Meredith Guthrie | EPOCH Editorial Board
In 1960, four local college students discovered one of the most expansive and magnificent subterranean landscapes quietly resting below the rugged surface of the Texas Hill Country. The soaring underground chambers and speleothem formations developed when an underground river cut through the prehistoric sedimentary rock. Natural Bridge Caverns, so named for the limestone bridge at the entrance to the caves, is now a lively destination for tourists exploring the local area.
The caverns are vast and dense with formations, and recently animal remains were found at a length of 5,945.7 meters (3.69 miles) in the cavern system. Last January, researchers and members of the Natural Bridge Caverns team spent several days recovering the cache of wildcat fossils that could be anywhere from 200 to 20,000 years old. The remains are currently being evaluated by palaeontologists at the University of Texas at Austin, who expect their findings to be published within the next two years. We reached out to John Moretti, a postgraduate researcher at UT, and Winter Prosapio and Brad Wuest from Natural Bridge Caverns to discuss the logistics of the excavation, the potential significance of the remains, and the role that Natural Bridge Caverns has in preservation and research.
Meredith Guthrie: To get us started, can you sketch out the geological features of the area that allows for these types of caverns to form in the first place?
Brad Wuest: Caverns in this region of Texas are solution caves, and the formations are created by the process of calcite deposition. Basically stated, this region was once covered by a shallow sea, which created layers of limestone. When the sea receded, rains continued and dissolved limestone, creating voids. Those voids collapsed, forming larger chambers. Once all the collapse settled and the cave structure was stable, the formations we see now had vast expanses of time to grow. After all, the rain still falls above ground, continuing to dissolve limestone along the way. As water still seeps in through the soil and rock above the cave, it gathers minerals, which will eventually drop into voids and leave behind minerals (this is the calcite deposition), creating formations. This process is responsible for the highly decorated caverns we have today.
MG: How expansive is this cavern network, and what distinguishes the areas where the animal tracks and remains were found?
BW: Natural Bridge Caverns is the largest cavern system by volume in the state of Texas. Currently, only 20% has been developed for trails. There are two distinct caverns within the system with separate entrances and exits, each with unique features. The first is the Hidden Cavern, which never had a natural opening to the surface and, as a result, is packed with incredibly large, delicate formations. It has two large breakout dome rooms, and the deepest point in the Hidden Cavern is 230 feet from the surface.
The second is the Discovery Cavern, which is where the wildcat bones were found and has large rooms filled with massive formations. It has the largest breakout dome in the state, which is bigger than an American football field and has visibility of a regional aquifer. This is where we offer two ‘adventure’ tours where guests can have an actual caving experience. It’s in the area of one of these adventure caving tours where the first cat trackways were found. No one had ever noticed them before.
MG: What are the logistics of excavating in such a deep and extensive cavern network? I imagine it takes specialised training to complete a dig of this nature.
BW: Vertical training on what is referred to as SRT (single rope technique) is critical for this type of caving. Many caves can be explored without this expertise; however, the bones were located at the bottom of a sheer eighty-foot pit. That made rope training mandatory for our palaeontologist, and John was kind enough to get the necessary skills so he could take part.
People outside the caving world don’t always realise that many of the features you find above ground – cliffs, rock faces, crawlways, cracks – are in a cave. And those features can be even more dangerous in a cave. Our cave is wet and has very sticky and often deep mud, which makes the hiking tough, tight crawl spaces and a lot of pits to descend and ascend which adds to the excitement and challenge. The humidity is really high at 99% and the air temperature is 21 degrees Celsius, which feels hot when exerting yourself. However, when you remain still and wet for long periods of time it’s possible to become hypothermic. It’s also pitch black. You need not only rope training, but experience to bring the safety equipment and backups necessary to stay safe in the cave environment.
John Moretti: Fieldwork deep within a cave system is not terribly different from any archaeological or palaeontological fieldwork on the surface. Like other researchers, I need to collect certain materials and associated data and transport those objects and information back to a base or lab. In any fieldwork, the landscape or physical setting does a lot to dictate how we conduct that work.
