Oakland geology ramble 6: Chabot to Leimert via the Oakland Conglomerate reference locality

11 June 2018

In 1914, UC Berkeley professor Andrew Lawson published the first decent geologic map of our area, the San Francisco Folio of the Geologic Atlas of the United States. That’s where the Oakland Conglomerate got its name. But while that fine rock unit lives on in name, the concept it represents has changed. Let’s look at a hundred years of progress in geologic mapping the area just west of Redwood Peak, the heart of the Oakland Conglomerate.

Here’s Lawson’s map from 1914. Orient yourself by finding Redwood Peak. The belt of rocks labeled “Ko” is the Oakland Conglomerate, the road winding along it is called Skyline Boulevard today, and the road heading leftward from it is today’s Castle Drive.

Lawson called Ko the “Oakland conglomerate member of the Chico formation,” a name and classification by which he meant that these rocks were notable but not important enough in the big scheme of things to single out. To be fair, he was dealing with a very large, detailed and cryptic field area at the time.

Fifty years later, along came James Case, who picked this area for his Ph.D. research at UC Berkeley. Lawson, who died in 1952, was no longer around to intimidate his graduate students, so Case was free to argue that Lawson had erred, having lumped too many different rocks in Ko. Some of them were really KJk, the Knoxville Formation, and others Case put in his brand-new Kjm, the Joaquin Miller Formation. (This is similar to what I was saying the other week about the Orinda Formation.) “It is therefore proposed,” he wrote, “that conglomeratic beds exposed along Skyline Boulevard west and northwest of Redwood Peak be considered the reference locality of the restricted Oakland Conglomerate.”

Case’s thesis was approved in 1963. His map, published a few years later in USGS Bulletin 1251-J, is so badly reproduced it’s embarrassing. Instead I’ll show Dorothy Radbruch-Hall’s elegant map of 1969, which incorporated Case’s work.

And for completeness’ sake here’s the current standard map, by Russ Graymer from 2000.

It’s a cool area. I recommend visiting it the way I did: take the 339 bus up to Chabot Observatory and hike down to Montclair or the Leimert Bridge to catch the 33 bus back downtown.

First, go behind the observatory buildings and find the fire road, which is part of the West Ridge Trail in Redwood Regional Park.

The trail exposes the conglomerate beautifully. The cobbles are exceptionally well rounded, a sign that they once tumbled a long way down a steep river into the sea.

You could meander your way to Moon Gate, on Skyline, where you’ll take the trail going left, or you could follow the steep and tempting path up to the water tank. From there the views look north past a wooded hill, with Round Top peeking up behind it, to the Briones Hills . . .

or northwest toward Grizzly and Vollmer Peaks.

A tiny trail on the other side will get you to some proper outcrops of massive sandstone, also part of the Oakland Conglomerate.

Either way, you’ll then be on the Scout Trail heading south, then angling east down to Skyline. Along the way you’ll pass a young and vigorous redwood stand, planted in 1978 thanks to good old Jerry Brown.

Cross Skyline and take the Castle Park Trail west. It’s a lot safer than walking on Skyline, and a nicer walk. When you hit Castle Drive, take the pavement down to the secret fire road called the West Trail.

This is the original century-old road shown on Lawson’s map. Go on, you’ll thank me later.

There’s a particular kind of peace to be found on abandoned roads.

Once the trail ends, back at Castle Drive, you can pick your own best way down, an exercise left to the reader. This was my way — 3.5 miles long, 1100 feet downhill. . .

through the redwoods and across Leimert Bridge.

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The human presence, benign to sanctified

28 May 2018

In my last post I described the spectrum of waste to be seen around the landscape. Piles of garbage are easy to see and judge as litter. Bones and downed trees are natural, but for citydwellers it takes an effort not to wish them removed.

Then there are all the things on the spectrum of art, like these amaryllis bulbs donated to undeveloped Knowland Park. Yeah, they’re an alien species, but neither is their presence a heedless desecration. People decorate things; it’s what we do. We make spaces where there were none before.

We make local habitations, some even with names, in wild Oakland.

I’ve come to enjoy the signs of artistic impulse out in the field. Many are harmless roadside attractions, too small to catch attention except at walking speed.

Others turn the nondescript into something . . . descript. I wish I knew more about this little assemblage that was up in Knowland Park a few years ago.


I think a solo artist made that.

I think a small group adopted this tumbledown shed on Grizzly Peak; their affection shows.

And then there’s the communal gallery up in Leona Heights Park; been going on for years; different every visit. Anyone can participate, but you can tell the standards are intimidatingly high. The first time I found it I went yeesh; now I take my cue from the Grateful Dead: “nothing left to do but smile, smile, smile.”

The best place for this kind of art is the Albany Bulb. I haven’t been in a few years. Wikipedia suggests that the art has been fading and is no longer what it was. With that in mind, here are some photos from about ten years ago.

The general scene is a former dump site for construction waste. Today, most material of this sort is recycled — old concrete can be used as aggregate in new concrete and rebar is easily melted into new steel. At the Bulb, this material was recycled into art.

Lots of the work was crude, but effective.

The setting, on the Bay shore, was a crucial part of the work.

