Archive for the ‘Leona volcanics’ 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.

The pyrite orebody of Leona Heights

10 May 2021

Through historical accident (or fate), I’ve been a longtime reader of the late Oakland fiction author Jack Vance. As it happens, Vance was exposed to geology by coursework in mining engineering at UC Berkeley, and one of the most charming and memorable features of his Planet of Adventure series, written in the late 1960s, was the mineralogical currency of the planet Tschai, called sequins.

In volume 3 of the four-book series, we learn that sequins grow in a locality controlled by the alien Dirdir species, who amuse, enrich and feed themselves by hunting the sequin hunters. Sequins come in a range of colors, the clear ones being worth the least and the rare purple ones the most. I no longer have the text in front of me, but I remember them growing out of the ground, literally cropping out. Over at Tor.com, reviewer Paul Weimer does have (and loves) the text and reveals the additional detail that sequins consist of “a uranium mineral called chrysospine.”

The name is mineralogical fantasy, and possibly misleading in that “chryso-” refers to a golden or light green color. But come to think of it, uranium impurities often turn minerals brown from radiation damage, and radiation damage to an originally clear or golden mineral might result in a fair purple by analogy with “sun ripened” glass. An analogy with ripening fruit, too, is irresistible.

In populating his planet with this precious crystalline substance that grows in the ground like mushrooms, Vance evokes truly ancient geological notions that are natural among people who know nothing beyond the most basic alchemy. Gold Rush California saw a lot of that pre-industrial thinking among the amateur prospectors who scoured the state, and the Cornish miners who worked in the hard-rock Mother Lode mines brought along their own ancient customs and superstitions.

In Oakland, the people who exploited the pyrite in the Leona Heights mining district didn’t have the advantage of magic. But Fritz Boehmer, the canny Prussian immigrant who spearheaded mining in these hills, was apparently prone to dreams of earthly wealth, a deep California trait. He was not especially well educated, having apprenticed in metalworking. When he learned of the ore underneath his ocher deposit — one story is that he was digging postholes for a cattle fence, another that “a Japanese” was seeking water for a large fish pond — he thought he had an iron and copper mine, but the professionals set him straight. The copper was only a few percent (although later it was by-produced in paying quantities) and the iron was waste; the money from pyrite (FeS2) was in the sulfur. He let the Stauffer Chemical Company run the mines and gave scientists of the time free access to them.


Pyrite on quartz

The mines ran, interrupted by fires, for about 30 years starting in the 1890s. There were at least three of them. Records are confusing and I’m still trying to sort them out.

The best ore in Leona Heights was in pods of hard, dark, solid pyrite yielding 50 percent clean sulfur that sat, like layers of frosting in a chocolate cake, within a zone 12 to 30 feet thick that tilted into the hillside. The people who published papers about this district scratched their heads at the deposits. They all concluded that the Leona volcanics (“Leona rhyolite” as they knew it) was so jammed with pyrite that the upper part weathered into iron oxides (which stayed behind as the ocherous “iron cap” or gossan) and iron sulfate, which leached down in solution and was reduced back to pyrite beneath the water table in the so-called vadose zone, where it was exposed to a lot of carbonaceous material.


Fine-grained pyrite concentrated in the Leona volcanics, Campus Drive

The trouble with the kind of intermittent research these geologists pursued in the operating mines is that each person who visited the workings saw a different set of rocks. The Leona Heights mines were also prone to fires, so parts were off limits for years at a time, or abandoned.

Henry Mulryan, in a 1925 Master’s thesis, summarized the previous work and consulted their authors, but with several parts of the mines closed off by fires he failed to find any of that carbon-rich rock in the areas he had access to. Unable to prove anything one way or the other, he was forced to punt, saying he would rather wait and see what further digging revealed at depth. “If the Leona Orebody is derived under vadose conditions, then it is the only one known to the writer and should take its place in the world’s literature on ore deposits.” (I too am skeptical about this carbonaceous rock, but the Oakland Hills are complicated here so who knows?)

That was a hundred years ago, before geologists made huge strides in understanding this class of “volcanogenic massive sulfide deposit,” not to mention a scientific revolution, in the years between then and now. Meanwhile the mines are long closed and will never be reopened. The samples, if they still exist, are gathering dust in obscure cabinets. I’ve read all the contemporary literature (except for some theses — Leona Heights seemed to be a handy subject for Stanford and Berkeley students at the time), which is an absorbing chore because the records are sketchy by modern standards and the terminology has changed. But there are rewards; Mulryan had some good photos of the Leona sulfur mine circa 1924.