In the case of our recent Wild Cat Expedition, I wanted to collect two partial skeletons of ancient wild cats nearly a mile deep inside Natural Bridge Caverns. Getting that far into the cave required leaving the paved public trail in a cavernous room called the Hall of the Mountain Kings and descending down a steep slope into the Valley of the Fallen Lords. From there, our trail took us up and down slick, slippery slopes of ancient bat guano and into a long, tunnel-like horizontal passage. Almost immediately, we began accumulating mud – mud on our boots, our gloves, packs, everything. As we proceeded, the thick mud gathered and quickly had an effect on how we moved and worked.
A lot of our gear is specialised cave equipment made to handle the mud, water, jagged rocks, and tight squeezes. Those supplies hold up where normal field gear fails and help us, at a basic level, get the work done. For most of this journey we can walk upright, but there is a tight, narrow space called the Birth Canal where we can fit through only by laying down flat and sliding forward on our bellies, pushing our packs along in front. Exiting the Birth Canal, we can return to walking upright.
Throughout the horizontal passage, in the undisturbed deposits, there are the tracks – footprints – of small cats pressed into the clay. Those tracks lead up to two successive holes in the floor, pitfalls that open into two enormous rooms, each with an ancient cat skeleton at the bottom. The first of those pitfalls drops ~50 ft. into the Inferno Room. The second pitfall is a narrow, ~20 ft. shaft that leads down to another precipitous portal, this time a nearly 60 ft. drop into The Dungeon. Both of those pits are dangerous and present a logistical challenge. Safely reaching the floor requires us to rappel into both the Inferno Room and The Dungeon.
The predominant method used today is called SRT – Single Rope Technique – and it allows us to rappel (also known as abseiling) down and ascend back up one fixed piece of rope. That was a new skill for me, and I spent a lot of the preceding year practising, first in trees and then in caves, to achieve a basic level of competency. I was trained by several people but learned primarily from the Underground Texas Grotto, my local caving club here in Austin, Texas. Friends and others helped along the way, and eventually I assembled my very own vertical kit, all of the gear I would need to make the final leg of the journey safe and sound. Descending on rope gets us in, but once we collect the bones, those fragile, ancient, skeletal remains must traverse the same obstacles on the way out. We brought a variety of containers, gauze, foam, and plaster bandages to package the material for a safe journey. One of the skeletons exited in water-tight, heavy plastic cases that were pulled up the approximately eighty-foot drop out of The Dungeon and then carried, slid, and packed back to the Hall of the Mountain Kings.
MG: Asking on behalf of our archaeology editor, Alex Rome Griffin, how are you approaching the archaeological context? Are you able to excavate stratigraphically, or are you digging in spits? If so, what are the challenges of piecing the puzzle together without stratigraphy as a guide?
JM: The depositional context of palaeontological (or archaeological) material in caves can be exceedingly challenging to interpret and understand. Caves are notorious for ignoring the basic depositional patterns that govern most surface deposits. Superposition may not (and often is not) present or reliable. To give you an example of these challenges, some of the strange depositional patterns typical of caves are outlined in an excellent short paper by Sutcliffe (1970). Caves also diverge from surface environments in that adjacent areas can have entirely distinct depositional settings and histories. In our case, we could not expect to be able to correlate sediments between the Inferno Room, The Dungeon, and the horizontal passage that connects the rooms. Each setting was distinct and idiosyncratic.
The depositional patterns of caves can be complex, but the situation facing us was relatively straightforward. Little deposition had occurred in both rooms, another odd pattern common to caves, and there was little to no associated biotic material. That made collecting the skeletons fairly straightforward. I set simple datums and recorded the relative position of the skeletal remains in vertical and horizontal space. Brad Wuest and others on the team used a compass and other cave survey instruments to plot the location of the skeletons on detailed maps of each room. With that control established, we could begin to collect the skeletons.
