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Aspects of the Quaternary Geology of Santa Rosa Island, California
The landscape of Santa Rosa Island, California, has been influenced by multiple factors during late Quaternary time, including pygmy mammoths, climate change, and historic grazing.
Extinction of the pygmy mammoths occurred about 12,800 years ago during a period of rising sea level. Mammoths, competing for shrinking resources, stripped the island of vegetation, then perished. Increased erosion resulted in formation of large dune fields, now largely stabilized, at Carrington Point and Skunk Point. These dunes contain the only complete skeletons of pygmy mammoths found on the island, perhaps the last of their kind.
Most of the large canyons on the island have steep-sided barrancas incised into terrace deposits. The chronology of filling and cutting is poorly understood. Some of the cutting may have started during the mammoth event.
The onset of sheep grazing in 1844 accelerated erosion and sedimentation, and an overgrazing/flood event, probably between 1860 and 1880, caused widespread gullying. Cores cut in a marsh on the east coast show an increase in sedimentation beginning in the 1840s, reflecting the impact of sheep grazing, and show a period of relative stabilization during the cattle era (post 1901).
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