On September 26, 1934, Adrey E. Borell drove his National Park Service vehicle along the south ridge of the Grand Canyon, answering a strange call. As an associate naturalist technician for the Service’s Wildlife Division, Borell was often called in to deal with crises and other matters, and, on that day, had received a report about birds in distress.
Borell soon saw the carnage for himself.
That day alone, Borell recorded 123 birds recently or long dead. Among them were woodpeckers, jays, sparrows, and several large hawks. The cause was immediately obvious to him: all had been caught in a thick, sticky black liquid seeping from the ground. They had become the newest victims of a natural phenomenon that had been similarly trapping and killing animals for tens of thousands of years. These unlucky birds in Arizona had stumbled into what’s commonly called a tar pit.
Black Gold
Petroleum is one of the fundamental materials of the modern world. Formed by the breakdown of ancient algae and other living cells, it’s the source of not only fuels like gasoline and diesel but also plastics, paint, fertilizers, and a long list of other chemicals ubiquitous in our everyday lives. In its natural form, often called crude oil, petroleum is a dark viscous substance sometimes tinted yellow. Humans mostly drill for it deep underground, always seeking out new deposits of this “black gold” to feed the hunger of human industry. But in some places, no such effort is necessary. There are places in the world where petroleum bubbles freely up from the Earth.
Petroleum is a soup of different hydrocarbons, organic compounds made of carbon and hydrogen atoms. The specific composition of crude oil varies by source. These hydrocarbons are all different weights, and the lighter ones don’t long survive contact with the air. They can easily evaporate and degrade, and in a relatively short time out in the open, the loss of these light hydrocarbons leaves behind a thicker, stickier substance. Cooler air can make the substance harden faster, eventually causing a durable crust to form. We use several names to describe this material in its various liquid and solid forms, including asphalt, bitumen, pitch, and tar—though the composition of each varies.
Petroleum tar is incredibly sticky, and what’s commonly referred to as a “tar pit” would, in fact, be more accurately described as an “asphalt” pit. Most things that come into contact with naturally occurring asphalt and tar are immediately painted with this natural glue. Insects are ever at risk of being caught by it, as are particles, like pollen, traveling through the air. But if a seep is wide and deep enough, it can catch a lot more than a gnat. Even the largest animals unfortunate enough to step in the black muck can find themselves stuck and eventually sink into the deadly trap.
The Deep Human History of Tar
Records of tar seeps, pits, and other natural petroleum products go back at least as far as Biblical times, and likely much further. The story of Noah mentions the use of pitch to reinforce his ark. The parable of the Tower of Babel, a narrative set at about 2000 BCE, states that “they had brick for stone, and slime had they for mortar.” This slime is likely the same petroleum pitch. Both stories almost certainly borrow from the common shipbuilding and construction practices of their time. The use of petroleum might, in fact, have already been ancient when these stories were first told. Bitumen-treated flint tools have been found in Syria from as far back as 68,000 BCE.
“Bitumen was the adhesive of antiquity,” writes J. Connan, in his comprehensive examination of prehistoric petroleum use and trade. Bitumen
was used extensively to repair broken statues or pottery; to fix flint implements on the wooden handles of sickles; to manufacture handles for small tools, such as chasing-chisels made of sharpened bones; to cover surfaces of artefacts subsequently decorated with mother-of-pearl, black shale, pink carbonate or lapis lazuli.
The most common ancient use of petroleum tar seems to be in construction, the “slime” of the Babel story. Mixed with clay, sand, and straw, it was an easy, available, and durable mortar for binding together stones or bricks. Long before it fueled the human world, petroleum-based materials helped to build it.
A Lost World, Preserved
In 1769, a party of Spanish colonists trekked into what is now California’s Los Angeles County. According to Florence Josephine Seaman’s historical summary of the site, among the group was a Catholic priest, Father Juan Crespí, whose diaries record the findings of the expedition as they searched for the best place for their new settlement. As they traveled north, they passed what Crespí described as “extensive swamps of bitumen.” Local Indigenous people told him that they used the pitch, or la brea in Spanish, to seal their canoes.
Over the next century later, the property acquired the title Rancho La Brea and a new owner took possession of it. Henry Hancock began to harvest the material there after the Civil War, shipping it off to San Francisco and other ports for construction projects. As his crews dug deeper into the ground, they quickly discovered a different sort of treasure, a scientific gold mine that would redefine our knowledge of North America’s prehistoric life.
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For about 50,000 years, the pit at Rancho La Brea had been capturing animals and other life, just as the Grand Canyon tar had caught Borell’s birds. But here, the excavated skeletons were often huge and unfamiliar. As experts arrived to study the finds, they discovered relatives of bison, sloths, and other large mammals that once walked California’s coasts and valleys. The pits were incredibly prolific, write William Akersten, Christopher Shaw, and George Jefferson. Between 1906 and 1915, workers and scientists unearthed roughly 2 million bones of large animals from La Brea. Immediately, a pattern started to emerge in the bones that both excited and confused the paleontologists. There were too many predators.
Dire wolves, saber-toothed cats, and other long-toothed hunters were everywhere in the La Brea pits, write Lillian Spencer, Blaire Van Valkenburgh, and John Harris. In ecosystems in the modern world, predators generally make up a relatively small percentage of both the individuals and biomass. Every predator needs many more prey to support it. A savanna with too many lions would quickly run out of gazelle. But here, the top predators dominated. More than half of all excavated skeletons belonged to carnivores.
