Oakland geology ramble 7: South Orinda to Montclair

22 July 2019

It’s been a while since I brought you a geology ramble — a no-car-needed hike that starts and ends at different places with public transit. Ramble number 7 connects Montclair and Orinda over the spine of the Oakland Hills. Three years ago, when I presented ramble #2 from Rockridge to Orinda, I said “I have a vague scheme for a southern route,” and this is that scheme fleshed out and walked both ways.

Here’s the 6.4-mile route superimposed on Google Maps. It climbs from about 600 feet elevation at either end to 1600 feet, in Sibley Regional Volcanic Preserve.

The route starts in Orinda, for reasons I’ll get to later. From the Orinda BART station, take the County Connection number 6 bus and get off at Woodland Road, across from St. John Orthodox Church. The first mile is a gentle downhill into the valley of Moraga Creek, on Woodland and then Valley View Drive. You’ll pass this hillside, which exposes lava of the Moraga Formation.

If you have the time and energy, this is an interesting exposure, but there’s a long walk ahead.

The road turns into Lost Valley Road as it turns right. Follow it to Edgewood Road. At the end of Edgewood is this gate leading to the open space of the Orinda Wilder development.

Go on in. The road straight ahead leads to the new homes of Orinda Wilder, but you’ll turn left, up the hill.

Two miles farther and 800 vertical feet up, there’s another gate at the boundary of Sibley. On the far side of the gate is a sign saying “private property.” That’s the sum total of the access restrictions along this ramble.

The full route takes you through 12 different geological map units, by my count.

They are the Mulholland Formation, Moraga Formation basalt (Tmb) and interbedded sedimentary rocks (Tms), the Siesta Formation (Tst), Orinda Formation (Tor), Claremont Shale (the golden stripe from corner to corner), Sobrante Formation (Tsm), the unnamed Eocene mudstone (Tes), the teeny, cryptic Paleocene sandstone (Ta), Redwood Canyon Formation (Kr), Shephard Creek Formation (Ksc), and the Oakland Conglomerate (green).

What particularly interested me about this hike was the stretch through the Siesta Formation and the Moraga Formation sediments, which aren’t exposed in Oakland proper.

The Siesta consists of fine-grained stuff: sand, silt, clay and some limestone. The limestone is what stands out, because it’s white.

Outcrops of the limestone aren’t obvious, but my little acid bottle always reveals it by the telltale fizz.

This large limestone boulder reveals a lot of broken-up structure, including some shale chunks mixed in. Landslides will do this to unlithified sediments — and around here, even 9-10 million years ago, that means earthquakes.

The Siesta also includes a little conglomerate, as seen in this roadside exposure, but in general the rock isn’t highly visible. It likes to turn back into dirt.

Farther up the road, the Moraga sediments show themselves as coarse sandstone, rough stuff that gathered around whenever the lavas weren’t erupting.

That little magenta bit brings me to the other scenery: it’s always more than rocks that brings me to these heights. In early summer it’s wildflowers.

And views.

And views of rocks.

By the time you’re on the ridgetop, you’re well into the volcanic rocks that Sibley is famous for. Look carefully and you may spot the mineral-filled lava bubbles called amygdules. (I left this one for you to find.)

The high point in Sibley is the midpoint of this ramble. It’s all downhill from here to Montclair (where you catch the “geologist’s express” 33 bus), heading south on Skyline and then following the route of my Shepherd Canyon circumambulation. Be sure to look back at that lovely ridge as you start down Skyline.

I’ll end this post with a big fat map showing the topographic contours, more road details, and mile markers.

Stones of the Broadway Valdez District

8 July 2019

The Broadway Valdez District is a swath of territory, named by city planners, centered on Broadway and extending from 23rd Street up to I-580. The area is undergoing redevelopment at a furious pace. For my purposes, the area will give birth to a showcase in the use of building stone in current architecture. Here’s the first chapter in that story.

