Archive for the ‘Oakland rock types’ Category

Return to Pine Top

25 April 2022

A brief visit to Mills College for the recent pow wow reminded me of some business — not unfinished business, but rather an inquiry ready to renew. The upper end of the Mills grounds is very different from the lush central campus with its beautiful floodplain setting, and it has the possibilities of bedrock and fault-related findings. And it’s been seven years. To refresh our memories, here’s the geologic map.


Pine Top is labeled Jb next to Lake Aliso at the east end of Mills College. Qpaf, Pleistocene alluvium (the Fan); Qhaf, modern alluvium; Jsv, Leona volcanics (Jurassic); Jb, basalt; Jgb, gabbro

I’ve always wondered about Pine Top. It stands up so steeply and dramatically at the foot of the high hills, right on the Hayward fault (which is poorly localized here). The digital elevation model of the hill makes it look as if it had been quarried, and indeed there are records of a quarry on the college land.

I’ve also wondered about Pine Top because the basalt “Jb” is hard to find, and I came up empty on my first visit. The hill appeared to be fully mantled in soil.

The campus is especially pretty right now. I hope they can get past their problems and resume their long successful history in Oakland.

Lake Aliso is its usual self, thanks to the late-season rains.

Supposedly the lake is a sag basin related to the Hayward fault, but I’m starting to think that it owes its existence entirely to damming.

This old photo of the lake, from around 1893, shows the side of Pine Top nicely forested in oaks, which would not be the case had there been a quarry there. I think the quarry was located north of the lake where the freeway now runs.

Source

This time I found the original footpath up the hill. Students used to have costumed processions up this path, bearing torches and regalia.

At the top, they would assemble around the Hearth and enjoy their celebrations.

Maybe some alumnae with long memories can add comments about how it used to be.

The view from the top has closed in as the trees have grown, but in the old days it was surely fine.

But anyway, this time I found bedrock — well, pieces of it, around the big microwave tower that was emplaced up here since my last visit. Here they are arranged for a portrait. I also found a little in the old footpath.

This is not basalt by any means, not even a highly altered basalt. This is the highly altered ash of the Leona volcanics, what the old-time geologists with their eyeballs and hand lenses called the Leona Rhyolite. That calls into question not only the “Jb” label for Pine Top, but the whole stripe of Jb drawn on the geologic map. Just some more things to go and check out this summer.

McAdam’s quarry

11 April 2022

One of my first outings during the pandemic era was a hike up Shepherd Canyon in search of Alexander McAdam’s sandstone quarry. But I was missing a telltale clue. Now I think I’ve found it.

I wanted to locate the quarry because it produced the stone used in the historic First Unitarian Church in downtown Oakland, the only example I’ve found of a truly local rock used as dimension stone in a building instead of crushed stone in an anonymous construction.

The clues I had pointed to a location “at the head of Thirteenth Avenue” somewhere “in Medos Cañon, back of Piedmont.” I thought this meant some place in present-day Montclair, but all the old maps I checked didn’t show any land there belonging to McAdam. Then I tracked down the 1894 Wagner map of Alameda and Contra Costa Counties, and here’s where his ranch was: over the ridge above Shepherd Canyon, in the valley of Redwood Creek.


Shepherd Canyon in the center; the old Thorn Road runs around its north edge. The Heyland property at lower left is in Dimond Canyon and the Hays School is in present-day Montclair. The McAdam property is at lower right past the county line, shown by stippling.

The land is within Redwood Regional Park, so I surveyed the territory last week. Here’s a closeup of the map, showing McAdam’s 131.24 acre holding.

And here’s where it sits in the geologic map.

This is very steep country, in an area where the redwood groves had been logged out forty years before. McAdam used it as ranchland, like others in this remote district, but he also operated a successful quarry here somewhere. It wasn’t in the coarse, crumbly Oakland Conglomerate (mint green) or the shaly stuff of the Shephard Creek Formation (pale green), but somewhere in the thick-bedded, fine-grained sandstone of the Redwood Canyon Formation (Kr on the map, olive green).

Fortunately, Redwood Regional Park maintains the old trails and logging roads, so I superposed McAdam’s land on the park’s trail map and gave it all a good look. Note: the poison oak is very healthy this year.

