Archive for the ‘Leona volcanics’ Category

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.

Arroyo Viejito

6 January 2020

Some of Oakland’s most interesting land is also its most inaccessible; I’m speaking of our streambeds. And on the whole, the largest remaining stretches of wild streambed belong to Arroyo Viejo. Just to orient you, here’s the Arroyo Viejo watershed, as it’s mapped today by the Alameda County Flood Control District. The red stripe, which I added, represents the Hayward fault. (I’ll return to that.)

Here’s a zoom-in to the lower right corner, showing the upper part of Arroyo Viejo and the valley of a defunct little stream that I’m calling Arroyo Viejito.

The peculiar feature that caught my eye several years ago is how Arroyo Viejito runs parallel to Arroyo Viejo, very close to it, with a distinct rocky ridge between the two streams. Today the two valleys are very different, and a century’s worth of maps hints at what happened. Here’s the 1897 topo map showing the two streams, underneath the word “Viejo.”

In 1915, the area was more accurately mapped, and the two streams are shown as extremely close together at one point.

Everything changed after this. The country club was expanded and the adjoining land was subdivided and developed into the very exclusive Sequoyah district starting in the early 1920s. At that time Golf Links Road was pushed through to what would become the Grass Valley district in the 1950s, and Arroyo Viejito was diverted into the large stream at their closest approach and a sewer line inserted into the abandoned valley. It was very handy for the developers. As of 1947, the little stream had vanished and the land lay open for a new wave of luxury homes.

As of 1980 the buildout around Arroyo Viejito was complete.

The sewer line is accompanied by a maintenance road that is now a nice place for the locals to walk, and it connects with the little-visited creek trail at the north edge of the zoo’s property. I featured this area, in passing, three years ago in Ramble 3.

The reason these two streams ran so close together is related to the Hayward fault. It’s been dragging the lower, western half of Arroyo Viejo north, and for the last few hundred thousand years the stream has stretched out along the fault line before turning toward the Bay. Models of landscape evolution suggest that the headwater streams have been getting squeezed, aligning themselves and crowding together.

The combination of an especially large earthquake and a major flood could cause Arroyo Viejito to break through the narrow waist and join Arroyo Viejo farther upstream, abandoning the stretch with the sewer line and leaving the ridge standing there for a few more thousand years until it erodes away. But impatient developers have short-circuited all of that, and now the little stream is defunct, its former catchment part of a sterile golf course.

As I said, it was the ridge between the two streams that caught my eye and dared me to set foot on it. It’s in the middle of this Google Earth view looking west.

Its sides are very steep; it’s like an island. One day I found that it has a tiny trail running along its top, and signs of an old road and excavations. My guess is that the ridge was dug up for fill material when the sewer line was put in. The high-resolution lidar data acquired along the Hayward fault a few years back covers the west half of the ridge, and the resulting digital elevation model (with the trees and buildings stripped away) shows these features plainly.

Lately I’ve visited this ridge and the stream valleys of both Arroyos, in search of access and ultimately in search of rocks. Access beyond what I’ve already mentioned is difficult, and I have paid dearly for it in poison-oak rash. But I shall return.

The bedrock map looks like this, but I am suspicious of all of it given the difficulty of access and the paucity of outcrops. One big goal of mine has been to inspect the stream bed where bedrock might be exposed, for some real ground truth. I suspect that geologists, while doing their best, have resorted to drawing lines based on the topography.

The green zone marked KJk is shale and conglomerate of the Knoxville Formation, and that’s what I’ve always found in the eastern chunk of it. This shale is just west of Golf Links road where it crosses the creek.

And the conglomerate is abundant as loose boulders (not bedrock) downstream. It’s beautiful stuff.

But I have found none of it yet in the western section. Instead, everywhere I’ve looked the rock is either coarse sandstone shot with calcite veins, interpreted as the very oldest part of the Knoxville . . .

. . . or familiar rocks of the Leona volcanics (Jsv).

This includes up on the little ridge and down in the Arroyo Viejo streambed.

I still have a good bit of territory to visit, though. The streambed will have to wait until dry season, when I can poke around this weird-ass lime-cemented breccia.

And there’s more ridge to check out. Outcrops like this are so crusted with lichen that I might need to bring a rock hammer for some very careful, unobtrusive chipping.

There are some other charms in this northernmost stretch of Knowland Park. Every time I’ve visited there are fresh deer bones, indicating a mountain lion’s sphere of influence. And the cries of exotic animals occasionally drift down from the zoo’s hilltop center.

No other place in the world exactly like that.