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.