One theory for this predator glut arose from the way that tar and asphalt kill. It’s a slow death, with one foot, then another caught in the muck, leading to a long and probably loud struggle against the deadly pull of the black trap. Spencer and her colleagues explain that when large prey like a bison was caught in a pit, it may have gained the quick attention of passing big cats, wolves, and bears. As they pounced on the wounded animal, a seemingly easy meal, they too became caught, and the scene would only become more noticeable to the next predator. One bison might be responsible for capturing several predators, leading to the apparent dominance of carnivores in the pits. This also explains the large number of scavengers like coyotes and vultures. Indeed, an extinct group of vulture-like birds called teratorns are the largest birds found at La Brea, with a wingspan of up to twelve feet.
For some of the time that La Brea has been capturing dire wolves and giant birds, another skilled hunter has also patrolled its shores. The Los Angeles area is home to to many Indigenous groups, including the Chumash and Tongva, whose people populated the region, dotting it with villages, camps, and homes. Indigenous communities have lived alongside the La Brea pits for thousands of years. It should be no surprise that they too are part of the ecological record laid down in the pits. In 1914, workers at La Brea found the first, and to date only, human remains, a skull and other bones from a young woman who died about 9,000 years ago.
The Tar Pit that Paved America
Rancho La Brea may be the most famous such pit in the Americas today, but that wasn’t always the case. On the southwest coast of Trinidad, a 100-acre lake of asphalt has preserved a slice of Indigenous history while also contributing to the birth of modern cities.
This so-called Pitch Lake has a long and storied history, summarized in Megan Gannon’s 2018 piece in Archaeology Magazine. In 1595, English colonist and captain Sir Walter Raleigh landed in Trinidad. Raleigh and his crew may have been the first outsiders to learn of the huge lake, but it had been known and used by local Indigenous people for millennia. According to the local Arawak people, the site had once been a thriving village. But the villagers broke a sacred taboo by killing hummingbirds, which carried the spirits of their ancestors, and for this offense, their homes were swallowed by a lake of black pitch, never again to be habitable.
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The English carried home stories of the asphalt, which they reported to be both more abundant and of higher quality than any in Europe. Still, for 250 years, it remained a resource mostly for locals and occasional maritime visitors. It wasn’t until 1857 that commercial exportation of the tar of Pitch Lake began in earnest. By the 1890s, more than 175,000 tons of pitch from Trinidad was sent to Europe. But it was the rapidly growing and industrializing United States that became its biggest consumer. In 1870, Newark, New Jersey became the first US city to use the asphalt made from Trinidad pitch for pavement. Orders from city after city followed. Over the next thirty years, 90 percent of the asphalt used around the world, and especially in American cities, came from Pitch Lake.
To date, over 11 million tons of Pitch Lake asphalt have been exported from Trinidad. Like La Brea, what began as a commercial enterprise also unearthed scientific and historical treasures.
Indigenous people have lived in Trinidad for at least 7,000 years. Several overlapping cultures lived and worked on the island, but because they likely made the vast majority of their tools and other goods out of wood and fiber, very few artifacts of this rich history remain. Though there are animal remains preserved there just as in La Brea, the real treasure of Pitch Lake are several wooden artifacts, including two sets of weaving tools from thousands of years apart. One was carved around 600 CE, while the other is over 5,300 years old, probably carved by the first people to inhabit Trinidad, and so far the oldest wood carving ever found and dated in the Caribbean.
Other pits around the Americas have served up paleontological gold mines of their own. In Cuba, a pit at Las Breas de San Felipe has provided proof that giant ground sloths (now extinct) survived much longer there than scientists previously believed, and that the massive extinct mammals likely still walked the island when its first human inhabitants arrived.
In Peru, the Talara Tar Seeps are South America’s answer to La Brea, preserving a window into the late Pleistocene era, between 15,000 and 17,000 years ago. Some of the same predator species have been found at Talara as La Brea, including dire wolves and saber-toothed cats, but, as Jessica A. Oswald and David W. Steadman explain, one of the most important finds there has been a wealth of bird remains. Some of these are species that still fly over South America but now live hundreds of miles away, helping us to understand not only a history of extinction but also the migration of species over time. Today, Talara is a port city on the Pacific coast, but during the last ice age, write Oswald and Steadman, lower sea levels moved the coastline kilometers further from the site, and pit excavations have helped scientists to understand how different the ecosystem was at that time.
Big animals, especially predators with impressive fangs, will always be the top museum attraction from the various tar and asphalt pits of the Americas. But especially as excavation techniques have improved, some of the most important finds have been much smaller. Seeds, leaves, and pollen that landed in them give researchers new insights into the plants that dominated these sites tens of thousands of years ago. Caught in nature’s own flypaper, insects are preserved more perfectly than almost anywhere else; some beetles even retained the color of their shells.
Tar and asphalt pits may be a death trap for individual animals, but their unique properties have allowed lost species and stories to live on. Many of these sites are still undergoing excavation, so there are almost certainly new and as yet unimagined discoveries waiting, buried in lakes and seeps, for their moment in the sun.
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