Gone are the days when stone was a load-bearing structural material in Oakland buildings. (A few of our older churches survive from that time.) Gone are the days, even, when stone was fashionable for large-scale cladding (see: Kaiser Center’s dolomite and Lake Merritt Plaza’s granite). Today, at this moment, stone is a color and textural accent framing the offset planes of metal and matte components by which today’s designers react to the bland slabs and faceless facings of yesteryear.

First out the gate in the race to develop the Valdez Triangle (framed by Broadway, 27th and 23rd Streets) is the Hanover Company’s two big residential properties, Hanover Northgate and Hanover Broadway.

Hanover Northgate stretches the length of Valdez between 24th and 26th, and its exterior has two different treatments. Here’s the south end.

The part on street level is faced with stone. The blond stuff, which echoes the top level, is an interesting fossiliferous marlstone. And I mean, fossiliferous.

Marlstone is not your traditional white or gray limestone; it owes its hues to clay minerals mixed with the carbonate minerals that define lime rocks. The variegated texture and color is integral to its charm. Notice among the cross-sections of fossil bivalves is a little brown gastropod shell.

Beneath this, at foot level, is a dark gray marlstone.

No obvious fossils here, but there are veins of calcite to lend visual interest. Carbon, not carbonate, provides the black color. Such a rock forms in anaerobic settings, where organic carbon is reduced to elemental carbon rather than being oxidized to form carbonates or CO2 gas.

The other end of the Hanover Northgate has a different palette: a warm buff-gray over a crystalline black footing.

In between the two parts, the carport features this rugged paving. However, it’s just dark-tinted concrete — cheaper than stone, just as tough and more controllable.

On to the real stone: a nicely polished laminated limestone. This panel displays two little faults cutting vertically through it; the middle part has been displaced upward. There’s also a thin vein, the result of a later fracture.

Interestingly, all the panels I took a close look at appeared to be placed stratigraphically upside-down.

One of the first things a geologist looks for is evidence of stratigraphic up and down. After seeing a few hundred outcrops, doing that becomes automatic, especially in the Oakland hills where many of the steeply tilted rocks are overturned past vertical. Sometimes the evidence can be quite subtle. The most obvious sign in this example is at bottom center, where the thin beds sag into the gaps in the thick bed. This rock — in this panel anyway — accumulated in the downward direction.

If I’m right, then the stone supplier made sure to mark this stock with arrows so it would be set in a consistent direction. Although the result might confuse geologists, it would avoid giving observers of the walls, even ordinary folks, a vague sense of disorder.

The dark rock at the base of the wall is “black granite,” better known to geologists as something like amphibolite or gabbro. It has a sandblasted finish for a more matte appearance.

Between the two Hanover buildings is a lovely plaza, open to the public and graced with stainless-steel sculptures by HYBYCOZO: a stylized oak tree and eight California poppies.

Not much stone is in evidence — the plaza is mostly tile and concrete and metal. But the benches are topped with solid black basalt.

Which leads us to the Hanover Broadway building, soon to have a Target store on the ground floor.

There’s not much stone on this building either. Like “Whistler’s Mother,” it’s a study in black and gray. Here’s an odd irregular panel on the side facing the plaza: brecciated limestone.

Here’s a closer look at this intricate material. (All images click to 800 pixels, as is my usual practice.)

And around the side in an entryway are some panels of gneiss, complete with quartz veins. The squashed-and-squeezed veins feature what are called ptygmatic folds, because geologists have come up with the damnedest words for things.

These stones, all of them, are things no human artist or artisan can duplicate. Though it may go in and out of fashion, stone will never go out of style.

Oakland and the Coast Range ophiolite

24 June 2019

A commenter asked, in connection with a recent post, if I’d written anything about Oakland’s ophiolite. The answer is, not specifically until now. The Coast Range ophiolite (OH-feel-ite) is a string of mostly disconnected outcrops of unusual rocks that extends north almost to Redding and south almost to Point Conception, rather like the way my writing about it runs through the ragged string of posts on this blog between late 2007 and today. In Oakland, the serpentinite patch is part of it, the San Leandro gabbro is part of it, and the Leona volcanics are part of it (see posts about that part here and here).

