A deep dive into the Ice Age cave hiding inside a San Antonio subdivision
Getting into Friesenhahn Cave requires descending a 30-foot creaky metal ladder, through a narrow shaft that opens into a dark one-room cave. Water drips sporadically from its limestone roof, and the cave floor is a mixture of clay, mud and bones — some of them tens of thousands of years old. Some of the most fearsome Ice Age predators used this cave as a den, dragging prey back into the cavern, accumulating one of the most well-preserved fossil records in North America. These predators included giant short-faced bears, dire wolves — and most famously — the scimitar-toothed cat (Homotherium), which banded together to hunt young mammoths and other vulnerable animals. Jennifer Hofmann, a professor at Concordia University Texas who manages the cave, pointed out fossils illuminated by her head lamp. “That’s probably the bottom of a tibia … of a mammoth. It’s big,” she said, pointing to a bone lodged in sediment. “(And) that is probably a Homotherium leg or arm. Just in the last year, that has uncovered itself.” Friesenhahn Cave is located just 20 minutes north of downtown San Antonio. For researchers who study life during the late Pleistocene, “Friesenhahn Cave is unavoidable,” said John Moretti, a paleontologist at the University of Texas Jackson School of Geosciences. Despite being internationally known within the field, most San Antonio residents have never heard of the cave. To some extent, the cave’s owners since 1998 — Concordia University Texas — have preferred it that way. Its entrance is located on 4 acres of private land in the middle of an affluent subdivision, separated only by a wooden fence and brush. Keeping curious neighbors away has not always been easy. Concordia is not the first guard of this Ice Age treasure. Long before the university knew the cave existed, the family that owned the land above it had its own reasons for keeping people away. Old Man Friesenhahn Around the turn of the 20th century, two German brothers by the last name of Friesenhahn arrived in Central Texas looking to buy land. One of them, Alfred Friesenhahn, established a sprawling ranch in Bexar County, near the town of Bulverde, property that included the entrance into an unexplored cave. It’s unclear who first descended the mossy sinkhole that leads down into the cave. One family legend, Hofmann recounted, is that one of the Friesenhahns first explored the cavern and initially thought the large bones that lay in it belonged to their cattle that had fallen down the vertical shaft. The cave was officially surveyed for the first time in 1915 by D. V. Schuchardt, a student at the Agricultural and Mechanical College of Texas (now Texas A&M), who was hired by a research associate at the Carnegie Institution of Washington to study the cave and bring back a “trial collection” of fossils. Schuchardt described the floor of the cavern — which he referred to as “Bone Cave” and “Bulverde Cave” — as being buried beneath 3 feet of fossil-rich sand and clay. He retrieved fossils belonging to 18 different animal species, about half of which were extinct. In 1920, Elias Howard Sellards, a geologist and paleontologist with the University of Texas, had planned on conducting a more thorough excavation of the cave following Schuchardt. But that trip never happened. In fact, no other researchers would be allowed back into the cave for another three decades. The reason why is still debated. What is clear is that the owner of the property wouldn’t let Sellards or anyone else onto his ranch. According to retired University of Texas paleontologist Ernie Lundelius, the reason “old man Friesenhahn” gave for denying researchers access to the cave was that he felt like it was wrong to disturb bones that had been laying there untouched for so long. Lundelius, now in his late 90s, recounted this story last year during an interview with Texas Cache, an online Texas history magazine. But there was another explanation. When paleontologists with the University of Texas were able to conduct the first thorough excavations of the site in 1949 and 1951, they uncovered a plethora of fossils, but also large chunks of “concrete castings.” Opinions on what these large chunks of concrete belonged to are mixed. The initial opinion of some researchers was that they were rotten water troughs from the ranch that had been pushed into the cave. Others, though, thought they resembled castings used to distill or store whiskey. According to Hofmann, a Friesenhahn family member — the nephew of Alfred Friesenhahn who grew up on the ranch — explained that his uncle was selling alcohol to U.S. Army personnel in San Antonio during prohibition, which lasted from 1920 to 1933. The theory is that he had been using the cave to hide or distill the contraband, and that this was the real reason he didn’t want anyone near the cave for so many years. According to David Barkalow, a historian at Wayland Baptist University San Antonio who wrote the entry for Friesenhahn Cave on the Texas State Historical Association website, there aren’t any written historical accounts he’s found corroborating the story. Retired Concordia professor Larry Meissner, who was the cave coordinator before Hofmann from 1997 to 2014, said he was originally skeptical of the story until he heard the Friesenhahn family member’s