Understanding the Functioning of the La Brea Tar Pits

Scientific excavations at the La Brea Tar Pits began in earnest in the early 1900s, but the origins of the pits date back millions of years. The region now known as Los Angeles was once underwater, where marine life and sediments collected on the ocean floor. Over time, pressure transformed these organic materials into fossil fuels. Approximately 40,000 years ago, as the ocean retreated, the petroleum began to rise to the surface.

The substance in the pits, more accurately termed asphalt, is the residual material after the lighter elements of petroleum evaporate. This highly viscous material, particularly in warmer conditions, has the ability to trap even large animals. Around 90 percent of the fossilized mammal remains extracted from the pits belong to carnivores [source: The Natural History Museum of Los Angeles County]. This has led paleontologists to theorize that the scenario at Rancho La Brea often involved prey animals, especially those that were weak or injured, becoming stuck in the pits, attracting predators who would then also become trapped.

The most recent inventory of the La Brea collection, conducted in 1992, revealed over million specimens from more than 600 species of plants and animals [source: The Natural History Museum of Los Angeles County]. Since then, ongoing excavations, particularly those under Project 23, are expected to possibly double the number of specimens in the collection.

We'll delve deeper into Project 23 later, but first, let's explore the historical background of the tar pits.

The History of the Tar Pits

Long before the Industrial Revolution, humans utilized fossil fuels, including the asphalt from the La Brea Tar Pits. Indigenous tribes, for instance, applied this asphalt to waterproof items such as canoes and baskets.

After the Spanish took control of the region, they established cattle ranches on the land. The Hancock family acquired the property in 1870 and began drilling for oil. Subsequent studies and minor excavations occurred, but significant activity didn't begin until the early 20th century. In 1913, the Natural History Museum of Los Angeles County (then known by a slightly different name) was permitted to explore the area, leading to a comprehensive two-year study that recovered many of the specimens now in the collection. Despite digging 96 pits, the excavations were disorganized and unsafe, with a focus primarily on larger animal bones, while smaller fossils, such as those of plants and invertebrates, were often ignored.

L. E. Wyman spearheaded the initial major excavations, but Chester Stock, a paleontologist from the California Institute of Technology, conducted much of the early research on the findings. Certain pits, notably Pits 3, 4, 9, 61, and 67, yielded particularly fascinating discoveries. However, Pit 91 emerged as the most significant over the years, undergoing intermittent excavations ever since. More details on this will follow on the next page.

Pit 91

Among the 96 pits mentioned earlier, Pit 91 stands out as the most renowned and extensively studied. For almost four decades, it was the sole pit under active excavation at La Brea. In the late 1960s, researchers decided to refine their methods by collecting all fossils from the pit, including those of smaller species, to gain a more comprehensive understanding of the Pleistocene Epoch's conclusion.

On June 13, 1969, known as "Asphalt Friday," excavations resumed with a focus on gathering remains of amphibians, reptiles, insects, small birds, shells, and plants. Alongside these less conspicuous fossils, Pit 91 has yielded numerous well-known Pleistocene species, such as dire wolves, sabertoothed cats, western horses, ground sloths, and mammoths—all from a pit just 15 feet (4.5 meters) deep.

Most of these remains have been radiocarbon dated to between 10,000 and 40,000 years old. Like many pits, Pit 91 contains fossils spanning a vast period. While 30,000 years is a long time for animals to become trapped, the millions of fossils found are still astonishing. Researchers explain that if just 10 large animals were trapped every 30 years, the current fossil count is easily justified, especially if such events occurred once every decade.

Work on Pit 91 is currently paused due to the unexpected discovery of what has been named Project 23.

Project 23

In 2006, Project 23 began with the unassuming start of a parking garage project. The Los Angeles County Museum of Art (LACMA) planned to build an underground parking facility near the tar pits. Due to the area's historical significance, a salvage archaeologist was required to oversee the work. This proved fortunate, as 16 artifact-rich deposits were discovered during construction.

To avoid significant construction delays—estimated at 20 years for thorough excavation, which LACMA officials opposed—salvage archaeologist Robin Turner devised a solution. Within three and a half months, 23 wooden crates containing the deposits were extracted using cranes and delivered intact to the Page Museum. Wrapped securely in plastic and weighing up to 125,000 pounds (55,000 kilograms), these crates were transported to the museum's main research facility, known as the "fish bowl," where visitors can observe researchers meticulously examining the contents through glass walls.

The most remarkable discovery from the project to date is "Zed," a Colombian mammoth skeleton that is 80 percent complete, including its tusks. Unlike earlier mammoth discoveries at the tar pits, where bones were haphazardly mixed and reassembled, Zed's remains offer a rare opportunity for curators to study the life of a Pleistocene mammoth in unprecedented detail. The matrix surrounding Zed's fossils is rich with microfossils, symbolizing the countless mysteries still waiting to be uncovered at the La Brea Tar Pits, one of the most fascinating remnants of the Pleistocene Epoch.