Looking west on the Leona Mine. The hook in the road is at the end of McDonell Avenue. The rail line carried ore cars to the crusher, then to a 1600-foot aerial tram that carried the ore to the train in Laundry Farm canyon. Chabot Observatory in the background.

I’m still scratching my head about the Leona Heights pyrite, and I find myself envying Jack Vance’s freedom of imagination. Reality can be tough; you can’t just make up something wonderful.

Lake Temescal, the west side

12 April 2021

For one of Oakland’s most rugged places, the west side of Lake Temescal doesn’t expose a lot of rock. But what’s there is unusual for Oakland, and interesting.

I’m talking about this ridge — tectonically, a shutter ridge — across the lake from the swimming beach.

The map of Lake Temescal Regional Park shows two trails there, the low one along the water and the high one up in the woods. An even higher trail, not marked on the map, is off limits and doesn’t expose much rock anyway.

I often wish I’d lived here in the 19th century when Oakland was new. It was in 1868 when Anthony Chabot acquired a steep little canyon back of the hills and built a dam to supply the young city with dependable water service. His technique, perfected in the gold fields of the Sierra, was to take a high-pressure water hose and wash down the sand and gravel from the hillsides to build the dam. I wish I could have inspected the scrubbed slopes at that time, but there were almost no trained geologists in the whole state, let alone me.

The canyon has a flat floor now, after decades of sedimentation, but you can see from the high trail that it’s still steep and narrow.

And the action didn’t end when Chabot finished the dam. He built a control tower in the new reservoir, but a landslide soon took it out. I’m guessing that was probably near the sluicegate where the beach house is today, and I’m guessing that the rainy winters of 1868-69 and 1869-70 plus afterslip and aftershocks of the big 1868 earthquake on the Hayward fault had something to do with it.

Speaking of which, two major strands of the Hayward fault run right through the reservoir. They’re helpfully shown on the map above. This is where the main strand crosses the dam. (Don’t worry, the massive dam will not fail even under the largest possible quake on this fault.)

The dam itself was raised and then lowered during the next few decades, and presumably the lake rose and fell too. Meanwhile trees and brush moved in upon the slopes where the Ohlone had previously maintained grassy meadows, and the rocks decayed and soil built up.

And the rocks themselves embody the complicated history of starting out in a vigorous subduction zone, being deeply buried and exhumed probably more than once, then being torn up and shoved around by the San Andreas fault system of which the Hayward fault is part.

All that is to say that Lake Temescal is a dynamic area at all time scales. It’s more complex than the small-scale geologic maps can show, even though it’s complex enough on that map.


KJfm, Franciscan melange; ch, chert block; af, artificial fill; sc, silica-carbonate rock; sp, serpentinite; Jsv, Leona volcanics; fs, Franciscan sandstone; KJkm, Knoxville Formation

The west side is mapped as melange, which is basically a mess of mashed-up sandstone with big blocks of other rocks, each with its own separate history, suspended in it. The little blip labeled “ch” is this block of chert at the top of Hill Road. So when I visited the west side trails last week, I expected to find things I didn’t expect. And most of the rock along the trails appears to be nondescript sandstone — I say appears because since hammering is forbidden, it’s hard to find a fresh surface. But lo and behold, along the high path coming down from Broadway Terrace, there’s the telltale gleam of blueschist in the exposed subsoil.

The color really comes out when you wet a piece.

Other apparently high-grade metamorphic rocks include this hard, glittering schist. Not having access to thin sections and petrographic microscopes, I can’t check for the presence of jadeite, which has been reported in blocks from this melange.

And over at the north end of the train is a distinctive outcrop of another schist. Hard rock supports slow-growing lichens, and the species differ depending on the rock’s chemistry.

Underneath the lichens, the rock is a bluish-gray mixture with a strongly folded texture, both signs of a rock that’s been through a lot of distortion at high pressure and temperature. These photos are from exposures by the lawn. The first shows the folding and the second shows fault-related crushing on the left side. The crushed material is called gouge, and bits of it are common in this sector.

My authority on Oakland’s Franciscan rocks, John Wakabayashi, holds that the west side of the lake hides the same ancient major thrust fault — a megathrust, in fact — that’s famously exposed in El Cerrito above the recycling center, where high-grade rocks have been pushed above lower-grade rocks. Unfortunately the fault itself appears to be in “a brush-filled gully with no exposure,” so it may be a while before we ever learn more.

But I did learn something more last week, about the beach house.

