In the first two decades of the twentieth century, hundreds of thousands of fossils were excavated from the Rancho La Brea Tar Pits. Prior to that time, these pits were unknown. When Spanish settlers first arrived in the area of Los Angeles in the eighteenth century, they found a number of tar springs located in the middle of a large plain at the foot of the Santa Monica Mountains.
Surrounding the springs was a scattering of animal bones visibly embedded within a layer of asphalt. It was not until the mid 1870s that people began to realize the remote antiquity of these bones. Soon after exploratory excavations began in the early 1900s, scientists were finding tar pits containing large numbers of fossils.
The conventional explanation for the occurrence of these fossils is that thirsty birds and mammals, deceived by water-filled pools of tar, had blundered into these viscous traps and died in them. Although widely accepted, the entrapment theory has failed to give convincing answers to some key questions, including the physical characteristics of tar pits, the fragmentation and chaotic intermingling of the bones, and the numerical preponderance of the carnivores. Since these issues cannot be resolved by the entrapment theory. The evidence seems to be pointing toward the possibility of flooding as the agent for fossil deposition at the La Brea Tar Pits.