<P> The tar pits visible today are actually from human excavation . The lake pit was originally an asphalt mine . The other pits visible today were produced between 1913 and 1915, when over 100 pits were excavated in search of large mammal bones . Various combinations of asphaltum and waggler have since filled in these holes . Normally, the asphalt appears in vents, hardening as it oozes out, to form stubby mounds . These can be seen in several areas of the park . </P> <P> This seepage has been happening for tens of thousands of years . From time to time, the asphalt would form a deposit thick enough to trap animals, and the surface would be covered with layers of water, dust, or leaves . Animals would wander in, become trapped, and eventually die . Predators would enter to eat the trapped animals and also become stuck . As the bones of dead animals sink into the asphalt, it soaks into them, turning them a dark - brown or black color . Lighter fractions of petroleum evaporate from the asphalt, leaving a more solid substance, which encases the bones . Dramatic fossils of large mammals have been extricated from the tar, but the asphalt also preserves microfossils: wood and plant remnants, rodent bones, insects, mollusks, dust, seeds, leaves, and even pollen grains . Examples of some of these are on display in the George C. Page museum . Radiometric dating of preserved wood and bones has given an age of 38,000 years for the oldest known material from the La Brea seeps . The pits still ensnare organisms today, so most of the pits are fenced to protect humans and animals . </P> <P> The Native American Chumash and Tongva people living in the area built boats unlike any others in North America prior to contact by settlers . Pulling fallen Northern California redwood trunks and pieces of driftwood from the Santa Barbara Channel, their ancestors learned to seal the cracks between the boards of the large wooden plank canoes by using the natural resource of tar . This innovative form of transportation allowed access up and down the coastline and to the Channel Islands . </P> <P> The Portolá expedition, a group of Spanish explorers led by Gaspar de Portolá, made the first written record of the tar pits in 1769 . Father Juan Crespí wrote, </P>

When were the la brea tar pits discovered
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