West Liberty, northern
margin of Champaign County, western Ohio, USA.
The cave is developed in an
NNW-SSE elongated knob of Columbus Limestone (Eifelian Stage, lower Middle
Devonian) that is capped by a little Ohio Shale (Upper Devonian). Western
Ohio generally has Silurian-aged surface bedrock. However, the
Bellefontaine area of western Ohio has an "island" of Devonian-aged
surface bedrock. This is the Bellefontaine Outlier, the highest
point in all of Ohio. The Columbus Limestone knob at Ohio Caverns is near
the southern margin of the Bellefontaine Outlier.
Above: Looking S at northern edge
of Columbus Limestone knob, southern Bellefontaine Outlier. Ohio Caverns
is developed near the crest of knob on the eastern side of the road (left side
in this picture), behind the tall tree at left.
Above: Looking NNE down the
northern margin of Columbus Limestone knob. Ohio Caverns is developed in
the subsurface of the area behind the photographer here. The hills in the
distance are part of the central Bellefontaine Outlier.
Above: Looking ~SE. Ohio
Caverns occurs directly below the land surface here. The small whitish
building behind the two tall trees is the cave exit house.
COLUMBUS LIMESTONE
The Columbus Limestone
outcrops in a north-south trending belt from southern Ohio to central Ohio to
the Lake Erie islands. Like all limestones, it is prone to dissolution by
acidic fluids & cave development. Not all areas with limestone
bedrock will have significant cave development, however.
Above: Columbus Limestone along
walls of Ohio Caverns, near the modern entrance.
Many of the walls of Ohio
Caverns have readily observable Columbus Limestone bedding. In places,
the limestones are coated & obscured by secondary cave mineral
deposits. The Columbus Limestone here has the same general lithology
& appearance as surface outcrops in central & northern Ohio. The
formation consists of light brownish-gray, fine-grained limestones to
fossiliferous packstones with several well-developed stylolite horizons &
chert nodule horizons.
Above: Wall & ceiling of Ohio
Caverns showing bedding nature of the Columbus Limestone. The subrounded
to eye-shaped structures along the wall are chert nodules.
Above: The jagged dark line is a stylolite
developed in the Columbus Limestone & exposed along a wall in Ohio
Caverns. Stylolites are moderately common in limestone successions.
They form by limestone dissolution from significant tectonic and/or burial
pressure.
Above: The cave ceiling here is a
stylolite surface. Stylolites represent surfaces of weakness. At
Ohio Caverns, stylolites often define the upper separation planes for collapsed
limestone blocks.
Above & below: Blocks of cherty limestone
that have collapsed from the ceiling of Ohio Caverns.
CHERT NODULES
Chert nodule horizons are
moderately common in Columbus Limestone outcrops of central Ohio. They
are also common along the walls of Ohio Caverns. They often protrude from
the cave walls. This is a result of differential weathering &
differential erosion. The chert nodules are hard (H = 7 on the Mohs
Hardness Scale) and not easily prone to natural acid dissolution. The
surrounding limestone/calcite matrix is relatively soft (H = 3 on the Mohs
Hardness Scale) & readily dissolves in carbonic acid & organic acids in
downward percolating groundwater.
Above & below: a fossiliferous chert
nodule with an obvious strophomenid brachiopod shell cross-section & a
large solitary rugose coral.
PASSAGES
Ohio Caverns does not have a
very long cave system. Most of the passages are well-developed canyon
passages, characterized by being relatively tall & narrow. The
canyon passages at Ohio Caverns tend to follow straight lines for some
distance, presumably due to limestone dissolution along planar subvertical
joints.
Above: Canyon passage in Ohio
Caverns.
Tubular passages have elliptical to rounded
shapes in cross-section.
Above: Tubular passage at Ohio
Caverns with multicolored staining on the walls & ceiling plus minor
speleothem.
Many of Ohio Caverns'
passages have shape characteristics of both canyon passages & tubular
passages (see pics below).
Below: Dripping & standing
water can be observed in many of the passages at Ohio Caverns, which is still
an actively-forming cave.
Much of the passage system
in Ohio Caverns was filled with muddy sediments when first discovered in the
late 1890s. Some of the smaller side passages still have muddy sediment
fills (see pics below).
STALACTITES
Stalactites are mineral icicles hanging
down from cave ceilings. They are one of many varieties of speleothem,
the general term for all secondary cave mineral deposits. Speleothem that
forms by dripping water is called dripstone. Stalactites and their
equivalents on cave floors - stalagmites - are the most common & easily
recognizable forms of dripstone.