Here’s a recent simplified geologic map from a 2005 paper by ophiolite savant John Shervais (doi: 10.1130/B25443.1, available here) showing the most important bits of the ophiolite in black. It’s kind of a privilege to have a piece of it in our city.

The details in California are intricate and I’m about ankle-deep into them at the moment, so I’ll be pretty general here. Ophiolites were first recognized as a suite of related rock types over 200 years ago, at the dawn of scientific geology, in the Alps. Alexandre Brongniart gave them the name, which means “snake rock” in scientific Greek, because serpentinite (“snake rock” in scientific Latin) is so typical of them. About 50 years ago, at the dawn of plate tectonics, they were recognized as pieces of oceanic crust that somehow have ended up on land during the elephantine dance of the tectonic plates.

The oceanic crust of most ophiolites is not as well organized as the standard oceanic crust formed at mid-ocean ridges. It forms in the roiling setting near subduction zones, where subducting plates sink straight down and draw the other side toward them. (This situation, called slab rollback, is the opposite of what we’re taught in popular accounts that talk about subduction in terms of colliding continents and mountain-building.) As the plate on the other side is stretched thin, new magma forms beneath it, rises and freezes into fresh oceanic-style crust (the ophiolite). It’s because most ophiolites form near land next to subduction zones (the “suprasubduction-zone setting” in academic lingo), not way out to sea at mid-ocean ridges, that we find scraps of them plastered onto the continents in a couple hundred places around the world. There are some other tectonic schemes that make ophiolites, but this is the typical one.

Ophiolites consist of rocks that correspond to the major layers of oceanic plates, which are a deep base of peridotite, a middle layer of gabbro, an upper layer of basalt and a cap of mixed seafloor stuff: red clay, seamounts, volcanic chains and the odd limestone basin here and there. And most of these can be found in Oakland. The peridotite, when seawater reacts with it, turns quickly into serpentinite (but you can see rare remnants in places).

The gabbro, a coarse-grained rock of the same composition as basalt lava that has cooled slowly enough for visible mineral crystals to grow, underlies much of San Leandro and the deep-East Oakland hills.

And the Leona volcanics is a big pile of volcanic ash, shot through with dikes of basalt and now strongly altered, that sat on top.

The Coast Range ophiolite is highly disrupted now. It’s been caught up in millions of years of squeezing, stretching and kneading North America’s western edge — and that was before the San Andreas fault system arose and smeared everything sideways.

A handful of intrepid specialists continue the work that Brongniart started, reassembling and correlating and extrapolating and collecting ever more data. Ophiolites are important in the bigger scheme because some of them are the only pieces of seafloor rock that are older than the present ocean floor (which barely covers the Cretaceous period, back to about 140 million years). But if ophiolites are born poorly organized, unlike proper deep seafloor, attempting a perfect restoration may be an delusion, a will-o’-the-wisp. Our insights may always be a string of fragments, and we may have to let mystery be, but we have to try.

I also wrote this introductory piece about ophiolites for another website, once upon a time.

The Lincoln Square landslide of 1958

10 June 2019

The Lincoln Square shopping center, which I featured in my previous post, has nothing to do with President Lincoln, just as Lincoln Avenue has nothing to do with Honest Abe (it was named for Lincoln Rhoda, son of landowner Frederick Rhoda). It wasn’t on the Lincoln Highway either. Nope, it was named by the prominent citizen and developer, Luther H. Lincoln, on whose land it was built, to honor himself. Admittedly, Lincoln had his own measure of fame from serving as Speaker of the state Assembly in the late 1950s.

The shopping center, which opened in 1963, sits on the site of a messy, sensational landslide.

