large surface deposit of natural asphalt, a mixture of heavy oils that is left after the lighter, more volatile components of a crude-oil seepage have evaporated. An example is Guanoco Lake in Venezuela, which covers more than 445 hectares (1,100 acres) and contains an estimated 6,000,000 tons of asphalt. It was used as a commercial source of asphalt from 1891 to 1935. Smaller deposits occur commonly where Tertiary marine sediments outcrop on the surface; an example is the tar pits at Rancho La Brea in Los Angeles (brea and “tar” are synonymous with “semisolid asphalt”). Although most pitch lakes are fossils of formerly active seeps, some, such as Pitch Lake on the island of Trinidad, continue to be supplied with fresh crude oil seeping from a subterranean source. Pitch Lake covers 47 hectares (115 acres) and contains an estimated 6,700,000 tons of asphalt. The asphalt is a major export of Trinidad and Tobago and is used mostly for road building.