The cat skeleton in the Inferno Room appears to have disarticulated on top of a large talus/debris cone composed of cobble to boulder-sized rocks. The skeletal elements were scattered vertically throughout the voids and crevices between the rocks, some buried in clay, others resting loosely on stone or mud. Recovering the bones required moving rock and excavating through small, discontinuous pockets of clay. Along the way, we were vigilant in observing the sediment and watching for any depositional patterns that could help us understand the taphonomy of the Inferno Room cat. The talus cone was a jumbled pile of rock and clay. Documenting the depositional history of it all would have required a much more intensive investigation. Still, we were able to detect some patterns relevant to the cat skeleton and that data that will help us reconstruct, in a general way, how the elements of the skeleton dispersed and were deposited. The situation in The Dungeon was entirely different. The skeleton in that room was disarticulated and strung out across the surface of an active flowstone (speleothem) formation. Most of the individual bones were embedded in the flowstone cemented down and partially encased in the mineral deposit. In general, flowstone accumulates very slowly (~1 in2/100 years) and offers an initial indication that the skeleton is old, certainly predating the discovery of the cave in the 1960s.
MG: What evidence have you obtained, and how can you date these fossils more precisely?
JM: The assemblage in Natural Bridge Caverns presents a remarkable set of associated evidence – two cat skeletons located in adjacent pit-rooms with cat tracks preserved in the horizontal passage leading to both pit-rooms. That assemblage appears – on the surface – to document the final moments, long ago, of two wild cats as they wandered through the darkness. That apparent narrative needs to be examined and tested with evidence. Are the skeletons actually the same age? Could the trackways have been created by the individuals whose skeletons were in the Inferno Room and The Dungeon?
We are planning to determine the age of the two skeletons using radiocarbon analysis. Radiocarbon has the unique ability to date the time elapsed since the death of a particular organism, making it ideal for our investigation. We will be dating two segments of bone from both skeletons to ensure that the resulting chronology is accurate and robust. Those radiocarbon ages depend on the presence of endogenous collagen in the fossil bones. Modern radiocarbon dating utilises collagen, a protein, and associated amino acids. Those biomolecules are far less likely to be contaminated with younger carbon than the inorganic mineral components of bone. Collagen is far more easily destroyed, however, than the inorganic portion of bone. Large cave systems, with stable temperature and moisture conditions, act as sheltered environments that are capable of preserving biomolecules like collagen and even ancient DNA. As a result, we have a good chance here of obtaining preserved collagen. The resulting dates will help us determine the temporal relationship of the two skeletons – if those two cats really were walking through the cave together, their ages should be statistically indistinguishable.
Still, the cave has provided us with a backup approach if collagen is absent and radiocarbon dates prove impossible or unreliable. The flowstone (i.e., calcite deposit) coating the skeleton in The Dungeon can also be dated radiometrically with Uranium-series analyses. The bones must be older than the flowstone covering them, so a U-series age would provide a useful constraint on the age of that skeleton, but would not be direct evidence of the age of the ancient cat.
The tracks themselves cannot be dated, nor can the red clay that they are typically pressed into. I will be studying the morphology of the limb and foot bones from the two recovered skeletons. We have photos of the individual tracks already, and soon we will be making another trip into the cave to use a custom LiDAR system. The Wild Cat Track Scanner (our LiDAR device) will give us precision laser scan renderings of the tracks complete with quantitative spatial data. Studying the size of the prints and the stride length (i.e., the distance between each track) will tell us about the cats that made those trackways. Comparing the skeletal and LiDAR data gives us a means to determine if the two lines of evidence are consistent and determine if the cats represented as skeletons could have made the nearby tracks.
MG: On the spectrum of archaeology to palaeontology, this dig clearly falls towards the latter. Has there been any interdisciplinarity?
JM: The Inferno Room, The Dungeon, and the overlying passage all lack any cultural materials and likely were never visited by humans prior to exploration in the early 1960s. Even now, few people have ever entered The Dungeon or the Inferno Room. The cats that ended up in those pits are our focus, and they offer to teach us a great deal about the past diversity, distribution, and natural history of small wild cats in Texas. The research here, then, is entirely paleontological. Still, cave research is inherently interdisciplinary because it requires understanding regional sub-surface geology, groundwater dynamics, and the shifting physical interaction between the surface environment and the cave below. Moreover, the research cannot focus solely on the cat bones. From the team constructing the LiDAR scanner, the radiocarbon experts that will turn the bone to graphite and analyse the sample, to the ancient DNA lab where I will be working to extract ancient molecular remains, we cannot do this work alone and rely on our partners with world-class, specialised expertise in a variety of topics to achieve success in our research project.
