Archive for the ‘Oakland conglomerate’ Category

Museum-quality rocks from Oakland

30 January 2017

I keep saying that Oakland has geological features worthy of being put in textbooks. Today I’m here to show you that Oakland has rocks worthy of being in museums, and I’ve put them there.

In 2012, I was asked to put together a set of teaching rocks for the Chabot Space and Science Center. After all, other planets are made of rocks, right? It took some doing, but some of the rocks were easily available within Oakland’s borders in roadside exposures. The conglomerate of the Orinda Formation was one.


The red chert from the Franciscan Complex was another.


And of course there was our serpentinite.


All told, I made five sets of 15 rock types for the kids.

The next year I got a request from Las Positas College, in Livermore, for a boulder of blueschist. Turns out this little college teaches geology, because every citizen will benefit from a course, and students can get a head start on a 4-year degree there. I struggled one out of this streambed, where it wouldn’t be missed.


They installed it in their teaching garden as Rock J, on the left. It’s small compared to its mates, but that thing weighs a ton because high-grade blueschist is pretty dense.


My reward included a visit backstage to see their cool collections.


Then last year, I got a note out of the blue from the under-construction Maine Mineral and Gem Museum asking my help in building their collection. Maine is well known for its gemstone and mineral mines, but the state has no blueschist. I went to a quiet outcrop where it’s just lying around.


Got two nice boulders and couldn’t choose between them, so I sent them both. They told me one will go on display and the other will go in their teaching collection.


None of these are precious collectibles or gemstones. They’re just cool and educational.

I’ve pretty much stopped collecting rocks for myself because I’m not important enough. But museums are important enough.

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.


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.


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.


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.


For the best exposures, look down into the streambed.


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.


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.

“Geoseki” at an exhibition

21 March 2016

Last Friday at the Oakland Museum of California, I had the pleasure of giving a pop-up talk billed as “Artful Rocks and Rocky Art” that riffed off of my backstage experience with the UNEARTHED: Found + Made exhibit (going on til April 24). This was the only chance I’ll ever have to show my rock collection in a museum, and I’m very grateful to the museum staff for helping it happen.

I laid my four chosen specimens on a table and did an alas-poor-Yorick thing with each one. The point was to say something about what a geologist might see upon contemplating these stones, as a counterpoint to what a suiseki practitioner might see in a suiseki stone.


I went from stage right to stage left, starting with this piece of Orinda Formation conglomerate.


It exemplifies a lesson from Earth Science 101 that’s still the most profound thing geologists teach the rest of us. Conglomerate is a rock made of preexisting rocks — pebbles — and sand. The pebbles signal that a long-vanished mountain range once stood nearby, an upland which crumbled slowly into gravel that washed down riverbeds to rest in the sea. They were buried by more and more sediment deep in a seafloor basin, where the gravelbeds turned into new rock. And somehow, that rock was raised again above the water and became part of a new mountain, the Oakland Hills near Claremont Boulevard.

I summarized that with the singer Donovan’s rendering of an old Zen saying: “First there is a mountain, then there is no mountain, then there is.”

This sexy piece of serpentinite came second.


I explained how serpentinite arises when seawater invades the hot deep crust beneath the ocean floor, transforming its minerals from dark pyroxenes and olivine into the soft, scaly green translucent mineral serpentine, named for its resemblance to snakeskin. Later this material was vomited up in a seafloor mud volcano, then transported onto the land by plate tectonics where I found it by the road near Lake Berryessa.

Third was my pet cobble of laminated chert, mascot of my Facebook page. I found it long ago on a San Mateo County beach.


The multiple sets of layers in this silica-rich stone mark different events in its history. The earliest set is the fine laminations; perhaps they were annual layers left by a rich microscopic rain of dead diatom shells, or layers of them made by large storm events. The material, once buried, transmuted into chert under relatively mild conditions. Subsequently, and repeatedly, cracks formed across the laminations that filled with the same silica-rich material — earthquakes like today’s were the likeliest triggers. These veins are evidence of geologic conditions that extended across a whole region for a prolonged period in the deep past. Then the rock was uplifted. And then finally the pounding of cold surf sculpted the stone away until this smooth little nubbin was left. Nevertheless, it held enough evidence for me to visualize that whole lost land and history, as surely as the conglomerate told its tale of a mountain range.

Last was this unprepossessing bit of sandstone from Mountain View Cemetery.


It looks as ordinary as the sand in a riverbed, even under close inspection with a hand lens. But in the lab, the geologist can interrogate it with various microscopes and radiations that go far beyond the visible. People who have done that learned that this sandstone, the high-grade Franciscan graywacke found just up the hill, has its grains cemented together with prehnite. This is a mineral forms at great depth, and a testimony that rocks can be taken very far from their birthplaces and brought back to the light of day.

