Robber Baron Cave is one of the most significant of the over 500
Bexar County (San Antonio), TX caves. It is the longest known cave
in the county with 4,961 feet of mapped passages. This cave has
many interesting features including a large sinkhole entrance, a
geologically complex two-dimensional maze of passages, a rich
history, and several unique species which live only
underground.
Robber Baron Cave probably has an origin related to the Edwards
Aquifer, even though the cave is in the Austin Chalk limestone.
Biological studies have discovered several blind invertebrate
species found nowhere else on Earth. In December 2001, two of these
species were listed as endangered by the US Fish and Wildlife
Service (USFWS). The cave is also the only one in the county that
has been open to the public as a tourist attraction, when from 1926
to 1933 an estimated 300,000 people toured the cave. As San Antonio
grew, the cave became part of a densely urbanized area and subject
to problems of vandalism, graffiti, and unauthorized
visitation.
Acquired by TCMA in 1995, Robber Baron is now protected as a
karst resource to not only preserve its unique biology, geology and
history, but also to provide a place where people can learn about
and experience the underground environment.
Robber Baron Cave is a maze cave with a complex set of
interconnected passages that generally intersect at right angles.
The cave is found within the Austin Chalk formation which underlies
must of north-central Bexar County. The Austin Chalk is one of the
upper confining layers of the Edwards Aquifer, and in the location
of the cave is an upthrown fault block. It is a soluble formation
meaning that water flowing through small cracks can, over long
stretches of time, dissolve the rock (in contrast to physical
erosion.) Austin Chalk is also relatively soft and rather clay-like
(and less soluble) in its upper layers. It appears that the cave
may have formed in two periods, the first one of which established
the basic layout of passages. A second period followed after the
cave opened to the surface when runoff played a role in enlarging
some passages, while partially filling others with sediments.
There are several theories to explain the origin of the cave
(its speleogenesis). One involves aggressive water flow near
locations of constrictions in the main flow (floodwater mazes).
Another states that slow and dispersed flow through an upper rock
layer that is not soluble could form such caves (diffuse recharge).
Recently a new theory has been proposed that explains maze caves as
a result of ascending transverse speleogenesis. In this theory,
water from an underlying aquifer is hydraulically forced up into
overlying beds of rock. The water then travels horizontally through
fractures in this bed for long distances along multiple paths. If
fractures that are differently oriented exist in the overlying bed,
the water may be forced up into these and form crossing passages.
Characteristic features of this theory of speleogenesis are the
presence of small orifice-like feeders, which are small rounded
"ear canal" like openings in the floors of passages, along with
cupolas, which appear as a series of small domes in the ceilings of
these passages. Robber Baron has numerous such feeders and cupolas,
and local conditions make the other theories unlikely.
The cave maintains a near constant temperature of 68-70 degrees
F. Although this may sound pleasant, the relative humidity is near
100% which leaves no place for sweat to evaporate. Air flow in this
cave, as with most caves, is dependent on local barometric
conditions. As air pressure falls (such as from a passing storm)
the lower pressure outside will draw air out of the cave causing a
breeze in passages toward the entrance. During periods of rising
pressure, the reverse is true. Some small passages deep inside the
cave also exhibit airflow indicating the presence of cave passages
beyond that which are known.
One of the main features of the cave is the presence of
significant amounts of carbon dioxide (CO2) in certain parts of the
cave, known as "bad air". These areas tend to be in cut-off or
lower sections of the cave where circulation is less and where CO2,
which is heavier than air, can pool. CO2 levels generally seem to
be worse in the summer months than the winter. More frequent
pressure changes from cold fronts along with a greater temperature
differential may combine to help flush out the CO2 in winter.
During the summer, some portions of the cave may be problematic to
enter as the high CO2 levels can result in extreme shortness of
breath, even when not moving, along with dizziness and
disorientation. These symptoms disappear quickly when returning to
areas near the entrance (although a headache may remain after
entering an especially bad area). One theory regarding the origin
of the CO2 is that it may arise from chemical reactions of water
with the surrounding rock, or alternatively, that it may be
out-gassing from lower rock layers within the aquifer.
The formation and development of caves is known as
speleogenesis. Caves are formed by various geologic processes.
These may involve a combination of chemical processes, erosion from
water, tectonic forces, microorganisms, pressure, atmospheric
influences, and even digging.
Most caves are formed in limestone by dissolution.
Speleothems in Hall of the Mountain King, Ogof Craig a Ffynnon,
South Wales.
Solutional caves form in rock that is soluble, such as
limestone, but can also form in other rocks, including chalk,
dolomite, marble, salt, and gypsum.
The largest and most abundant solutional caves are located in
limestone. Limestone dissolves under the action of rainwater and
groundwater charged with H2CO3 (carbonic acid) and naturally
occurring organic acids. The dissolution process produces a
distinctive landform known as karst, characterized by sinkholes,
sinking streams, and underground drainage. Limestone caves are
often adorned with calcium carbonate formations produced through
slow precipitation. These include: flowstones, stalactites,
stalagmites, helictites, draperies, soda straws and columns. These
secondary mineral deposits in caves are called speleothems.
The world's most spectacularly decorated cave is generally
regarded to be Lechuguilla Cave in New Mexico. Lechuguilla and
nearby Carlsbad Cavern are now believed to be examples of another
type of solutional cave. They were formed by H2S (hydrogen sulfide)
gas rising from below, where reservoirs of oil give off sulfurous
fumes. This gas mixes with ground water and forms H2SO4 (sulfuric
acid). The acid then dissolves the limestone from below, rather
than from above, by acidic water percolating from the surface.
Karst topography is a landscape shaped by the dissolution of a
layer or layers of soluble bedrock, usually carbonate rock such as
limestone or dolomite.
Due to subterranean drainage, there may be very limited surface
water, even to the absence of all rivers and lakes. Many karst
regions display distinctive surface features, with sinkholes or
dolines being the most common. However, distinctive karst surface
features may be completely absent where the soluble rock is
mantled, such as by glacial debris, or confined by a superimposed
non-soluble rock strata. Some karst regions include thousands of
caves, even though evidence of caves that are big enough for human
exploration is not a required characteristic of karst.
To get credit for this cache answer the following
questions:
- This cave is found in an upraised section of what
formation?
- What was discovered here that is found nowhere else on
earth?
- Using your GPS find the depth and width of the sink hole in
front of you.