MG: Coming slightly back up to the surface, are the caves delicate ecosystems? In terms of ecology, what does Natural Bridge Caverns have to take into consideration when pursuing exploratory or commercial objectives?
BW: Stewardship of the caves has always been a core value of the family, and all development has been done with an eye to balancing access and protecting the cave environment. Our cave is still growing, and we take great care with the work above ground to ensure we don’t impact the cave below. We also have doors at each entrance and exit to maintain the 99% humidity, temperature and natural air flow of the cave. We installed weather stations in the cave so we can constantly measure and monitor the environment. Recently we installed millions of dollars of state of the art, programable LED lighting that allows us to greatly reduce energy and minimise the impact of bringing light to the cave environment – and we hired a team from Germany with the necessary expertise to ensure the installation was done carefully without damaging formations. We also partner with scientists to conduct biological inventories throughout the cavern. The diversity and prevalence of cave-adapted species is an excellent indicator of a healthy cave ecosystem.
It's worth noting that the Wuest family was recently awarded a Land Steward Award from the Texas Parks and Wildlife Department, which recognises the work of private landowners who are dedicated to keeping their property suitable for native plants and wildlife.
MG: Natural Bridge Caverns is an institution in Central Texas. How do you connect with local communities, schools, state agencies, universities, etc. to form cooperative partnerships with the area?
BW: It’s through our desire to always learn, improve and give back. To do that we stay engaged on the not only our local level but also state, nationally and internationally. We are blessed to have worked with everyone from enthusiastic science teachers to geologists researching million-year-old climate data. We do this because we are as passionate and curious as they are, and we do all we can to search for answers. What kinds of cave-adapted species are in our cave, we wondered. We reached out to biologists for answers, conducting a major biological survey of the cave (which, by the way, is how we ended up learning about the wildcat trackways). Did the cave serve as a shelter for people in the past? We asked archaeologists to come out and study the area. Due to the significant archaeological finds in the Natural Bridge Caverns entrance sinkhole the site is now in the National Register of Historical Places. How can we teach children about this region beyond the cave walls? We partnered with the Texas Wildlife Association, a wildlife education group dedicated to teaching kids to value the land and the animals who also make it home.
We are fortunate that our curiosity about this incredible natural wonder is shared by so many, and we’ve been dedicated to ensuring that we learn all we can and add to the body of knowledge when possible. Never stop discovering is a brand pillar of Natural Bridge Caverns.
MG: Are additional areas under exploration?
BW: Our cavern is the most actively explored cavern system in the state. At the time this article was being prepared we started the recovery of a third wildcat skeleton. We plan to continue exploring as long as this cavern continues to present opportunities for us to look beyond the next bend. We have no formal programs at this time, but we are open to hearing from the scientific community about ways our cave can offer insights into problems or persistent questions.
We would like to thank Winter Prosapio, John Moretti, Brad Wuest, and Lexi Wuest for taking the time to discuss the recent developments at Natural Bridge Caverns and for sharing many of the images that have appeared in this article.
Further Reading:
Sutcliffe, A. J., ‘A section of an Imaginary Bone Cave’, Studies in Speleology, 2, ii (1970), pp.78-80.
John A. Moretti is a PhD Candidate in the Jackson School of Geosciences at The University of Texas at Austin. John is a paleontologist studying patterns of vertebrate diversity and distribution throughout the past 3-5 million years with a particular focus on the Quaternary and the natural history of extant species and associated communities. John is currently engaged in a number of projects examining the temporal distribution and climatic context of vertebrate taxa based on remains preserved in caves and karst across the Edwards Plateau of central Texas in the United States.
Editorial note: This article was compiled from several text-based exchanges between the EPOCH team and members of the excavation team. The correspondence has been edited and collated for clarity, and the final format has been reviewed and approved by all contributors. All sections of text have been revised to British spelling for continuity throughout the article.