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Subsurface exploration of two landslides in southern California yielded carbonized wood fragments below the slide masses. One slide, just east of the active Portuguese Bend slide in Palos Verdes Hills, Los Angeles County, in rocks of the Altamira Member of the Miocene Monterey Formation, overran alluvium containing wood fragments; the radiocarbon age of the wood is 2915 ± 205 years B.P. The slide mass and scarp show minor erosion with rills and channels about 3 feet deep, and at least 25 feet of clay and silt have been deposited in the tension cracks at the head of the slide. The other slide, in southern Orange County in the Miocene-Pliocene Capistrano Formation, overran brush on the lower valley slope; the radiocarbon age of the wood is 17,180 ± 750 years B.P. Tributary valleys more than 40 feet deep have been eroded in the slide debris since movement, and more than 35 feet of sandy and silty alluvium has been deposited against the toe.

Movement of the San Juan Capistrano landslide is correlated with the maximum late-Wisconsin low stand of sea level, about 20,000 to 17,000 years B.P. postulated by Curray (1965). Greatly increased precipitation during this time, coupled with an extremely low sea level, probably accelerated the rate of downcutting in the San Juan Valley, and oversteepened valley slopes resulted in extensive landsliding. Stream erosion during this time probably established equilibrium with the lower base level.

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