Lake Pit - La Brea Tar Pits EarthCache

Welcome to the La Brea Tar Pits

At the Lake Pit in Hancock Park, you'll encounter the most prominent feature of the La Brea Tar Pits: a bubbling, tar-covered pool of water that has come to symbolize the prehistoric world preserved beneath Los Angeles. This EarthCache invites you to explore how this seemingly calm body of water is actually part of an active natural asphalt seep. The Lake Pit gives a dramatic look into the geologic and paleontological processes that have shaped this site for tens of thousands of years.

Note: The La Brea Tar Pits are only available during the open hours of Hancock Park which are regularly 6:00 AM - 10:00 PM. Look online for further details.

What Is the Lake Pit?

The Lake Pit in front of the museum was originally dug during asphalt mining in the late 1800s. Rainwater and groundwater later collected above the rising tar, forming the shallow lake visible today. The bubbles, surface sheen, and strong odor come from gases and tar seeping up from the Salt Lake Oil Field below. A life-sized display of a mammoth shows how animals may have become trapped in tar at this site thousands of years ago.

The Formation of Tar

Tar is a dense, sticky form of petroleum that forms at the surface through a long natural process that begins deep underground. It starts with organic matter, primarily microscopic marine organisms, that accumulated on ancient seafloors millions of years ago. Over time, this material was buried by layers of sediment and subjected to increasing heat and pressure. These conditions caused the organic matter to chemically transform into crude oil, which then migrated upward through porous rock layers and faults in the Earth's crust.

As the crude oil moves closer to the surface, it encounters oxygen, sunlight, bacteria, and changing temperatures. These environmental factors cause the lighter portions of the oil, including gases and liquid hydrocarbons like methane, ethane, and propane, to evaporate or break down. Microbial activity may also consume some of the more volatile components. What remains is a thick, heavy residue known as tar. This substance is too dense to evaporate or flow easily, so it tends to accumulate in surface depressions or areas where water or clay-rich sediments slow its movement.

Tar does not erupt or spill out rapidly. Instead, it rises slowly and steadily, sometimes over centuries, and collects in shallow pools or seeps. In many cases, the tar becomes mixed with groundwater, covered by sediment or organic debris, or hardened into a crust at the surface. These pools can be difficult to identify visually, as they are often hidden beneath what appears to be solid ground or standing water. In warm climates, evaporation increases, causing more tar to surface and become visible. Natural tar seeps like these are relatively rare and tend to occur only where oil reserves are close to the surface and geologic conditions allow oil to migrate upward.

The Story of the La Brea Tar Pits

At La Brea, tar rises to the surface from deep beneath Los Angeles due to the unique geologic structure of the area. The site is located above the Salt Lake Oil Field, a major petroleum deposit formed during the Miocene epoch. Crude oil from this reservoir migrates upward through fractures and faults that cut through the sedimentary layers of the Los Angeles Basin. One of the most significant fault systems in the region, the Los Angeles Basin Fault Zone, helps transport this oil toward the surface in areas such as Hancock Park.

As the oil nears the surface, it is exposed to the open environment. Sunlight, oxygen, and bacterial activity gradually remove the lighter hydrocarbons, leaving behind thick, sticky tar. This material accumulates at or just below the ground surface and can collect in natural low points such as shallow basins or streambeds. The Lake Pit is one of several places within Hancock Park where tar continues to rise today.

Rainwater and groundwater often collect in these depressions, forming what appear to be ordinary ponds. However, beneath the surface, tar continues to seep upward, sometimes bubbling or forming slicks. This ongoing seepage makes La Brea one of the most active natural tar seeps in the world. Unlike most petroleum deposits that are tapped and removed for commercial use, the tar at La Brea continues to emerge naturally and visibly, offering a rare opportunity to observe part of the petroleum cycle in a modern urban environment.

Tasks for This EarthCache

To log this EarthCache, visit the site and complete the following tasks. Please send your answers via Geocaching or email.

1. Include “Lake Pit - La Brea Tar Pits - GCB5ERJ” as the first line of your message.

2. Look at the surface of the Lake Pit. Describe at least two visible features that indicate active tar seepage.

3. Describe the smell around the Lake Pit. What does the presence of this odor suggest about the activity below the surface?

4. Look at the terrain around the Lake Pit. Why do you think tar collects specifically in this part of Hancock Park? What role might the landscape’s shape and elevation play in where tar pools form?

5. In your log, attach a photo of yourself or a personal item with the Lake Pit in the background. (Note: photos predating the publication of this EarthCache are not accepted.)