Tar pits are composed of heavy oil fractions called gilsonite, which seeps from the Earth as oil. In Hancock Park, crude oil seeps up along the 6th Street Fault from the Salt Lake Oil Field, which underlies much of the Fairfax District north of the park. The oil reaches the surface and forms pools, becoming asphalt as the lighter fractions of the petroleum biodegrade or evaporate. The asphalt then normally hardens into stubby mounds. The pools and mounds can be seen in several areas of the park.