When it comes to landslides, the blame usually lies uphill. And in the late 1950s one of Oakland’s largest suburban developments, Crestmont, was under construction on the steep hillside above Lincoln’s land.

The hillside of Crestmont was acquired and developed by Andres Oddstad’s residential construction company. He made his name building whole neighborhoods of “economy homes” in South San Francisco, Pacifica (Linda Mar was his work), Redwood City and other West Bay localities. Crestmont was Oddstad Homes’ big splash in the Oakland market, a luxury development tagged “Riviera of the East Bay.” The ads in the Tribune cried, “Grand, sweeping panoramic views from your home in Crestmont leave you breathless day or night. Here is the charm and freedom of country living only 15 minutes from downtown Oakland!” The redwood-and-stucco houses cost $30,000, a premium price in those days. And the views truly are terrific.

Oddstad worked big and fast, leveraging its economies of scale. This aerial photo shows the state of things in early 1957.

And here’s a similar oblique view from Google Maps with the street names. The landslide I’ll describe was on Van Cleave Way, down at the bottom of the development. You can see from the airphoto how much digging and grading was involved. The serpentine rock making up the hillside was . . . mostly strong. The homesites built up on filled land were . . . mostly reliable.

Luther Lincoln and his family lived on the large lot of 4000 Redwood Road, just below the bottom of the image, as early as 1952. As Crestmont went in on the hillside above him, Lincoln built a big new home and arranged to have part of his land rezoned from residential to commercial. It was an ideal site for a shopping center to serve the new residents. And the land was largely waste already: The defunct Alma Mine, with its 5000 feet of abandoned tunnels and piles of waste rock, sat next door.

Oddstad’s project went well until the winter of 1957-58, the wettest season in 50 years. Ten inches of rain occurred in February, another ten in March. Two more inches fell during the last weekend of March, and just past midnight on 30 March, in the midst of a pounding rain, about 300 feet of landfilled hillside on the west side of Van Cleave Way began to crumble.

The Tribune reported that Mrs. Walter Horberg was moving the furniture out of 79 Van Cleave Way. “At 2:45 a.m., as beams groaned and snapped, the rear portion of the handsomely designed ranch-type home sagged and then, with a mighty crash, tumbled down the hill. The rear rooms of the house tumbled 100 feet, most of it straight down, and were carried along by the mud slide. The front section dropped a lesser distance. Somewhere in the rubble, the Horbergs’ family parakeet, Nickie, chirped on.” No one was hurt, but six homes on the block were lost or endangered; two of them hadn’t even been sold yet.

This photo from the next day’s paper, one of many from the catastrophe, was reproduced in US Geological Survey Professional Paper 944, “Relative slope stability and land-use planning in the San Francisco Bay region, California,” published in 1979 and still a good read. You can see that there’s no bedrock visible in the landslide scar, just dirt.

Here’s the scene below Van Cleave Way today. The lots for the five lost homes were rebuilt, turned into four larger lots, and developed 20 years later. The leftmost house, its roofpeak just visible, is one of the original ones from 1957.

This was not the last slide in Crestmont. Two new houses on Kimberlin Heights Drive were lost in June 1958 when the concrete piles holding them up failed. (A mild earthquake on 31 May was made the scapegoat.) And in 1962 a mudslide from the hill above Kimberlin Heights Drive swept a 5-year-old girl to her death.

But back to the Van Cleave slide. The wall of mud poured onto Luther Lincoln’s new home directly below, destroying the house and all of its contents except for a car. A few years later, Lincoln turned the scene of ruin into the Lincoln Square shopping center, and the textbook exposure of serpentinite in the hillside behind it that I showed you in the last post dates from that time.

To my knowledge, no slides have occurred in Crestmont since 1958. The streets look sound to my eye. But some empty lots remain below Van Cleave in the landslide scar that could be developed some day.

The pressure to fill open land with traditional suburban houses is relentless. And all the land left open today is precarious.

Edited to correct the date of the fatal slide in 1962, not 1955.