Archive for the ‘Oakland rocks’ Category

Conglomerate in the upper Arroyo Viejo streambed

9 May 2016

Oakland has three different bodies of conglomerate: from youngest to oldest they’re in the Orinda Formation (of Miocene age, maybe 10 million years old), the Oakland Conglomerate (Late Cretaceous, maybe 80 Ma) and the Knoxville Formation (Late Jurassic, about 165 Ma). Recently I’ve been getting fixated on the last one. I hope these photos from upper Arroyo Viejo will help you understand why.

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Above I-580, Arroyo Viejo runs along the road as you drive up Golf Links Road and into the woods of Knowland Park. Halfway through, it takes a sneaky dogleg to the left, then another left turn through this tunnel under Elysian Fields Road. Beyond is the stream’s source, now in the Sequoyah Country Club golf course.

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Between those two left turns, the stream valley exposes the Knoxville Formation. I haven’t walked the whole section yet, just visited both ends. The lower end has wonderful conglomerate outcrops.

High on the valley walls, the rock is heavily vegetated. It gets that way because between the pebbles and cobbles of the conglomerate, the matrix is a fine-grained sediment that supplies nutrients to plants and offers space to their roots.

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As you get close to it, the conglomerate reveals the abundant well-rounded cobbles beneath its green coat. Geologists like their rocks clean, but nature prefers them this way. You have to admire them.

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For the best exposures, look down into the streambed.

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A little bit downstream is a section of creek choked with boulders of this stuff, some as big as sofas, that I showed you a few months ago. Those monster rocks weren’t carved out of the streambed by the creek. Instead, they rolled down the valley walls, which are very steep (35 to 40 degrees) as most are in Oakland.

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The valley does have some landslide scars, so we know that those happen. But mostly I blame earthquakes.

Both events provoke the stream into washing the boulders away. Landslides create instant dams, which build up the water pressure in the lakes that pond above them. When the stream bursts through, usually in a matter of days, it makes short work of the rock pile. Earthquakes, for their part, give the whole underground a shakedown and cause a weeks-long surge of water afterward that likewise gives the boulders a good head start.

In between these disturbances, maybe once in a century, rare cloudbursts pour enough water into the watershed to roll the biggest rocks downstream and grind down the streambed an inch or so at a time.

Given enough time — and geology always provides that — the boulders break down into pebbles and clay and wash out to sea, eventually to become new rocks. The Knoxville Formation conglomerate has waited some 165 million years to start that journey. The Earth is almost thirty times older.

Rocks and views of Fairmont Ridge

2 May 2016

Fairmont Ridge is the grassy upland that forms the backdrop to San Leandro. As it happens, East Bay Regional Park District owns much of it as part of Lake Chabot Regional Park. It has some rocks, which I’ll show first, and also some fine views.

Here’s the aerial view of the ridge from Google.

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And the geology of the same area is here.

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We’ll look at rocks from three different units: the green area is underlain by the Knoxville Formation, a shaly sedimentary unit; the light-brown area labeled Jpb is basaltic lava; and the pink area labeled Jsv is Leona rhyolite, which you’re familiar with by now from Oakland.

The Knoxville is well exposed around Lake Chabot. Here, to the east of the access road at locality 1, it appears to be strongly sheared, suggesting that its contact with the structurally underlying Leona and basalt is a fault. This view is facing north, parallel to the contact.

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The basalt unit is formally described as pillow basalt, the kind of balloon-shaped flows you’d find where lava erupts beneath seawater. But these rocks have been shoved around a lot since they were erupted in Late Jurassic time, and I have yet to see decent pillow morphology in any exposures. Still, the outcrops, like this one at locality 2, are picturesque.

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The windbreak of giant, mature eucalyptus is visible in the photo. This is a naturally breezy park, and the line of trees offers some welcome shelter.

Across the ridge on the Bay side, there are more outcrops of the basalt. Around locality 3 it’s well displayed.

