Lava rampart at Grizzly Peak

The other week I noticed a big outcrop of basalt of Moraga Formation right next to Grizzly Peak and remarked that it would be worth a visit. And so I did, while preparing my post for KQED Quest Science Blogs last week on the Moraga Formation. It’s easy to reach from a couple of different pullouts on Grizzly Peak Boulevard. Here’s a view looking south.

The rock is thoroughly covered with lichens, so you can’t tell much about it, but it appears to be a massive flow of lava that, like all the rocks in the Berkeley/Oakland Hills, has been tilted nearly upright with the top toward the northeast. Stratigraphically above it, beneath the road, it’s mapped as sedimentary rocks associated with the Moraga Formation. The area was a lake or wetland at the time, about 9 million years ago, with high hills on the west where the Bay sits today. And on the other side of the road, including Grizzly Peak itself, the bedrock is the Bald Peak Basalt, a slightly younger volcanic unit.

Here’s a better view of the west face.

There’s quite a large rock face exposed on that side of the lava bed, but almost nothing exposed on the uphill side. The rock has a lot of paint on it here and there, including some marks that look like rock-climbing instructions. The rock is pretty sound for climbing, but be careful anyway because its strong exposure to sun and weather can weaken this basaltic rock without showing. (The rhyolitic rocks in Berkeley proper are better that way.)

It’s a well-used outcrop, but still, please don’t take a hammer to it as it’s located on UC Berkeley land. It’s the headwaters of Strawberry Creek.

Here’s the view back to the place where I first spotted this feature: in the row of trees just to the left of the rock.

Hayes Creek – Dracena Park walk (#34)

Last week I bought a copy of Secret Stairs East Bay, by Charles Fleming, and met the author in person at a book event at the Solano Street branch of Pegasus. As I leafed through the book and heard the author, it was clear that while the walks offer lots of insight into Oakland’s history and culture, the geologic stories to be seen on these walks were yet to be told. I thought, Well, I can try that.

Sunday my wife and I took one of the walks, number 34, to Dracena Park (featured here before) in Piedmont. It begins at Chapel of the Chimes on Piedmont Avenue and takes you on a pathway across the valley of upper Glen Echo Creek (which I’ve called Mountain View Valley). Here’s the view back toward the Chapel from the other end.

The stream was culverted long ago, but on the 1897 topo map it’s shown as Hayes Creek. Today it’s Glen Echo Creek. The valley floor is so flat because it was graded and planted to houses. But in my unprofessional opinion, its flood hazard today is as low as anywhere.

Onward! The book directs us to the head of this little gorge, part of Pleasant Valley Creek’s watershed, and thence to the old quarry pit now known as Dracena Park.

You should always suspect humans as a land-shaping agent in Oakland, and indeed Walter Blair, who ran the quarry and before that a dairy at this site, may have had a flume or a transport line of some sort here. But its original form appears to be intact.

We turn into the park proper, and glorious bedrock appears—Franciscan sandstone, ready-fractured for its purpose.

Go ahead and inspect the stuff; no hammer is needed (and none allowed anyway) when it crumbles so readily. Fracturing and tectonic movements—and surely some seismic work, like a bartender’s cocktail shaker—has rubbed and even polished parts of the stone.

Fleming says that the stone went into the homes of Oakland, but that is not true. This is not dimension stone by any means, but rather the usual quarry of Bay Area stone hunters in general: crushed stone and aggregate for roadbeds, underlayments and concrete mixes.

What was once a noisy scene of dynamite and dust is now a green bowl punctuated by the cries of children.

The walls of the park are pretty well greened over, but watch out anyway: bedrock exposures are not forever. Maybe in a marble or granite quarry, where solid rock is sawed away in blocks, but here rockfall is a continuing potential hazard.

These are not decorative boulders emplaced by landscape designers, but fallen rock. And that ivy-covered fence at the left? It’s really a safety measure to keep landslides away from picnickers. Here’s the whole thing.

Dracena Park is a worthy way to remake an old quarry. But if you’re here when an earthquake strikes, get away from the walls.

Grizzly Peak and Moraga basalt

In northernmost Oakland is the small, steep Panoramic neighborhood, which perches on the high ridge between Strawberry and Claremont canyons. At the top of Panoramic Way is this view north across Strawberry Canyon to Grizzly Peak, which at 536 meters (1758 ft) is the highest point in Oakland. Click the photo for a 1000-pixel version.

The bottom of the photo grazes the UC Botanical Gardens. The lowest slopes in this view are underlain by the Claremont chert, up to the powerline tower directly below the peak. Above that is the Orinda Formation, and the ridgeline is lava of the Moraga Formation.

To the right of the peak, on this side of Grizzly Peak Boulevard, is a big, towering outcrop of basaltic lava; here’s a closeup.

That seems like it would be worth a visit, if the poison oak isn’t too bad.

Caldecott Tunnel and the Orinda Formation [updated]

Those of us in the geology community have been eager to hear more from the Caldecott Tunnel’s fourth bore project. The bore is cutting through the Berkeley Hills in a perfect transect, allowing geologists to sample every meter of the rocks along the way. This weekend, I heard some news in a presentation at the CalPaleo 2011 meeting.

The east end was where the boring began. The Orinda Formation is beautifully exposed there—most of it, anyway—on both sides of Route 24.

