The La Brea Tar Pits are a group of tar around which Hancock Park was formed in the urban area of Los Angeles. The natural asphalt (also called asphalt, bitumen, pitch or turtle in Spanish) has been filtered from this area for tens of thousands of years. Tar is often covered in dust, leaves or water. Over the course of many centuries, tar has preserved the bones of trapped animals. The pools of La Brea Tar are a registered national trademark. Since brea means tar in Spanish, "The La Brea Tar Pits" is an example of a tautological place name; "La Brea Tar Pits" literally means "tar". The Brea Tar Pits and Hancock Park are located within what was once the Rancho La Brea Mexican land concession, now part of the city of Los Angeles in the Miracle Mile neighborhood, adjacent to the Los Angeles County Museum of Art Angeles and the Artisan and Folk Art Museum. The visible tar pits are actually from human excavations. The lake was originally an asphalt mine. The other wells visible today were produced between 1913 and 1915, when more than 100 wells were excavated in search of large mammalian bones. Various combinations of asphalt and waggler, have filled these holes. Normally, the asphalt appears in vents, hardening as it detaches, forming full mounds. These can be seen in different areas of the park. The tar pits are composed of heavy oil fractions called gilsonite, which filter from the Earth like oil. In the Hanock Park, crude oil filters along the sixth road fault from the Salt Lake oil field, which is the base of much of the Fairfax district north of the park. The oil reaches the surface and forms tanks in different points of the park, becoming asphalt while the lighter fractions of the oil biodegrade or evaporate. This infiltration has been happening for tens of thousands of years. From time to time, the asphalt would have formed a deposit quite often to trap animals and the surface would have been covered with layers of water, dust or leaves. Animals roam, become trapped and eventually die. Predators would come in to eat trapped animals and even get stuck. As the bones of dead animals sink into the asphalt, it soaks in them, turning them into a dark brown or black. Lighter fractions of oil evaporate from the asphalt, leaving a more solid substance, which encapsulates the bones. Dramatic fossils of large mammals have been extrapolated from tar, but the asphalt also preserves microfossils: remains of wood and plants, bones of rodents, insects, molluscs, dust, seeds, leaves and even grains of pollen. Examples of some of these are on display in the George Page museum. The radiometric dating of wood and preserved bones has given an age of 38,000 years for the oldest material known from the La Brea deposits. The pits still trap organisms today, so most of the pits are fenced to protect humans and animals. The Native Americans Chumash and Tongva who lived in the area built boats unlike any other in North America before settler contact. Dragging fallen Northern California redwood trunks and bits of wood from the Santa Barbara Canal, their ancestors learned to seal the gaps between the axes of the large wooden plank canoes using the natural tar resource. This innovative form of transport has allowed access up and down the coast and the Channel Islands. As they crossed the basin, the explorers reported seeing some tar geysers coming out of the ground like springs. The explorers reported that they had encountered many of these springs and had seen large swamps, enough, they said, to caulk many ships.