The hike
Keeping with our tradition of completing the uphills first, we’ll be starting near the east end of the Caldecott Tunnel and hiking up to the visitors’ center (and restrooms), then returning back down a shorter trail. AllTrails lists the hike as 4.6 miles with 810 feet of elevation gain. The hills aren’t too steep.
After rains, the trails can be muddy, but we’re on fire trails most of the way, so it’s passable even when muddy.
Alltrails map
Geology
Things we’ll see/look for
From the self guided tour brochure (we’ll be doing this in reverse order):
Sibley Volcanic Regional Reserve, Oakland, CA; East Bay Regional Parks District
- Walk up the paved road to the EBMUD water tank to see a dark basalt dike, a feeder of lava to the crater, that cuts through a sequence of tuffbreccias (grayish brown) and pebbly mudstones (light gray), inside and near the crater bottom.
- This pit was made by quarry operations in which massive basalt lava was removed. The pit exposes the interior of the Round Top volcano. You are standing on bedded tuff-breccias, which filled much of the crater, settling at times into a small lake. The steep wall across the pit consists of lava that capped the crater after it was filled. Eventually the Round Top vent buried itself in basalt flows.
- This roadcut exposes Orinda Formation river gravels, sands, and mudstones. The red (when moist) streaks and layers in these river beds were caused by oxidation of iron in the sediments. Such varicolored “redbeds” sometimes contain fossils of plants and animals. Elsewhere in the preserve, bands of more intense red are found at the tops and bottoms of lava flows, where iron was oxidized and reddened by baking and steam action; these bands are called “bake zones.”
- Before you is a wall with basalt on the left and Orinda mudstones on the right. The bedding in the mudstones gives the appearance of dragfolding resulting from relative uplift of the lava occurring during the past 10 million years. Alternatively, the disruption of the mudstones may have occurred earlier, at the time of volcanic activity. This site was close to, or was in, the wall of the volcano, and would have been subject to slumping, sliding, and plowing.
- Massive basalt was removed from this major quarry pit. The north wall shows a set of thick lava flows tilted on edge, nearly vertical. The well-defined layers near the top of the face are jointing-units resulting from shrinkage caused by cooling. They are analogous to the basalt pillars of Devil’s Postpile in the southern Sierra.
- This is a sequence of basaltic tuffs (ash) lying on top of a dark lava (far left), all tilted steeply to the east. The base of the tuffs was baked red, probably because the lava below was still hot and steaming when the ash landed. At the top of the sequence there is another lava flow (far right, to the right of the post) that baked the underlying tuffs red
- Before you is a basalt flow, massive at left, rubbly to the right. The lower parts, coming into contact with cold land surfaces, harden early and are often jumbled by turbulence and drag. The rubbly part of the flow is filled with vesicles caused by gas pockets, which were later filled with chalcedony, opal, calcite, zeolites, and sometimes green celadonite.
- This huge, reddish-brown block of lava fell from the cliffs to the upper right. The lava cooled and hardened while it was still moving, resulting in dramatic fragmentation, known as “autobrecciation” (self-broken).
- See hard lava to the right and left, and soft, easily eroded tuff between, all tilted eastward almost to vertical. The lava to the left baked the top of the tuffs brick red. This lava looks as if it was probably 100-150 feet thick. These flows were not fluid, but contained enough silica to be more viscous. They probably moved at a slow walk, with glowing blocks of lava tumbling down a steep front and setting vegetation on fire.
- The brick-red knobs in the foreground, and rugged outcrops of the same color on the skyline, are made of cinder that flew from Round Top and landed while still hot, so the pieces welded together.
- The major valley to the north across Highway 24 is Siesta Valley. It coincides with the axis of the Siesta Syncline, a great fold that has lifted up thousands of feet of rocks on both sides. You are standing on the southwest limb of the fold, which includes all the rocks in Sibley
Transportation, getting there
- We’re meeting near the east side of the Caldecott Tunnel. You’ll want to get to the southwest corner of the road above the tunnel, then head south on Old Tunnel Road for about one-half mile.
- Parking is available at the Old Tunnel Road Staging Area, but if that fills up there is additional parking just past the turnoff (on Old Tunnel Road). Just watch the signs carefully and don’t park in a no-parking zone.
- If you use Google Maps, here’s a link to the starting point.
- There’s no public transit option, but if you want to arrive by BART (Orinda Station), I can pick you up there. For each hike I’ll post the train you need to be on so I can get you and make it to the hike on time. I can usually fit three people in my van.
- If you’re a Lyft/Uber user, you can also use that for the ‘last mile’ from the Orinda BART station.
Other web resources
- EBRPD guide to the park. This guide includes stopping points on a self-guided tour of the geology.
- Northern California Geological Society 2005 field trip guidebook.
- Oakland Geology: Deep Oakland chapter 9: Sibley Volcanic Regional Preserve
- KQED collaborated with the Exploratorium for this web page.
Videos
Student documentaries of college field trips
I couldn’t find any videos with instructional details about Sibley, but apparently there was a class there in 2011 that produced a few student videos.
Aaron Galbraith did a nice job documenting the 2011 Alameda/Laney College geology course field trip to Sibley.
Here’s Kamal Dabwan’s version
Redshadow2200’s version
Past Meetup hikes
Our past trips to Sibley (with photos and reviews)