When the Earth Opens
While sinkholes occasionally swallow homes and roads, they also play a positive role as wildlife habitat.
By Russell A. Graves
Standing at the precipice of the depression, I strain to peek over the edge while maintaining my balance.
"How deep is it?" I ask my longtime friend Dana Wright. She leans over next to me and in an easygoing West Texas fashion opines, "At least two lariat ropes."
"Sixty feet..." I ponder while simultaneously imagining what it was like the instant the earth cleaved and swallowed the hunk of red dirt and cactus. I can't quite grasp whether the hole formed all at once or over time. Either way, it is spectacular.
Dana is a biologist for the Texas Parks and Wildlife Department, and our friendship goes back to the early 1990s, when we both moved to Childress. Like me she loves natural oddities and is a great scout whenever she finds something new. Here in Cottle County, we stand at the edge of a vast vertical sinkhole she discovered on a patch of ranchland.
Sinkholes are enigmatic. They aren't that common but they aren't rare either. Part geologic anomaly and part mysterious apparition, sinkholes may be a lot more common than people think. Unlike a smoking caldera or voluminous mountain, sinkholes don't strike any romanticized chords in the popular press. And unless one of the chasms opens up and swallows homes or modern infrastructure, their appearance mostly goes unheralded.
Sinkholes occur over much of the United States. In states like Florida, sinkholes often make news because of the damage they impart when they form in populated areas. In Texas, however, most sinkholes go unnoticed. In areas where they are likely to form, people may pass by them all of the time and never know they exist.
A perfect mix of geography and geology
A sinkhole is a natural depression that's formed when subsurface limestone, salt or gypsum is slowly eroded away by groundwater. As surface water infiltrates the soil, it percolates downward and moves deeper into the soil. Over time, the water eats away at the rock layer until voids, or caves, form in the rock. As these voids grow, ultimately the spaces between the rocks become too big and the weight of the earth on top of the rock causes the chamber to collapse.
Natural sinkholes most commonly form in the karst regions of Texas. Karst is an area of irregular limestone in which erosion has produced fissures, sinkholes, underground streams and caverns. In Texas, high concentrations of karst rock occur in the soluble limestone areas of the Hill Country and the gypsum-rich Rolling Plains of northwest Texas.
It is possible, however, for unnatural sinkholes to form. In urban areas, water main breaks can erode the subsoil and cause the earth above to cleave.
Sinkholes are also formed when groundwater or oil is removed from the subsurface, leaving a void in the ground. Because of excessive water extraction and unstable soil in Mexico City, a sinkhole opened up in the summer of 2007 and killed one man.
Perhaps the most heralded human-caused Texas sinkhole occurred in 1980 near the West Texas town of Wink. On June 3, residents woke to find a 370-foot-wide, 110-foot-deep hole had formed 2 1/2 miles north of town. Geologists suspect the sinkhole, also known as the Wink Sink, formed as a result of historic oil production practices in the Permian Basin that pumped saltwater from below the surface, leaving a void beneath. In May 2002, a second sinkhole opened up nearby. The new sinkhole dwarfed the first one, at 900 feet wide and more than 300 feet deep.
While the huge sinkholes that seem to instantaneously swallow the earth around them get most of the media attention, most of the depressions form more slowly. Geary Schindel, chief technical officer for the Edwards Aquifer Authority, says that scientists classify natural sinkholes based on their means of formation. "The most common that occur in Texas are formed from a couple of processes," says Schindel. "They form when the overlying soil collapses or is washed into a cave - this is usually a gradual process or more rarely a catastrophic event. They also form through the dissolving of bedrock with water entering a closed depression." Schindel explains that they can form when the roof of a cave collapses, although that type of formation is relatively rare. Schindel is a man who knows his sinkholes. As part of his job with the water authority, he's studied caves and sinkholes for the past 30 years and has traveled to 35 states and five countries.
In July 2007, pilot Jason Smith and I flew to Lakeview to photograph a naturally occurring sinkhole that suddenly appeared about 10 years ago along the edge of a cotton field in Hall County. The cotton field isn't too far from the Red River and is rich in salt layers just below the soil surface. Though neither one of us knew where the sinkhole was located, we easily spotted it as we cruised in a helicopter 500 feet above the cotton fields.
