Six Oakland quarries

Oakland, like most growing cities, started its climb to prosperity with the resources it had on hand. Those were land, soil, water, timber and stone. Today we produce no domestic stone. Here are the remains of six different quarries, five in Oakland and one in Piedmont.

Sibley Regional Volcanic Reserve is a former quarry where the basalt of a Miocene volcano was exploited for traprock.

The Morcom Rose Garden is said to be a former quarry; that would have been a gravel pit given that there is no bedrock mapped there.

The Hiller Highlands neighborhood is built around an old quarry where the highly faulted rocks (the Hayward fault is just to the left of this photo) were handy for making crushed stone.

Part of the Serpentine Prairie was exploited for rock at some point, probably for fill material.

Piedmont’s Dracena Park is the former Blake quarry, yielding Franciscan sandstone for aggregate under Oakland’s streets.

And the Zion Lutheran church, off Park Avenue, was built in an old quarry where sandstone of the Franciscan Complex was dug for crushed rock.

There are more of these; I just need to sort through some more photos. I think it’s important to source raw materials like stone from nearby whenever possible.

Old friends in new places

I paid a visit to Alum Rock Park today, in San Jose just past the far end of the Hayward fault where it joins the Calaveras fault. It’s a lovely place. One thing I found especially pleasing was the historic warm mineral springs, which happen to be confined to the Claremont Chert.

Down here the Claremont is dark, perhaps because of a high carbon content. I don’t think I’ve mentioned it here before, but the Claremont is actually a small part of the Monterey Formation, a very widespread unit of siliceous sedimentary rock that accounts for a large share of California’s petroleum. The carbonaceous material, in combination with its high permeability and the presence of hot fluids in this highly faulted area, probably accounts for the concentration of stinky sulfur springs here. It’s a fascinating place that I’ll be describing in more detail Thursday on the KQED Quest Science blog. But I knew you’d like to know that I raised a little mental fist as I saw one of Oakland’s distinctive rocks here.

Pebbles and perils

I was writing a post on Pebble Beach for the KQED Quest site. The pebbles there are special because they represent a huge variety of source rocks, pieces of which ended up offshore in deep-sea rocks back in the Cretaceous. Those rocks were pushed up above the sea and eroded again, freeing the pebbles for several more rounds of polishing and recycling. Then they were sifted in the surf to isolate them in a pure deposit.

Oakland wasn’t lucky that way. The sediments here derive mostly from finer-grained rocks, and the big exception, the Oakland Conglomerate, didn’t include such a variety of gravel clasts. And even then, our sediments don’t seem to have enjoyed a vigorous winnowing and polishing in the Pacific waves. Rather than shiny pebbles, our shores are mostly clay and mud.

But not entirely. The Franciscan block that underlies Piedmont and neighboring parts of Oakland contains the grab-bag of seafloor rocks typical of melange, and the debris it has shed down Oakland’s creeks has made it to the bay. So have bits of the basalt in the high hills. One place that these pebbles appear is in a little spot along the shore of Lake Merritt.

Unfortunately the waves of Lake Merritt will never turn them into a dazzling display. However, neither will Oakland ever be ground into bits like the retreating San Mateo coastline. Our dwellings will never fall down the eroding coastal bluffs like they do in Daly City. I can live with that.

Crystals

Along the south shore of Chabot Reservoir, in the mudstone of the Joaquin Miller Formation, I spotted this exposure of tiny, enigmatic mineral crystals last year.

They’re sprinkled across what looks like a bedding surface in the mudstone. This shot shows an area just a couple inches across. Because I was just taking a walk, I didn’t have my hand lens with me and I didn’t try to bring some home, so this is all we have to work with until I, or someone else, finds the spot again.

A medical maxim that’s just as useful in geology is, “where you see hoofprints, look for horses and not zebras.”

Most of the time, large grains in a mudstone are quartz. The crystals are elongated and appear to be prisms with points, just like quartz. But the honey color appears to be part of the mineral and is most unusual for quartz. Any quartz in this rock would be detrital, not authigenic—by which I only mean that quartz would not grow here, but would come in with the other sediment that turned into this rock. And if it were detrital, it wouldn’t have this clean sparry shape. Instead it would be ordinary sand grains. So I’ve ruled out quartz.

My working hypothesis is that these are calcite crystals in the typical “dogtooth spar” shape that have grown here, or very near here. I could confirm that with a quick acid test, and one little tiny piece of a small puzzle would be clarified.

Fake rocks

The Hillcrest School, up in upper Rockridge/Broadway Terrace, has a beautiful campus with gardens, trees and this play structure.

I wish it were real rocks. Of course the artificial outcrop is good for encouraging unstructured play, confidence using one’s body and all the rest. The lessons it teaches will serve the kids well as they move on to indoor climbing walls and other safe amusements. But I cherish grit, lichens, little holes with secrets inside, moss, roughness that scratches to get your attention. I favor visual texture, variegation, the presence of minerals in their cryptic typicality, the possibility of fossils. I want kids to explore wild rocks in wild places, even if it’s only a roadcut or a vacant lot. I think these kids deserve a real, local knocker, the kind their neighborhood abounds in. We have enough fake rock in our Disneylands and along our freeways.

Thank you.

Northbrae rhyolite

Berkeley is full of interesting rocks, many of them preserved in pocket parks. The most prominent of these is Indian Rock.

Click the photo for a 1000-pixel version. Some 100 years ago this tough rhyolite, a volcanic rock very high in silica, was mapped as part of the same rock body as the one in Leona Quarry and farther down along the hills. But a master’s student at Cal State Hayward gave it a good look in the 1990s and determined that the Northbrae rhyolite, as this occurrence was named, is quite a different rock. (In fact the Leona rhyolite isn’t considered a rhyolite any more, but rather a high-silica welded tuff/volcaniclastic sequence.) Whereas the first mappers thought that both rocks were Pliocene, which is quite young (about 5 million years), the Leona was later shown to be Jurassic (about 150 m.y.). The Northbrae is not that old, but neither is it as young as Pliocene. It’s just a little older than the Moraga Formation basalt and the volcanic rocks of Sibley volcano, about 11 million years, making it Miocene. It came out in that same episode of eruptions, which today sits to the north around Clear Lake and The Geysers.

And it’s still definitely rhyolite. Rhyolite is the stiff, slow-moving lava that makes up little volcanoes like the young dome inside Mount St. Helens, or the rugged knobs of the Inyo Domes, over the Sierra in the Mammoth Lakes area, or farther south in the Coso Range. It makes great rock for climbers—strong, imperishable, full of handholds and rarely giving way under a person’s weight.

The rhyolite of Berkeley is well worth a visit. Just go at a time when the climbers aren’t busy; the rock parks swarm with them in nice weather. I don’t think Oakland has any of this rock, but it might.

Joaquin Miller Park geology signage: Thanks, Jean Quan

Oakland is going to get a new mayor soon: Jean Quan, currently on the City Council. It was her discretionary money pot that funded production of a set of interpretive signs for Joaquin Miller Park. One sign will be about the park’s geology, and it will feature a geologic map donated by Karen Paulsell and text and photos donated by me. When the sign is unveiled (I don’t know when), you’ll see some familiar pictures, including the basalt from this post:

the blueschist from this post:

the serpentinite from this post:

and the Joaquin Miller Formation shale from this post:

The fifth photo I haven’t posted here before—this shot of the Oakland Conglomerate exposed along the Montclair Railroad Trail:

I didn’t vote for Quan, but I do appreciate her care for the city’s parks and wish her well.