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Stop 33, Merced Formation: The Setting

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The final stop is at the south end of San Francisco's Ocean Beach near Fort Funston. (more below)
Classic sea cliffs
Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
The sea cliffs here consist of Pleistocene sediments (1.8 to 0.01 Ma) of the Merced Formation, another classic place for showing geology students the features of sedimentary rocks. The Merced is mostly unlithified, proving that geologists don't map only rocks. The San Andreas fault runs just a kilometer or so offshore from here, and the epicenter of the great 1906 earthquake is thought to be at this location. These sediments were laid down in a rapidly subsiding nearshore basin. They originate mostly from the Sierra Nevada, to judge from the gravel clasts; but slightly older sediments (about 0.6 Ma) were derived only from local rocks. The implication is clear: until quite recently, geologically speaking, the Golden Gate did not exist and the great rivers of the Sierra and Central Valley reached the sea elsewhere—possibly in Monterey Bay. The Merced Formation was raised to its current level only in the late Pleistocene, about 100,000 years ago, as the crustal extension that opened its basin gave way to compression and uplift. Today's California is a very active place.

Day 1, Coast Range: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Day 2, Sierra Nevada: 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18

Day 3, Sierra Nevada: 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28

Day 4, Coast Range: 29 | 30 | 31 | 32 | 33

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