during the Pliocene. The La Brea Tar Pits The La Brea Tar Pits are pools of stagnant asphaltum that have been found on the basin's surface. These "pools" are important because hundreds of thousands of late Pleistocene bones and plants have been found. These pits allowed scientists to better understand the ecosystem at that particular point in the geologic past. Petroleum Accumulations of oil and gas occur almost wholly within strata of the younger sequence and in areas that are within or adjacent to the coastal belt. The Puente formation has proved to be the most notable reservoir for petroleum in the basin. The primary reason for the high abundance of oil is because the oil sands are well saturated within the basin. The thickness of these oil sands range from hundreds to thousands of feet. Anticlines and faulted anticlines are the structural features that are also responsible for trapping oil. The first reported oil-producing well was discovered in 1892 on the land that is presently beneath Dodger Stadium. This basin was responsible for half of the states oil production until the (90's?). This is remarkable due to the relatively small size and youth of the basin. The basin currently has about 40 active oil fields that collectively have 4,000 operating wells. In 1904, there were over 1,150 wells in the city of Los Angeles alone. Tight spacing and continued pumping of the wells resulted in most of the wells to dry up. Most recent data indicates that 255 million barrels of oil were produced in 2013. This is a large decline from the almost 1 billion barrels per year produced in the late 1970s. See also References External links Significant LA earthquakes Valleys of Los Angeles County, California Valleys of Orange County, California Watersheds of California Landforms of Southern California Los Angeles metropolitan area Regions of California Geology of Los Angeles County, California Geology of Orange County, California Pull-apart basins Greater Los Angeles Los Angeles River Peninsular Ranges San Gabriel River (California) Santa Ana River Transverse Ranges Sedimentary basins of North America Structural basins of the United States Valleys of California Watersheds of the West Coast (U.S.)