story. “After hearing the story that Dr. Hofmann was told during an interview of one of the early Friesenhahn family members, I became convinced that there may be some truth to the legend,” he said. Much of the cave’s early history has unfortunately been lost to time, living only with a handful of secondhand accounts from Friesenhahn family members who couldn’t be reached for interviews. Nature’s time capsule Long before the Friesenhahns, around 15,000 to 20,000 years ago, ice sheets blanketed the upper half of North America, stretching as far south as Indiana. Central Texas was a dry, cool grassland with stable temperatures year round, home to mammoths, giant bears, saber- and scimitar-toothed cats and even early humans. It was what we colloquially refer to as the Ice Age, and what scientists would more specifically call the Late Pleistocene or Last Glacial Maximum. During this period, a long ramp declined 30 feet underground into the cave, offering shelter for a variety of animals. At some point, the entrance collapsed, sealing the cave and the bones that lay in it for an unknown period of time. This protected the cave’s fossils from the wind, heat and animals that would have degraded the fossils. A sinkhole later formed above the cave, offering vertical passage into a trove of fossils in pristine conditions — “nature’s time capsule,” as Moretti put it. The cave is not very large, about 30 by 60 feet. Researchers had to bend over to navigate much of the cave, but several feet of fossil-bearing material have been removed from the cave floor, allowing you to comfortably stand in most of the cave. The cave has been such a productive fossil site, in fact, that researchers have been able to make conclusions not just about the animals alone, but also how they interacted with each other and their environment. “Everyone wants to find another Friesenhahn Cave,” Moretti added. The cave is best known for its plethora of fossils belonging to the Homotherium serum, a large scimitar-tooth cat resembling the more commonly depicted saber-toothed cat, or Smilodon. Bones representing as many as 30 Homotherium cats were found in the cave, including full skeletons, Moretti said. Before these remains were recovered from Friesenhahn Cave, the Ice Age cat was known only from teeth and disparate bones “Fossils of those cats are rare everywhere, except Friesenhahn Cave,” Moretti said. “[Some of them] were found and buried in probably the same spot and position that they died in. That’s exceptional.” Researchers believe that over tens of thousands of years, the cats and other predators like bears, dire wolves and coyotes used the cave as a refuge, venturing outside to hunt and then dragging what remained of their prey back to the cave. The scimitar-toothed cats especially seemed to be hunting juvenile mammoths, likely in packs, as evidenced by the large number of mammoth remains in the cave that are damaged in ways consistent with the bite of Homotherium serum, according to a 2021 paper published in Current Biology. “Homotherium may have shared some affinities with the African cheetah,” researchers wrote. “However, Homotherium was truly a cat like no other.” The remains of camels, bison, long-nosed peccary and many other Ice Age animals have been pulled from the cave over the years. All told, the excavations so far have yielded more than 4,000 teeth and bones representing over 30 genera of Ice Age mammals, reptiles and birds. One challenge with Friesenhahn Cave has been getting a clearer picture of how much overlap there was between the various animals that died in the cave. When scientists have attempted to measure the age of the organic matter inside the cave using radiocarbon dating, the results have been inconsistent, Moretti explained. The hypothesis is that the remains accumulated around 15,000 to 20,000 years ago, toward the end of the Pleistocene, but they could also be significantly older, over 50,000 years, Moretti explained. A not-so-well-kept secret In 2001, then a biology undergraduate student at Concordia University Portland, Hofmann had the chance to visit Friesenhahn Cave for the first time. She and other researchers camped out in what was then remote wilderness, “just scrub and cedar,” pitching tents and telling stories around a campfire near the cave’s entrance, Hofmann said. Two years later, when she returned as a graduate student, the cave was essentially in the middle of a neighborhood, flanked on all sides by a subdivision. As development grew, so did rumors about a nearby cave full of Ice Age bones. Today, homes are packed tightly together, sitting right up against the property line. University officials have tried to keep nosey neighbors away. They cut down a sign off the road identifying the cave and even scrubbed local online maps labeling the cave’s location. Inevitably though, kids in the neighborhood — fueled by rumors of an urban legend of a cave filled with Ice Age bones — couldn’t stay away. A few years ago, when local firefighters visited the cave to establish emergency plans in the case of a fire or flood, two of them who grew up in the neighborhood sheepishly admitted to Hofmann that they had been spelunking down in the cave as kids. The researchers have found beer cans in the cave, both modern ones and Lone Star beer cans with a pull-tab from the 1950s — presumably artifacts from the University