Franciscan rocks | Oakland Geology

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Rock garden coming to Lake Merritt

29 March 2021

The Gardens at Lake Merritt are building a rock garden in the heart of the grounds. They have plenty of gardens with stones in them already, but this will be a proper rockery. This post is about the work in progress. I know almost nothing about the plants they’ll be featuring, but I do know a little about rocks.

As we enter yet another year of drought, it’s important to note that rock gardens are made to conserve water. The stones and gravel offer solid shade to the underlying soil, and the typical plantings are small, hardy species from alpine or desert settings. As you bend down to admire these plants, have a look at the stones.

The location is between the community plots and the Torii gate, the crossroads of our remarkable garden complex. Just across the path is the hill-and-pond garden, where the turtles hang out.

This view toward the lake shows the layout. In the background is the entry to the Sensory Garden, which has had a thorough going-over during the shutdown.

The foreground containers in both photos showcase rounded river stones, blue-green argillite most likely from the northern Coast Range. I would not be surprised if some of the rock nuts of the Suiseki Societies of Northern California contributed to this project.

The center of the garden is a mound of sandstone tablets, with some accent stones, oriented north-south for optimal sun. Rings of different colored gravel surround it. Note the “do not climb” sign.

Some of the basins echo the brown sandstone of the central mound, offering textural contrast.

Others contrast more strongly. Here rough greenstone is set in crushed marble.

And what would a Northern California rock garden be without some red chert?

All of these rock types are typical of the Franciscan Complex, a lithological scrapple that makes up the bulk of the northern Coast Range, including San Francisco and the hill that Piedmont sits on. Get to know them, and you’ll see them all over the place.

The Gardens at Lake Merritt have several sectors that artfully mix plants and stones. The water garden I mentioned earlier is one, and there’s the cactus garden and the bonsai garden (which has just added a suiseki section) and the enclosed Japanese garden by the Community Center building. They’re all looking great right now, but check the hours before you go; weekends are still closed.

A few years back I wrote about the remarkable rock garden assembled by Ruth Asawa in San Francisco. As the world reopens, I hope to visit it again.

Posted in Franciscan rocks, Oakland stone | 1 Comment »

Mapping rocks never ends

1 February 2021

A few days ago I took part in the latest monthly meeting of my local geological society — we do it via Zoom these days — and our speaker, Christie Rowe of McGill University, reported on three research projects her grad students are doing in the Bay area, specifically the Franciscan Complex. The Franciscan is a scramble of different rocks that has challenged geologists since they first came to California.

Fifty years ago Stanford’s Gary Ernst recognized that the Franciscan represents the mess of material that gathers around a subduction zone, where oceanic crust (a now-extinct neighbor of the Pacific plate, in our case) slides beneath continental crust (the North America plate). So now we know what it is — the tectonic equivalent of the dirt in a bulldozer’s blade — and prompted by that knowledge we can try to unscramble the mixed-up pieces and learn what they might tell us about California’s geologic history or what happens in subduction zones.

Rowe is a Marin County native who’s been working since her PhD days on the latter problem, in the Franciscan rocks of her home ground. Specifically, she’s been looking for preserved bits of ancient earthquake faults. Normally these are buried deep underground, but they’re important because subduction-related earthquakes, so-called megathrust events, are the largest on the planet. Think of the 2011 Tohoku earthquake in Japan, the magnitude-9 monster whose tenth anniversary is coming up on 11 March. The Marin Headlands are full of them, broken in pieces.

Rare bits of the Franciscan have survived being subducted deeper than 25 kilometers and then returned to the surface, without totally wiping out what happened to them down there. The work requires dogged persistence. You have to look hard to find these “high-grade blocks” in the first place, then put your face close to them, magnifiers out, detect signs of slippage, then bring samples to the lab and determine what that slippage means — whether it happened on the way down, on the way up or afterward as the San Andreas fault system wrenched it all sideways.

Heart Rock, at Jenner Beach up the Sonoma coast, is small enough to fit inside a living room. One of Rowe’s grad students is mapping it at centimeter scale, spending a master’s thesis worth of effort on this one outcrop looking at rocks like this.