Like the Wink Sink, the Lakeview sinkhole was of significant size. At more than 400 feet wide, the circular sinkhole enveloped 3.25 acres when it collapsed. What made the sinkhole easy to spot was the water glistening in the depression. In this semi-arid climate, it was apparent that groundwater - not surface runoff - filled the hole. Although nearly at capacity, striation marks were easily spotted in the dirt, signifying where the dirt sloughed off into the hole.
As we flew away from the sink, it struck me how the sinkhole served as a direct conduit to the underground aquifer.
"Most of the recharge occurring in the Edwards Aquifer occurs in the beds of streams. These streambeds contain conduits into the aquifer, but most of them are not apparent, as they have been covered by stream cobbles and gravels. However, these would technically be buried sinkholes," Schindel explains.
As beneficial as sinkholes are to aquifers, there are potentially negative impacts on underground water as well. According to Schindel, sinkholes are under the constant threat of contamination by outside influences.
"Since they are often considered nuisances and unproductive land by some, they have commonly been used for disposal of household trash and appliances, agricultural products such as herbicide and pesticide containers, old cars and dead animals," he says.
"Considering they provide a direct link to the groundwater system, they also are a likely source of contamination."
A unique wildlife habitat
On my tour of Texas sinkholes, I felt it important to see the most prominent natural sinkhole in all of the state - the Devil's Sinkhole. So in July 2007, I took my family to witness the nightly bat flight and see the sinkhole firsthand.
At the Devil's Sinkhole Visitors' Center in Rocksprings, we boarded a bus painted in a bat motif to take a tour of the Texas Parks and Wildlife managed property. On the way out, tour guide Ben Banahan gave us a brief rundown on the history of the sinkhole and how a local rancher discovered it in the late 1800s. We arrived at the property well before sunset and had a chance to explore the area around the sinkhole. The landscape was typical of the Edwards Plateau - plenty of juniper, prairie grasses and cactus. Heavy rocks rimmed the sinkhole, and as I walked onto the observation deck, I was amazed how deep the hole plunged straight down.
The sinkhole is 351 feet deep, although I could see only about 200 feet down. The smell of moist cave air and bat guano belches up from the sinkhole's depths. Understandably, the sinkhole is a source of local lore, and it almost became a source of wealth also. Guano miners tried to harvest the abundance of bat droppings that were once a valuable source of fertilizer as well as an ingredient in gunpowder. Mining the guano proved too difficult and the effort was abandoned.
Right at sunset, I noticed the first flicker of bat wings emerging from the sinkhole. A few minutes later, thousands exited and streamed to the agricultural fields to the southeast. In only about a half hour's time, more than 3 million bats emerged from the sinkhole's depths.
Randy Rosales, superintendent of the Devil's Sinkhole State Natural Area, understands the importance of sinkholes as wildlife habitat. "Sinkholes provide a unique environment which can support life for important plants and wildlife," he says. "These bats can consume up to 30 tons of beetles and moths per night, many of which are agriculture pests. That's the weight of about 20 mid-size cars or 15 elephants!"
Rosales says that other types of wildlife live in the sinkhole as well. About 3,000 cave swallows nest in the hole, as well as a myriad of beetles and insects that feed on dead bats and guano. At the bottom of the sinkhole, where a small subterranean lake formed, you'll find a small shrimp-like creature known as the Devil's Sinkhole amphipod (Stygobromus hadenoecus). It lives only in this one spot and nowhere else on earth.
"The bat flights at night attract many predators as well, some that even roost inside the sinkhole, like the great horned owl. Oftentimes you can see owls catching bats as they exit the cave," he explains.
Sinkholes all over Texas, both big and small, provide habitat for skunks, raccoons, snakes, salamanders and a host of other creatures. Ferns and other plant life can thrive in sinkholes as well. Archaeological records show that the Bering Sinkhole in Kerr County was used as a cemetery by people 5,500 years ago.
In August 2007, Jason Smith and I again headed to the Lakeview sinkhole to take some pictures. I scanned the ground below as Jason piloted the helicopter. From our bird's-eye view, I spotted numerous sinkholes. In fact, Jason has an oval-shaped one on his ranch atop a big hill.
As we circled the Lakeview sink, I noticed two more sinkholes in close proximity. While they probably won't grow to the size of the big sinkhole, their presence alters the way the farmer cultivates his land. It seems that no matter how much man manipulates his surroundings, he never is in complete control - especially in sinkhole country.