Much of it is coarse conglomerate of late Miocene age, about 10 million years old, that’s thought to represent landslides into a freshwater basin in the middle of volcanic terrain.

That part, naturally, has no fossils because the environment was too rugged for even large bones to survive. But there’s other stuff in the Orinda, like lava flows at the top of the unit belonging to the Moraga Formation. This is a view of the underside of the lava, where it flowed onto the moist sediments one day long ago.

And there’s a good amount of fine-grained sedimentary rock, suitable for preserving fossils, as you move down section toward the hills. Caltrans has a contract with PaleoResource, a well-regarded firm, to monitor the work and recover fossils that turn up. In the initial digs, the scientists found and cataloged thousands of items, including lots of fish fossils, clams and crabs, birds and the first leaf fossils ever found in the Orinda. The press tends to zero in on large mammals, though, so we’ve heard about bones from wolverines, horses, rhinos, camels, pronghorns and even the obscure oreodont.

However, PaleoResource scientists have told me that Caltrans has not allowed the paleontologists into the tunnel proper, in violation of the contract and the agency’s own policies. That’s the kind of thing that makes me awaken at night and grind my teeth. Not only are we not learning about the lower Orinda Formation, we’re not studying the transition into the underlying Claremont Formation and the Sobrante Formation on the west side.

I try to take the long view. The tunnel is being dug in three passes, and conceivably the rocks can be sampled later. But PaleoResource officials have told me their contract runs out this summer, before the next phase of digging.

So I try to take a longer view. Once upon a time, nobody cared about paleontology. Heck, they didn’t care about archaeology. Today, turning up a human bone will stop a job in its tracks. Fossils aren’t that disruptive; they can be salvaged and documented in a day or two. Many agencies and jobs go well, the paleo people and the construction people interacting well and yielding good science. Other jobs are jobs from hell, but the long arc of history is curving toward respect for science.

Update: I’ve put up some photos of new fossils from the dig on KQED Quest Science Blogs.

Rettig canyon

I paid a visit yesterday to part of the Hayward fault in Oakland, but while there I felt the pull of a neighborhood treasure I call Rettig canyon. The name is from Rettig Avenue, which traverses it. Here’s the topography, from Google Maps. This is just south of the LDS temple.

The fault runs from top middle down Jordan Road and exits where Victor Avenue leaves the map. The hills to the west are the southernmost part of the Piedmont block of Franciscan rocks. Rettig canyon cuts right through the hills thanks to Peralta Creek, which comes here from Butters Canyon on its way to the bay through Peralta Hacienda and Foothill Meadows Park.

Normally when you see a stream cutting through a bedrock ridge, you explain it as either stream capture or a water gap. That is, either the stream eroded its way headward through the ridge or was running that way already when the ridge rose underneath it. Given the intense tectonic activity here, I’m inclined to call it a water gap, as I do Dimond Canyon (with the addition of tectonic stream capture).

I saw some possible evidence of this in the streambed. But first, a look at the scene.

Rettig Road is a single lane through the canyon and is coned off as a landslide zone. It’s been that way for at least six years; I hope a local will say more in the comments. The canyon is steep, dark and thickly wooded. You can scarcely see the stream, but you can hear the water everywhere.

But there is a place to scramble down to the streambed. It’s well populated with rocks that appear to be local Franciscan melange, pretty jagged and hence not transported far.

I was looking for bedrock and found some candidates like this scaly schist. I didn’t have my hammer and was reluctant to disturb the scene anyway, so I can’t say much about it. It might be serpentinite.

This is the outcrop that excited me, showing what looks like a thrust contact.

Ignore the green patches; that’s just algae. The rock on the left is fairly soft and foliated parallel to the contact. I picked out a small piece and can’t say much about it, but in the hand lens it looks like a highly altered talcy kind of stone. At the base is a good centimeter-think layer of nice gray clay, then we hit clean tightly packed sediment with highly tilted bedding; indeed it’s tilted steeper than the contact above it. So my best guess is that it may be the contact between the Franciscan and much younger Pleistocene sediments. Due to squeezing along the Hayward fault, the older rock has been thrusted up and over the sediment. This isn’t unheard-of, but I haven’t seen it documented around here so I could easily be wrong. But that would explain the rising ridge, the topography of Jordan Road (which sits in a long trough here that may well be a sag basin) and the course of the stream.

I couldn’t resist bringing home a pocket-sized cobble of beautiful actinolite schist.

Five more Oakland quarries

I’ve scrounged through my files and come up with five more quarries in Oakland that I’ve managed to photograph over the years. I’ll show them from north to south. First, of course, is the old Bilger quarry at 51st Street/Pleasant Valley Drive and Broadway, which I’ve featured before on this blog.

Next we have the quarry in Piedmont that now houses the town maintenance yard, by the ballfield along Moraga Avenue. The little spur street is named Red Rock Road, and the quarry appears to feature some usable shale from the Great Valley Group. But I haven’t dropped by since 2003, and my memory is vague.

Next we have the old Leona Heights sulfur mine, at the end of McDonell Avenue, which I featured in a discussion of Lion Creek.

The big old Leona quarry comes next, which I’ve talked about before, once to discuss the site and another time to show the rocks.

Finally, we have a small quarry in Sheffield Village, right on the Hayward fault where the digging was surely easy. Afterward some of those lovely period homes were tucked in there, along Revere Avenue.

There are more quarries to be documented; I just need to consult the records and then visit the sites.

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.