Seeing all this during Rowe’s talk took my mind, among other places, out to the rocks of Shepherd Canyon and Redwood Peak. The last person to give those strata a PhD-level scrutiny, using all available tools of the time, was a Berkeley grad student named Jim Case around 1960. Yes, 1960, a time when researchers were stuck in a mental framework of now-forgotten concepts and plate tectonics was still years in the future, when optical microscopes, brass seives, fossil correlations and test-tube chemistry were the best tools we had.

Case got his PhD, demonstrating that he’d mastered these tools as well as the literature, but he didn’t accomplish much more than correct a couple of ideas from earlier studies, establish a few new rock units on the map and tentatively correlate them with other units scattered around the East Bay. He put his little brick into the Wall of Science, then went on to a long research and teaching career doing other things.

Since then, other distinguished geologists have been over this territory. Case collaborated with Dorothy Radbruch of the USGS, a sharp and able field geologist. And in the late 1990s when Russ Graymer was putting together the East Bay geologic map that I rely on, he tramped the area with the late Earl Brabb and was ably advised by the late David Jones. Each of these workers found new things and revised their predecessors’ achievements. It always paid to reinspect the rocks. Nevertheless, none of them pulled out all the stops and pioneered a new in-depth reassessment of this interesting area.

We could do much better today. Every tool has advanced. The jigsaw puzzle of ancient California is far enough finished that any piece, if studied closely enough, can be placed on the table near — or even exactly on — its correct position and joined to other pieces. This would be more satisfying than what Case could accomplish in his time. We just need another grad student to take it on, another local who has imprinted on his or her home ground.

Mapping never ends, and geologic mapping always improves. New bits of rock are being exposed all the time. Fresh eyes see new things, and persistence furthers.

Posted in Franciscan rocks, Oakland sandstone/shale | 5 Comments »

Upper Indian Gulch

26 October 2020

When I last featured Indian Gulch on this blog, it was about the easy part, mostly a stroll up Trestle Glen Road. It ended with this glimpse of the living Indian Gulch Creek, bounding down the rock slopes of the Piedmont crustal block on its way to culverted oblivion beneath the elegant Trestle Glen neighborhood.

Upper Indian Gulch lies within Piedmont and the west fringe of Montclair that looks down upon Piedmont. Nowhere is the creek up there accessible to passers-by; if you want to see it you have to buy a house whose lot includes it, or make friends with someone who owns such a house. You’ll have to imagine it running in the dark underneath the street, as it does along La Salle Avenue just above St. James Drive.

Here’s an overview of the upper creek from Google Maps terrain view. The creek has three branches; the west fork is the main branch. To be a stickler, that fork should properly be called Indian Gulch Creek and the other two are just tributaries. The old property line between the two middle Peralta ranchos ran up this valley, Vicente’s on the left and Antonio’s on the right. Later the same line separated the Oakland and Brooklyn Townships of Alameda County. Today neither the ranchos nor the townships are relevant any more, but the boundary influenced the pattern of land ownership a century ago as Oakland expanded its territory and developers shaped the outskirts.

A stroll here is a workout. Part of my Ramble 4, Uptown to Montclair, goes up the creek’s middle fork but the steepest part is pedestrian-unfriendly. Three years ago I featured an excursion into the valley of the east fork. That pretty much exhausts the possibilities in those two valleys. In the west branch, two dead-end roads will take you to the floor of the valley, though the creek is not accessible. Indian Gulch Road leads down from Glen Alpine Road, just above the word “West”:

And Calvert Court swoops from Blair Avenue down into the creek’s highest watershed, where Oakland’s most isolated properties lie.

Here and there, you can get a look at the bedrock under the watershed: sandstone and mudstone of the Franciscan Complex.

And it’s hard, on Piedmont’s winding streets, to grasp the contours of the land. This view across the middle fork at Hampton Park is about as good as it gets.

Really, the best experience of these headwaters is on the rim roads that encircle the watershed. They aren’t photogenic in ways that my camera have caught many times over the years, but the views glimpsed through the trees and past the homes have always pleased my eye.

Looking east-northeast up Hampton Road at Sea View is a good view of the high rim of the east branch, topped by Pershing Drive.