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San Diego Formation
The San Diego Formation is a coastal transitional marine and non-marine pebble and cobble conglomerate deposit and marine sandstone rock with Pliocene marine fossils, from a former bay deposited during the Middle to Late Pliocene of the Cenozoic (2 - 3 million years ago). This formation is found from the south side of Mount Soledad in San Diego County to Rosarito Beach in northern Baja California, including Tijuana, Mexico, and the southwestern corner of San Diego County from San Ysidro to Pacific Beach. San Diego Formation deposits were formed in a large, open, crescent-shaped bay similar in size to Monterey Bay that existed on the coast in Pliocene times.〔California Geological Survey, Regional Geologic Map No. 3, 1:100,000 scale Compiled by: Michael P. Kennedy and Siang S. Tan, 2008.〕〔(Scott Rugh, Clams of Champions: The San Diego Formation ), from sdnhm.org, the San Diego Natural History Museum website accessed November 26, 2013.〕
Besides those of clams and other mollusks, quite a few bird fossils (which are generally rare) have been found in this geological formation. Among them is a possible ancestor of Cassin's auklet (''Ptychoramphus aleuticus'') and the loons ''Gavia concinna'' and ''G. howardae''. The former was quite likely a close relative or even ancestor of the large black-headed〔Common loon (''G. immer'') and yellow-billed loon (''G. adamsii'')〕 loons; the latter was perhaps a small relative of the large grey-headed〔Pacific loon (''G. pacifica'') and Arctic loon (''G. arctica'')〕 loons.〔〔
==Fauna==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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