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If you pay attention, you’ll see bits of this rock with polished surfaces, or slickensides, on them. These are caused by motion on faults, which rubs rocks against each other. Here and there, proper outcrops enable us to see that the faults are oriented vertically and parallel to the ridgeline. I interpret these as forming recently as these rocks were folded and tilted upright by motion related to the Hayward fault.

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The peak of Fairmont Ridge is fenced off, but an informal trail leads north along the east side of the fence to locality 4. (Poison oak will very soon make it impassible.) That’s where this typical specimen of Leona rhyolite was.

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But sometimes rocks are just rocks. Lift up your eyes from the hills and sit a spell. You can gaze upon the Bay side . . .

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. . . or over the reservoir toward Mount Diablo.

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Looking due east is a nice prospect of the ridge known as The Knife, overlooking San Ramon.

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Its high point is named Wiedemann Hill, elevation 1854 feet, and I have a growing fixation with it.

A circumambulation of Claremont Canyon

25 April 2016

A few weeks ago I took a strenuous ramble to accomplish a small thing — returning a stone to its home. The journey took me through some beautiful and interesting country, and the whole thing was the point.

The blue line of my route started from its farthest western point, where the 49 bus dropped me off by the Claremont Resort. From there I went up the ridge forming the northern side of Claremont Canyon, across the canyon’s upper reach to save time, and down the ridge on the southern side. The elevation change was more than 1000 feet, so this was not an idle stroll.

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The canyon is one of our finest examples of a wineglass canyon, a landform typical of major faults, in this case the Hayward fault. In case a photo image says more to you than a topo map, here’s a grab from Google Maps showing the canyon.

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Now look at it in relation to the Hayward fault, which runs straight across the bottom of this image through Cal Stadium and right behind the Claremont Resort.

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Movement on the fault has lifted the eastern (hills) side relative to the western (bay) side, which keeps Claremont Creek cutting down hard where it meets the fault. The result is that the mouth of the canyon is restricted to a narrow, steep-walled breach while the upper part of the canyon is free to spread out sideways as it erodes. This shape resembles the narrow stem and wide bowl of a wineglass, hence the name.

OK, what about the rocks? Here’s the geologic map with the photo localities shown. You see the Hayward fault cutting the lower left corner.

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We start out (1) through an unexpected little exposure of the Leona rhyolite (Jsv), with the Chabot fault defining its eastern edge. Here’s the stone with its typical rusty tint . . .

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. . . and here’s the view north of the dramatic contact between the Leona and, on the right, mudstones of the Great Valley Sequence (Ku, for undivided Cretaceous rocks).

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Higher up (2), the sandstone and related rocks of the Great Valley Sequence appear in abundance. In Shephard Canyon and farther south, these rocks are subdivided into several formations.

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The bedrock seems to support chapparal rather than forest, although maybe that’s only a function of the recent history of fire here.

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On the horizon, left to right, we see bare 1684 Hill, Radio Tower Hill and the adjacent knob across Grizzly Peak Boulevard, and dark Round Top just peeking over the ridgeline.

Still higher (3) we can spot a fresh landslide running from a Grandview home down onto its neighbors.

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Up around 1300 feet (4) is this exposure of a fault that has pinched across these strongly bedded rocks. By now we’ve crossed a contact into much younger strata.

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And right around the corner appears the Claremont Chert in all its typical glory. This is directly above the formation’s type section along Claremont Boulevard.

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Achieving the top of the canyon (5) gets you the reward of one of Oakland’s finest views.

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Finally I got to my destination (6), the exposure of Claremont Chert at Radio Tower Hill. That’s where I put back the specimen I collected there some 10 years ago.
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If you manage to find it, you can be its next temporary owner.

The trudge back down will work your quads pretty hard, but you get nice views of where you’ve been (7).

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I hope it gets easier with repetition, because I want to come back.


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