Tar seeps form Ice Age time capsules

Tar seeps form Ice Age time capsules. . La Brea's unique tar seeps make each pit here a different Ice Age time capsule. This is because the tar oozes up in different places at different times. While one seep stops, another one starts up not far away. The types of plants and animals that get stuck in one seep are often different from those that get stuck in a seep that pops up later. This start- and-stop cycle has occurred many times here over the last 55,000 years. , Fossils form side-by-side, yet twenty thousand years apart, The pools of tar in front of you conceal three separate pits dug from 1913 to 1915. Even though these pits are close together, they contained fossils from tens of thousands of years apart. How does this happen?, 1) Underground and under pressure , The oil field that underlies La Brea is about 1,000 feet deep. Pressure forces some of the thick oil - asphalt - upward., 2) Asphalt oozes upward , As deep underground layers shift, cracks form in the overlying rock layers. Asphalt and methane gas ooze and bubble up through these vents., 3) Active seeps form surface pools , The asphalt can ooze all the way to the surface to form shallow pools that actively trap animals and plant pieces. Animals decompose and only their bones remain. , 4) Sediment buries inactive seeps , Over many thousands of years, sand and soil wash down from the mountains. This seals off the seeps and protects the fossils trapped inside., 5) Gas bubbles churn fossils , Methane gas continues to bubble up, churning the bones from different animals and time periods into complex jumbles., No Two Pits Are Alike , Each pit at La Brea is a unique time capsule from L.A.'s Ica Age past. These three pits give us clues to how this area changed from 35,000 to 13,000 years ago., Pit 3 , A juniper tree fossil from Pit 3 is about 17,000 years old. , Pit 4 , The oldest fossils from Pit 4 include saber-toothed cat bones from 35,000 years ago. , Pit 61/67 , Most of the fossils from Pit 61/67 are around 13,000 years old. One of the youngest fossils found here belonged to a dog that lived 7,000 years ago.