This is one of my favorites, now long gone.

The landmark of Mad Mark’s Castle was once a sublime place.

litterart-13.jpg

Today I’m told it’s not really there any more. Sic transit gloria mundi.

The litter spectrum, benign to unholy

14 May 2018

The litter on the land falls along a spectrum. This disembodied deer hoof does not qualify as litter because, as far as I know, a human didn’t leave it on this abandoned, overgrown fire road. I felt no obligation to do anything more than pause and contemplate it.

At the opposite end of the spectrum is this spectacle just off Pinehurst Road on well-posted watershed land. I felt ashamed on behalf of humanity, but could do no more than pause and contemplate it.

Fortunately, that same day East Bay MUD had a crew nearby with a backhoe and related equipment, cleaning up an even worse litter pile.

That leaves a wide space between the two ends of the spectrum with different judgments to make and responses to consider. This next photo of cannabis seedlings is a few years old, and maybe these days people are doing a little less surreptitious planting in our out-of-the-way places.

It’s possible to do this without making an unholy mess, but I’ve come upon several instances in Oakland’s hills and they were all unholy messes of degraded plastic, bags of spilled fertilizer and remnants of crude camping practices. And the plants didn’t look very good either. So these seedlings . . . I left alone with a sense of foreboding similar to what I’d feel upon seeing fresh bear scat.

Stuff like this dead mylar balloon, lost from some celebration and fallen to ground way off the trail in upper Grass Valley, makes me feel ashamed . . .

but I’ve learned to stop stewing and just pick it up. One by-product of my quest to reduce my collection of rocks (I no longer call it a rock collection) is a growing set of used baggies. I pack several and fill at least one each outing.

Turns out that while I’m not that much of a hiker — just a flâneur of the hills — I’m right up there with the hardcore wilderness walkers who carry trash bags as their eleventh essential. It’s a practice I urgently recommend you follow too. Maybe we can’t fix the unholy messes that way, but it beats shame.

And here and there, you come upon trash that is picturesque.

And occasionally you come upon a thoughtful act of self-expression, around some corner where it’s been waiting to meet you. That’s what my next post will be about.

In other news, I’ll be leading a walk for Oakland Urban Paths next month visiting some of our historic former rock quarries. That’s in addition to the Oakland Heritage Alliance walk I’m leading in July. Also in the works, a walk for the Friends of Sausal Creek.

How useful is the Orinda Formation?

30 April 2018

Walking along the paved trail north of Inspiration Point, I was brought up short by a splendid outcrop of conglomerate.

It’s strongly reminiscent of the Orinda Formation conglomerate exposed to the south in Claremont Canyon, in Sibley Volcanic Preserve and along Route 24 east of the Caldecott Tunnel.

Naturally I fired up the geologic map (I keep USGS map MF-2342 on my tablet) to see how the locality is mapped. It’s the little hill northwest of Inspiration Point, right above the word “Nimitz” where a power line runs.

But instead of Orinda Formation (the orange unit labeled “Tor”), which underlies Inspiration Point, it’s mapped as “Tus,” or “unnamed sedimentary and volcanic rocks (late Miocene).” Turns out there’s a major fault that separates two big blocks of young East Bay rocks — that thick black line with the teeth that represent the upthrown side — and even though the rocks look the same, we can’t say for sure they are.

The area of “Tus” rocks is rather large; in fact it’s the largest single rock unit on the geologic map.

I poked around the literature and found that the Orinda Formation has drastically shrunk over the years. As one example, here’s part of a 1973 map of the Lafayette area (Calif. Div. Mines & Geology Map Sheet 16) that classified a bunch of rock as Orinda Formation, drawn with the exquisite attention that emanates authority.

But the details are quicksand. First, the map is not about bedrock per se, but landslide hazards. Second, the author’s citations are generally very old, not that there’s anything wrong with that, but still. Third, the author’s idea of the Orinda is a unit that mixes lava beds (lumped today in the Moraga Formation) with the coarse sediment that defines the Orinda today. A long footnote explaining his thinking shows that he basically made an arbitrary choice of stratigraphic nomenclature to match the informal usage of local engineering geologists, who tend to talk about “Orinda-type” materials (like I was thinking at my outcrop) without making sure the stuff actually matches all the criteria for the Orinda Formation.

I’ve read my share of geologic engineering reports; any large construction project has to have one prepared. They’re good for their purpose — ensuring that the work is suited for the ground — but they don’t critically assess all the details of the science. And they probably shouldn’t. Instead, they line up the existing literature, outmoded and current alike, and discuss or dismiss it on the way to reaching their conclusions and advising their client.

Maps like Sheet 16 propagate obsolete or informal nomenclature, and thus stratigraphic concepts that are outmoded or discredited persist in the geotechnical literature like zombies long after research scientists have moved on. But I don’t blame people. The old idea of the Orinda Formation, widespread and simple, was very handy. The current idea of the Orinda, constricted and specific, is less handy because it leaves a large area of bedrock with the mumbly label “unnamed sedimentary and volcanic rocks (late Miocene).”

Geologic maps aren’t written in stone. Only stone is, and we’re still learning to read it.