It is made with the local rock, namely the Leona volcanics. Whether the stones came from the hill just upslope to the east, the one that collapsed in the 1870s landslide, or from across the freeway in the great Tunnel Road cut during the 1930s, I do not know. I’m guessing the former, but I would be happy to be wrong if it means I can be certain. Putting the building and the roadcut in the same photo, there is a resemblance.

The fun thing is, both locations can be closely inspected. That sounds like a good afternoon project.

The changing identities of the Leona Quarry

9 November 2020

Last week I finally gave in and returned to the high hills — for exercise, as permitted by the county health authorities — and couldn’t resist a reconnaissance of the Leona Heights area. It’s Oakland’s boldest and most rugged region. Here it is from Knowland Park, above the zoo.

Most Oaklanders may know it, though, as the mountainside with the huge scar on it overlooking I-580, the former Leona Quarry.

The quarry was first opened by the Ransome-Crummey Company in 1904 and ended operations under Gallagher & Burk in 2003, but it changed hands (and names) several times over the years, making its detailed history hard to trace. Also, newspaper accounts often confuse it with the Leona Heights quarry, which was at the site Merritt College occupies today.

The quarry was first made feasible by an extension of the Laundry Farm railroad, above Mills College. It was originally high up a steep grade, as shown by the pick-and-hammer symbol in the 1915 topographic map.

I believe it was up there because the bedrock was well exposed, making excavation unnecessary at a time of heavy reliance on hand labor. The 1947 map shows that operations had moved downhill, and quite a bite had been taken out of the hillside.

And the 1980 update of the 1959 map shows the quarry scar at its ultimate size.

The whole time, this hillside was being quarried exclusively to make crushed rock. There was a huge demand for coarsely crushed stone in the days before asphalt and concrete pavement. The gold standard for city streets in the late 1800s and early 1900s was macadam, which has completely disappeared since then. You’ll only see it in silent movies.

A macadam road started with a shallow excavation that was filled with several layers of crushed rock, of successively finer grade, topped with fine gravel or rock dust. The jagged, blocky texture of crushed rock made macadam roads exceptionally firm in comparison to plain dirt or gravel, and they didn’t turn to mud in the rainy season.

As Oakland grew, filling in the harbor and airport and covering East Oakland with suburban tracts on an ambitious street grid, its quarry owners prospered, especially the well-connected ones who could arrange favorable contracts and keep wages low. Plain old crushed rock — road metal — was in high demand. Although there were still good markets for crushed rock after the macadam era ended, things were not the same. The Leona Quarry outlasted all of its competition in Oakland thanks to its remote location, good rail transport and ease of production. But eventually the city expanded to the quarry’s doorstep, the quarry ran out of easy rock and the show ended in 2003, when I took this shot of the north end of the property.

That’s when the site took on its next identity — a townhome district. The rock no longer matters.

But it used to. I think the Leona Quarry started running into problems as the standards in the rock business grew steadily stricter.

Leona Heights, the mountain, consists of a body of much-altered volcanic material of Jurassic age that I refer to on this blog as the Leona volcanics. Its eventful history left it impregnated with pyrite, iron sulfide, in many places. A little farther northwest, in the valley where route 13 splits from I-580, there was enough pyrite to support at least two mines. Down at the Leona Quarry there wasn’t as much, but it does exist and, as it does in the old mines, pyrite decays in the air and rain into iron oxides and sulfuric acid. The oxides turn brown, staining the Leona volcanics this typical color.

They also stain the stream water, as seen here in the headwaters of Chimes Creek above the quarry (and elsewhere in the hills).

The west side of the quarry was full of this “red rock” while the east side consisted of a dense blue-gray siliceous rock, more like this specimen I collected there back in 2009.

Whereas the red rock was useless for things like concrete aggregate because of its pyrite content, this was the good stuff. Nevertheless, the market for excellent road metal came to be dominated by huge outfits like Granite Rock — whose co-founder, Arthur Roberts Wilson, started his career at the Leona Heights Quarry back in the 1890s.

Meanwhile today, the former quarry is now a townhome plantation, set at the bottom of a high, steep rocky bowl.

There is no guarantee that the quarry’s second identity will last forever. Fire, earthquake and rockfalls can overcome any defense given enough time (although the Leona Quarry development has a GHAD that maintains the defenses). Zoning changes and real-estate fashions can undermine such enterprises as surely as physical hazards. There is no guarantee that anything we build will last a century, like the quarry did. Like the ancient philosopher said, everything flows.