All four of these stones, then, tell stories that imply the action of slow, colossal forces that are constantly reshaping our planet’s surface. The real work of geologists is to understand those forces and work out their ramifications. The little stories lead to big stories that in turn shed light on the little stories. And that’s what these little geoseki mean for me.

My understanding of suiseki is as shallow as my understanding of rocks is deep. And suiseki practitioners don’t need any of my knowledge to pursue their ends. Our chosen beauties — their art and my science — are orthogonal to each other and that’s OK, because they still intersect. We are fellow appreciaters of rocks, and suiseki stones are as special as mine. Get yourself to the museum and take them in.

Oakland’s conglomerate cousin

26 October 2015

Last Saturday I took part in a field trip with the Northern California Geological Society, looking at conglomerates down at the San Luis Reservoir, in the mountains between Gilroy and Santa Nella. It sounded kind of dry and remote and obscure, but I always have a good time, and I got new insight into Oakland’s rocks.

First a quick refresher: the Oakland Conglomerate is made largely of cobbles in a matrix of sandstone. It crops out in a long belt starting in Montclair, crossing Shephard Canyon and stretching along Skyline Drive across Lake Chabot into Hayward and beyond. Here’s an exposure above the Montclair Railroad Trail, about as good as you’ll see it.


The Oakland Conglomerate dates from late in the Cenomanian Age of the Upper Cretaceous Epoch, a period of geologic time between 100.5 and 93.9 million years ago. At that time the west coast was a large subduction zone, rather like the west side of South America today. Eastern California and Nevada was boiling with volcanoes, with big bodies of granite rising up beneath them.

Huge amounts of sediment eroded off these rocks were delivered down rushing rivers and dumped into a deep-sea trench near the coast. Conglomerate is a typical product of this high-energy setting as coarse-grained sediments are deeply buried.

At the same time, volcanic island chains and seamounts and other bits of the earth’s crust were being carried into the trench from the west, on the back of the oceanic plate, as it approached and plunged beneath the North American continent. These also shed large amounts of coarse-grained sediment into the trench, along with their mangled remains. (Our Leona “rhyolite” is one of those remains.)

What I didn’t know before taking the field trip is that the same kind of rocks were being made during Cenomanian time down at the San Luis Reservoir. (Probably a bit earlier than in Oakland, although we can’t get very precise ages from rocks like these.)

Here’s the geology of the area, from the online state geologic map. The reservoir is at the center.


The three dashed lines in the reservoir are strands of the Ortigalita fault. We walked along the shore just east of the middle strand, starting at the Romero Visitor Center. Here, just like in Oakland, the set of rocks known as the Great Valley Sequence (light green, labeled “Ku”) is tilted toward the east and exposed edge-on in the hillsides. That means the oldest rocks, at the bottom of the pile, are exposed along the western edge. The tilted beds of these rocks are subtly visible in the grassy hillsides in this view north, just as they used to be in Oakland’s hills before we let the trees take over.


Also, just like in Oakland, the Great Valley Sequence is shoulder to shoulder with Franciscan rocks across a major fault. In Oakland the two bodies of rock are interesting examples of their type. At San Luis Reservoir, the two bodies are world-class examples of their type. The Franciscan rocks there are jadeite-bearing meta-graywackes that are famous in the literature. The Great Valley rocks there are the biggest conglomerates in the whole state. They aren’t yet famous in the literature, but our field-trip leader wants to raise their profile.

San Luis Reservoir is a storage tank, not a lake. It’s pumped full and drained under human control to regulate irrigation for the San Joaquin Valley. As the water has risen and fallen over the last 50-plus years, it’s washed the soil off the rocks and left fabulous exposures. Here’s the view from the visitor center, with the dam on the left (east).


You can walk all over these yourself, just as we did.


Oakland’s conglomerate generally contains stones of pebble size (16 to 64 millimeters) or cobble size (64 to 256 mm, plum to basketball size). The San Luis conglomerate has stones of boulder size, which is pretty phenomenal. The biggest of these are measured in meters.


Our trip leader, Todd Greene of Cal State Chico, helped us picture this area as sitting at the mouth of a big, long-lived submarine canyon like the Monterey Canyon off our coast today. Torrents of well-rounded river rocks roared down its channels to pile up in a deep-sea basin. So the biggest rough-edged boulders, like the one he’s sitting on, represent crags that have fallen from the sides of the canyon.

We worked our way along the shore, moving down through more than a kilometer of conglomerate beds. Near the bottom was a thick unit of modest-sized stones, something very much like our Oakland Conglomerate.


Imagine if we could see exposures of this quality in Oakland. Oh well, everyone needs a daydream.