ROBBER BARON PRESERVE

Biology

A large variety of species live in Robber Baron cave, although they might not be noticed by most visitors.  Besides the occasional pack-rat and very rarely, a bat, these species are all invertebrates. They include:

These species can be largely divided into Trogloxene, Troglophile, and Troglobite. A Trogloxene enters a cave just for reasons of food, to escape summer heat, or to hibernate, but does not live there. A Troglophile lives a portion of its life in a cave but also lives part of the time above ground. A Troglobite, on the other hand, is so well adapted to the underground environment that it is only found in caves and does not survive well on the surface.

At least ten separate troglobitic species have been found in Robber Baron. These include two species of isopods, two species of spiders, a harvestman species, two species of millipedes, one species of Earwiglike entotrophs, a silverfish species, and an antloving beetle species. [1]

In December 2000, two of these were federally listed as Endangered Species (along with seven other troglobite invertebrate species in Bexar County.)  These are the Cicurina baronia (Robber Baron Cave Spider) and the Texella cokendolpheri (Robber Baron Cave Harvestman). Both of these species have been observed only in Robber Baron Cave. They are very small in size, only a couple of millimeters, essentially eyeless, and mostly lacking in pigmentation. These species are well adapted to caves, having a low metabolism due to the lack of food, and long legs for efficient travel and to feel their surroundings.

More information about endangered species in the Edwards Aquifer karst can be found here.

Cicurina baronia (Robber Baron Cave Spider). Photo by Jean Krejca.

Geology

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. [1]

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).  [2] Recently a new theory has been proposed that explains maze caves as a result of ascending transverse speleogenesis. [3]  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.

Meteorology

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.

References

[1] Veni, G., "The Caves of Bexar County", Speleological Monographs. 2, Texas Memorial Museum, Univ. of Texas at Austin, 300 pp., 1988.

[2] Palmer, A.N., "The Origin of Maze Caves", NSS Bulletin, v37, n3, pp. 57-76, July 1975.

[3] Klimchouk, A., "Hypogene Spelelogenesis: Hydrological and Morphogenetic Perspective", National Cave and Karst Research Institute, Special Paper No.1, 106pp., 2007.

For more information please e-mail joe.mitchell@tcmacaves.org

Preserve Manager: Joe Mitchell

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