of Texas researchers. To tamp down on trespassing, a developer built a heavy, locked metal grate above the cave and put up security cameras and no trespassing signs. Still, security remains a challenge. Kids from the neighborhood broke the lock and damaged the ladder years ago. While walking around the perimeter of the property, Hofmann paused to pick up an empty BuzzBallz container, and later found an area of broken fence behind a neighbor’s home. “This is a new one,” Hofmann said, pulling the broken fence back in place. “Not good.” The university has engaged with neighbors and the homeowners association to communicate the importance of preserving the cave. A few neighborhood volunteers help maintain the property, picking up litter and feeding the stray cats that have commandeered the area. There’s also environmental challenges, Hofmann said. The cave’s entrance was once covered in moss and ferns, but the surrounding development has altered the flow of water into the cave. The university purchased additional property around the cave in an effort to prevent more disruption. Concordia, a small Christian college in Austin, is an unlikely owner of Friesenhahn Cave. Located two hours away from San Antonio, the college offers biology and environmental science degrees but doesn’t have a paleontology program of its own. Hofmann herself is a molecular biologist. In 1998, the cave property was donated to them by the previous property owner whose daughter attended Concordia. The owner, who preferred to not be named, was also active in the Missouri Synod denomination of the Lutheran Church. The Austin university is affiliated with — but not governed by — the denomination. According to Hofmann, the owners chose Concordia as the cave’s new steward in part because they wanted to spur conversations between faith leaders about a debated topic within the denomination: the age of the Earth. Although the church’s official position on the age of the Earth is that it doesn’t have an official position, many pastors in the church accept a Young Earth creationist view. Young Earth creationists believe the Earth is 6,000 to 10,000 years old, based on a literal interpretation of the Bible’s Book of Genesis — which stands in contrast to the scientific consensus that the planet is 4.54 billion years old. Concordia taking ownership of the cave has led to some challenging — but very much needed — conversations between faith leaders, faculty and scientists, Hofmann said. Meissner and Hofmann have even brought some church leaders to the cave to discuss what they have found, and what it tells them about the past. They have also brought K-12 students and teachers to the cave with the help of grant funding to teach them about paleontology. “When we acquired the cave we assured the donor that we would engage with both the scientific and theological communities, being as objective as possible and letting the data take us where it takes us,” Meissner said. “I have received overwhelmingly positive responses to this approach from our constituency and the science professionals.” Puzzle pieces Concordia is now gearing up to tell the cave’s story to a wider audience. The university partnered with San Antonio’s Witte Museum to feature Friesenhahn Cave in its recently opened “Adventures in Texas Deep Time” exhibit. It includes a light detection and ranging scan that gives the public a 3D view of the cave. “Now they can really see a three-dimensional picture of what the cave looks like,” Hofmann said. “They can examine some of the specimens from the cave. They can kind of see … where this cave fits into the bigger story of Texas history, which is what this exhibition at the [Witte] Museum is doing.” Although the university is keen on making the cave’s story more visible to the general public, officials still want to keep its location off the map as best they can. That’s because there’s a lot more digging and exploration to be done in Friesenhahn Cave. Researchers have confirmed the existence of a narrow opening in a section of the cave floor that leads into a bus-sized cavern, where many fossils have likely washed into. And there could be many more enclosed limestone-carved caverns in the area. “There are caverns probably all under this, and the question is are they under somebody’s house?” Hofmann said. “It would be a good thing to know. You don’t want a sinkhole opening up.” Moretti has his own list of open questions: How long did the cats actually live in the cave? Is there another complete Homotherium skeleton waiting to be uncovered? What lies behind the original caved-in entrance? Getting answers means doing more fieldwork — something that has fallen out of favor within the field of paleontology since around the 2000s. Research priorities have largely shifted to using new technology to examine fossils already sitting in university and museum collections, he said, and fieldwork is slower, more expensive and often doesn’t pay off for a young researcher the way it may have in the past. Even so, a site as significant as Friesenhahn Cave should be able to attract funding. “It’s Friesenhahn Cave,” he said. “You can get funding.” “We’re trying to build the whole puzzle, but … we don’t have the box lid to know what we’re building toward,” Moretti said. “We know it’s incomplete, but we don’t really know how and in what way it’s incomplete.”