Most stalactites at Ohio
Caverns are composed of the mineral calcite (CaCO3 - calcium
carbonate), as many of the world's stalactites are. The rock making up
these calcite structures is often considered to be a variety of limestone
called travertine. It really shouldn't be called a variety of
limestone. "Travertine" as a stand-alone term is probably best.
Stalactites form as a result
of downward-percolating groundwater dripping from the ceilings of subterranean
cavities (see photos below). Groundwater tends to partially dissolve
limestone bedrock, which is composed of calcite. This happens because
groundwater is slightly acidic - it contains some carbonic acid & organic
acids. The carbonic acid in groundwater is acquired as water percolates
through soils & fractures & pores in bedrock, where the partial
pressure of carbon dioxide gas is higher than it is at the surface. A
higher partial pressure of carbon dioxide gas results in a rightward shift of
the following equation:
H2O (water) + CO2
(carbon dioxide gas) ¬¾® H2CO3
(carbonic acid)
So, groundwater becomes
slightly acidic. It dissolves acid-soluble minerals, such as calcite (see
equation below).
CaCO3 (calcite) +
H2CO3 ¬¾® Ca+2
(calcium ion) + 2HCO3- (bicarbonate ion)
Once downward percolating
groundwater reaches the ceiling of a subterranean cavity, the water encounters
air with a lower partial pressure of carbon dioxide gas. The two chemical
equations given above then shift to the left, and calcite precipitates.
Most people think that the calcite precipitates as water evaporates.
Well, most caves are quite humid (cave air is close to being water saturated),
so the water isn't evaporating. The calcite in stalactites & other
speleothems gets precipitated as a result of a CO2 partial pressure
change.
Crystal King Stalactite, the largest known in Ohio
Caverns. It's just shy of being five feet long. It is composed of
nice, white travertine.
STALAGMITES
Stalagmites are dripstones that form on
cave floors. Many stalactite-stalagmite pairs are asymmetrical in size
(i.e., large/long stalactite on the ceiling with a small stalagmite on the
floor or a small stalactite on the ceiling with a large/tall stalagmite
on the floor). Slowly dripping water results in large/long stalactites on
the cave ceiling and small stalagmites on the cave floor. Quickly
dripping water results in small stalactites and large/tall stalagmites.
SODA STRAWS
Soda straws are narrow, cylindrical
stalactites. Ohio Caverns has some well developed soda straws, although
many are broken. Some of them are thicker than typical soda straws, and
are slightly tapering, rather than cylindrical.
Above: a long soda straw with a
stalactite at its distal (bottom) end.
COLUMNS
Columns are dripstone speleothems
that form when stalactite-stalagmite pairs become physically joined by calcite
precipitation. They vary considerably in height and width.
Above: Soda straw column in Ohio
Caverns having an irregular width.
Above: Nice soda straw column in
Ohio Caverns.
Above: An odd column that is
barely connected to the cave ceiling.
Above: Lateral & bottom views
of a suspended column. This is a column that appears to be
floating in air - it's just attached at the ceiling. This is not a
completely natural feature. The base of this column represents the
original floor of the cave. This passage was partially filled with mud,
which has since been dug out in the process of making Ohio Caverns a tourist
cave. Additional suspended columns are shown below.
LOTS O’ DRIPSTONE
HELICTITES
Helictites are twisted, contorted
speleothems growing from cave walls & ceilings that appear to defy
gravity. They do not form by dripping water, as stalactites &
stalagmites do, nor by flowing water, as flowstones do. Helictites form
principally as a result of water moving by hydrostatic pressure &
capillarity. Hydrostatic pressure pushes water from pores in a cave wall,
and a CO2 partial pressure change results in calcite
precipitation. A small calcite protrusion is now present on a cave wall.
Capillary action draws additional water sideways through the protrusion, and
more calcite precipitation occurs. The spiralling, twisting, and
bifurcation seen in many helictites is caused by rotation of calcite
crystallographic axes during precipitation events and by the presence of
impurities in the precipitated material.
The three pics shown below
have nice soda straws & helictites on the ceiling of a room in Ohio
Caverns. These are composed of calcite. Many of the helictites here
started forming at the distal tips of soda straws. Some have grown upward
& attached to the ceiling, apparently in defiance of gravity!
DRAPERIES
Draperies are planar to subplanar to
wrinkled speleothem curtains hanging from the ceilings or inclined walls of
caves. They form by calcite precipitation as individual water drops run
down inclined surfaces. In many caves, draperies have reddish-brown &
whitish color bands parallel to the lower surface. Such strutures are
appropriately called "cave bacon" or bacon draperies.
Some info. provided by Hill
& Forti (1997 - Cave Minerals of the World, Second Edition).