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Stop 30, Orinda Formation and Moraga Volcanics: The Setting

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Stop 30 is at 37°51.904´N, 122°12.561´W, in a spectacular roadcut on route 24 just east of the Caldecott Tunnel. (more below)
The dramatic approach
Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
As you approach the Bay Area proper on route 24, the road winds past Orinda up a narrow defile toward the steep wall of the Berkeley Hills, which it pierces in a three-bore tunnel. This locality (the outcrop at the center of the photo) is visited by crowds of geology students from local colleges. The rocks there include thick beds of nonmarine conglomerate of the Orinda Formation (23–10 Ma) topped by basaltic lava flows of the Moraga Formation (10–9.2 Ma), all steeply tilted since their formation within a pull-apart basin related to the San Andreas fault. Similar basins exist today up and down the fault system, especially farther north. The rocks of the conglomerate arrived from the west, and some of them appear to have come from places that now lie to the north in Sonoma County. The tectonic disruptions recorded here are what has scrambled the earlier record of subduction and made Bay area geology so complex. The basalt is also well displayed in the Sibley Volcanic Preserve, a regional park in the Berkeley Hills a short distance south of here.

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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