unconformity in the Pliocene between the Repetto and the Pico indicates the top of the structure was eroded to a flat surface and later Pliocene and Holocene sediments were deposited horizontally. Subsidence Withdrawal of large volumes of oil from the poorly consolidated sediments resulted in compaction of the oil reservoirs, and resultant sinking of the overlying land surface. Subsidence became apparent in 1940, and exceeded 20 feet at the center. Water injection to maintain pressure in the oil reservoirs began in 1953, which eventually stopped further subsidence. References External links Los Angeles Almanac: Great Oil Fields NASA: Land subsidence in Long Beach from petroleum extraction Oil fields in California Geology of Los Angeles County, California Geography of Los Angeles Geography of Long Beach, California Geography of Los Angeles County, California Los Angeles Harbor Region Terminal Island Wilmington, Los Angeles Economy of Los Angeles Economy of Long Beach, California