The only place a quarry would make sense is at the very top of the property, but if you’re up for it, the Tres Sendas trail takes you down to some nice woods.

The rock here is appropriate, just like the stone in the church.

And some of it fractures nicely enough to be dressed into ashlar blocks.

But it would be nuts to operate a quarry down in the back forty. Up on the ridgetop in the northwest corner of the land is where I think McAdam had his pit. It’s the backdrop of this view over the spot where McAdam’s ranchhouse was, at the westernmost corner where the Waterloo Staging Area is today.

It was accessible via the Castle Canyon road, as seen in the 1897 topographic map. The McAdam place is in the center, at the end of the road.

The hilltop is just off high-lonesome Wilton Drive, where I last took you three years ago for the Shepherd Canyon circumambulation. The views from there are still wonderful.

The slope immediately below, too steep to think about descending, is where I think the quarrying went on. The outcrops of Redwood Canyon Formation display good rock.

I’m glad to put this little mystery to bed. And if the Unitarian Church must replace its stone with authentic materials, they know where to look.

Murieta Rock, El Cerrito

14 February 2022

In Gold Rush days, the Bay area was as wild as the rest of California: depopulated of Indigenous people and a free-for-all of frontier characters. One of those characters was the legendary outlaw Joaquin Murieta. His story, at least the version we have today, had all the makings of legend — a handsome, peaceable Mexican, viciously victimized along with his wife and family at the hands of Americans, who turned desperado and came to a bad end. As befits a good legend, every crime in California was added to his name — and this fine outcrop too in the hills of northern El Cerrito.

The rock stands out in early photos of El Cerrito, back when the hills were still bare, but today it’s unobtrusive in surroundings of trees and homes at the intersection of Cutting and Arlington Boulevards. It’s also smaller than it used to be; a rectangular quarry pit has been carved into its southwestern side.

Supposedly Murieta’s gang would watch the main road from up here and swoop down on victims. Or this would be their lookout when they hid out in Wildcat Canyon. That may have been. I think the name stuck because it looks like a broken-down haunted house made of a rare, unearthly-looking blueschist.

The area is geologically interesting. The rock is just south of Cutting, below the large “L” at the center (part of the name of the old San Pablo rancho).

All the bluish rocks are Franciscan, the orange (Tor) is the much younger Orinda Formation, and between them is the Hayward fault zone. “KJfy” is a metamorphosed sandstone and “spm” is the melange. Regular readers may recognize “Jsv” as the Leona volcanics, but this little pod is actually the northernmost occurrence of Northbrae rhyolite, the stuff of Berkeley’s rock parks. (Thanks to Karl in the comments for flagging my oversight.)

Murieta Rock is a high-grade block in a melange of serpentinite — a rare outcrop within a rare setting — and for background I refer you to this post from the last time I was up this way. Notice the large areas of the map labeled “Qls”; these are gigantic, slow landslides all of which originate in that melange. More of them are in north Berkeley (see my 2017 walking guide to the area, from the Berkeley Path Wanderers Association site).

Enough of that. You can reach Murieta Rock on the 7 bus line, from either the El Cerrito del Norte or Downtown Berkeley BART stations, or drive there yourself of course, but I enjoyed walking there through Canyon Trail Park — if I were Murieta, I’d swoop that way to carry out my robberies. The view from the top of the rock over San Pablo Bay is superb.

And since this is Valentines Day, why not consider the rock for a romantic geo-outing?

Open thread

8 November 2021

I’m so busy finalizing the manuscript of my book that I can’t find the bandwidth for my accustomed fortnightly post today. There won’t be one in two weeks, either. Next post will be on 6 December. In the meantime, the comments are open so we can talk to ourselves for the rest of the month. (“Did you even know you have selves you can talk to?” Bob Weir reputedly told the audience at a Grateful Dead show as they took a technical break.)

The photo is a fine specimen of mariposite rock from the Carson Hill quarry in Angels Camp. This popular landscaping stone is shot with a green chromian variant of the metamorphic mica mineral phengite. Portions of the deposit are worked for gold as part of California’s continuing gold crawl. The specimen is a recent addition to the rock garden at Lake Merritt.

What’s up with you?