Bay Nature - Sibley Volcanic Regional Preserve

Please refer to Sibley Volcanic Regional Preserve map and "Self-guided tour of Round Top Volcano" for detailed descriptions of stops and geologic highlights. This excellent guide was prepared by Stephen W. Edwards, Director, Regional Parks Botanic Garden.

http://www.ebparks.org/resources/pdf/trails/sibley_text.pdf


Subject
Photograph number
Potential Cover Photographs
3312, 3315 (both also applicable for lava flows with tilted layers)
Vistas and quarry pits
3010, 3314, 3344
Baked zone
3316
Lava flows with tilted layers
3197, 3312, 3313, 3315
3341 (Highway 24 roadcut east of Caldecott Tunnel)
Columnar jointing
3279, 3343
Basaltic lava flow
3347
Technical rock shots
3345, 3346
Paleosol
3340 (Highway 24 roadcut east of Caldecott Tunnel)

Potential Cover Photographs


3312 Steeply tilted volcanic breccia (left) and massive basalt dome (right).  Stop 4




3315 Steeply tilted volcanic breccia (left) and massive basalt dome (right).  Stop 4





Vistas and quarry pits


3010  Round Top Trail - view northeast after Stop 1




3314 Overview  of park, land bank area (closed to public), and view North towards Tilden Regional Park after Stop 1




3344 Large quarry pit at Stop 4, with massive (vertically tilted) basalt lava dome center and right





Baked zone


3316 Red-baked sequence of air-fall tuff (ash) at Stop 8





Lava flows with tilted layers


3197 Steeply tilted volcanic breccia (left and center) and massive basalt dome (right). Stop 4




3313 Steeply tilted volcanic breccia (left) and massive basalt dome (right). Stop 4




3341 Tilted layers of Miocene Orinda Formation (alluvial-fan deposits -  left gray) and Moraga Formation (basalt  lava flows  - right brown).
Roadcut along Highway 24 east of Caldecott Tunnel. Note poor natural exposures on upper slopes. These are the same rocks and along the same trrend as those exposed in Sibley Volcanic Regional Preserve.





Columnar jointing


3279 Columnar jointing developed during cooling of basaltic lava flow. Stop 7




3343 Columnar jointing developed during cooling of basaltic lava flow. Stop 7





Basaltic lava flow


3347 Basaltic lava flow. Note massive upper flow (left) and "jumbled" lower part (right) Stop 9

 



Technical rock shots


3345 Gravels (will provide more detailed caption, if necessary) OK to crop pen.




3346 Tuff (volcanic ash) breccias originated when fluid mixtures of ash, lava, and Orinda gravels, perhaps saturated by rain, slurried and flowed down the  slopes of the volcano. OK to crop knife, if necessary. Stop 1.





Paleosol


3340 Miocene Paleosols (fossil soils) formed on abandoned portions of an alluvial-fan, whose sediments were derived from coastal highlands where San Francisco Bay is today. The Walnut Creek - Mt. Diablo area was the coastal plain and shallow marine setting at that time. Roadcut along Highway 24 east of Caldecott Tunnel.




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