I first happened upon La Brea Tar pits on my bicycle trip around the lower 48 states (nearly 25 years ago, already!). As luck would have it I was there on a holiday and the museum was closed. The grounds were open and appear fairly much the same today.
Fresh tar oozes out of the ground in places. Seems that oil is somehow squeezed upward in certain areas and pushed through clay soil to make a sort of asphalt. Bacteria living in the asphalt eat the petrolium and produce methane gas which bubbles up into the tar and/or water making it look as though it is boiling when it is cool to the touch.
One pit was left open to show the excavation process. Unfortunately, they suspended the dig in 2007 in order to work on numerous crates of asphalt dug up for a parking structure on site. Bones are not found in the liquid tar but rather in the asphalt mix (or matrix). This particular pit is about 15 feet deep and it is believed the bone deposits continue another 5 to 8 feet down.
They left bones exposed as they were when they stopped the dig. The various colored flags each represent a different animal. Most bones are mixed in with and piled atop other animals. Fresh tar is beginning to ooze over the exposed layers.
Here's a larger tar ooze in the grounds and other ones also have sticks and garbage in them as people poke around in the tar. Animals continue to become stuck in the tar today. People probably would, too which is why they have fenced off the bigger areas.
Here's a picture of a dig back in the day with some of the tools in front and some of the jumble of bones as they would appear in the matrix.
Here is a matrix sample with (tags from left to right) a bison lower jaw, sabertooth teeth, sabertooth skull, and a bison horn.