Archive for the ‘Franciscan rocks’ Category

The great Tunnel Road cut

2 September 2019

The land on the south side of Hiller Highlands is far from its native state: it’s been extensively quarried for many years, and what’s left is a rocky, weed-choked waste. But the roadcut is also a geological treasure.

It’s one thing to look at a hillside and determine what it’s made of, another to study it carefully enough to determine what formation it belongs to. These are worthy accomplishments to be sure, but a more precious one is to find and study a place where different rock units come in contact.

Here’s where I bring out one of my favorite quotes from the history of geology, in the early 1800s when people were beginning to work out what the rocks were telling them. A party of geologists including the eminent Sir James Hall took the Rev. William Richardson, a notorious opponent of the newfangled Scottish school (now textbook orthodoxy), to the Salisbury Crags in the heart of Edinburgh. There they showed him a contact between traprock (a basalt lava flow) and sandstone, pointing in particular to bits of sandstone that were enclosed within the traprock. This contact was proof positive that the basalt was (1) a formerly molten rock that (2) had intruded into the sandstone long after the sandstone had formed:

“When Sir James had finished his lecture, the Doctor did not attempt to explain the facts before him on any principle of his own; nor did he recur to the shallow evasion of regarding the enclosed sandstone as contemporaneous with the trap; but he burst out into the strongest expressions of contemptuous surprise, that a theory of the earth should be founded on such small and trivial appearances! He had been accustomed, he said, to look at nature in her grandest aspects, and to trace her hand in the gigantic cliffs of the Irish coast; and he could not conceive how opinions thus formed could be shaken by such minute irregularities as those which had been shown to him.”

Contacts among Oakland’s rock units are hard to find because our rocks are poorly exposed to begin with. And even when you do find contacts, they may not preserve the small and trivial appearances that might tell you the most. But the Tunnel Road cut exposes a large area of rock, as seen in this aerial view from Google Maps. The slope is interrupted by several cutbacks that serve to stop runaway boulders and allow access for maintenance — most recently by the herds of goats that helpfully cleared away most of the French broom — and visitors like me last week.

This big cut exposes a significant contact right along the road, just west of the Gateway Emergency Preparedness Exhibit Center (under the word “Hiller”). The geologic map shows the spot as the contact between the Leona volcanics, in pink, and a teeny splinter of olive-green Knoxville Formation directly above the “o” in “Substation.”

The contact today is somewhat obscured by vegetation, so let me show it to you first as it appeared from Tunnel Road in December 2007, when the state last cleaned it up.

From lower left to upper right, the rocks gradually give way from highly altered volcanic rocks, containing some shaly beds, to dark-brown shale. Some of the experts heartily disagree on what exactly is happening here, but it’s widely taken to represent the very top of the Leona volcanics and the very base of the Knoxville Formation, lowest member of the highly sedimentary Great Valley Group.

Here’s how it looked in July 2019. You can tell in both photos that the shale is crumbling down the slope almost as fast as it’s exposed whereas the volcanics stand sturdier.

Along with a few other localities scattered around California, here’s proof that the two rock groups started out as peaceful neighbors, the shale laid gently down upon the volcanics under the Late Jurassic or Early Cretaceous sea. Although later tectonic events wrenched and stretched and broke all of these rocks as California gradually became its present self, this spot remained untouched for geologists to argue over. As an example, I captured Cliff Hopson pressing a point to the late Eldridge Moores at this very outcrop in 2005, no doubt discussing some minute irregularity.

It was a pleasure to stop here in July with John Wakabayashi, leader of that 2005 field trip. He noted how important it is to revisit outcrops: “When you come out, you notice things you didn’t notice before.” And he pointed out features of the volcanics I hadn’t picked up on my own. For instance, the locality is unusual in featuring fairly fresh volcanic glass, which can yield more faithful geochemical data than the altered rocks around it.

He also found something he hadn’t seen before: carbonate veins with shapes that reminded us of soft-sediment deformation. This suggested to him that they may have been original constituents of the rocks and not later alterations, in which case they might preserve microfossils.

Familiar features of the Leona volcanics are well displayed here, including its lumpy and fractured texture, a reminder that the unit is mostly not lava flows but ash beds and landslide deposits, fused and altered by hydrothermal springs.

Slickensides — polished fracture surfaces — testify to much later activity related to the Hayward fault and the rise of the Coast Range.

And I always take pleasure in spotting the green devitrified ash whose color is attributed to celadonite, possibly with other green secondary minerals like prehnite, chlorite and epidote.

Familiar places can still reveal new things, if you keep your eye on seemingly small and trivial appearances.

Oakland geology ramble 8: Piedmont Ridge

19 August 2019

I don’t always care about rocks; geology is about more than rocks. I don’t even always care about geology; sometimes I just want a vigorous, geographically arbitrary hike. Ramble 8 is one of those — a traverse of the ridge crest above Piedmont, our highest ground west of the Hayward fault. It goes from the Rockridge BART station to the 33 line bus stop at the Leimert Bridge, on the lip of Dimond Canyon, about five miles end to end. Nevertheless, it has geology and rocks along the way. Here’s the route. There’s a map with more detail at the end of the post, where I also have some announcements.

The BART station features the “Rockridge” destination signage, mounted above a selection of Sierran boulders. This time, some ten years after it was installed, I noticed that the lettering design is quirky, contrasting “rock” and “ridge.”

You could climb Keith Avenue and barge up upper Broadway to attain the heights at Contra Costa Road, but it’s more interesting to circle behind the College Preparatory School grounds on Brookside Avenue. The school nestles in a steep little stream valley — unlike many similar places, this is not a former quarry but, apparently, a former turn-of-the-last-century park that had a short existence.

Once you get up to Contra Costa Road via Eustice and Buena Vista avenues, the walking is pretty and level. While you’re here in this remote part of town, check out the blueschist outcrop at 6063, vacant since the 1991 fire. The entire ridge on this hike is in Franciscan melange, a mudstone matrix containing odd lumps of other rock types. You won’t see much of it.

At the very end of the street is Erba Path, a steep set of stairs down to the saddle in the ridge where Broadway Terrace peaks on its way past the south entrance to Lake Temescal and points east. Cross that busy road and head right back up again on Sheridan Road, visible here at upper left.

Turn right off Sheridan at Agnes Street, unless you want to explore the little-trod path that joins Sheridan’s two halves. (Everyone should at least once.) Take high-flying Cochrane Avenue, where views east of Thornhill Canyon and Glen Highland’s settled slopes beckon.

Then jog right again up the saddle of Florence Avenue (where I sent you in the last post) to Proctor Avenue. This is pretty steep, but short. By this point you’re well above 700 feet. You’ll go higher later, but first you clamber a hundred feet down to the next saddle where Moraga Avenue crosses the ridge. Take the well-marked pedestrian crossing, but beware — this is the most dangerous road crossing of the hike.

Proceed on Estates Drive, which climbs nearly to the 800 foot contour. Up here are two curious reservoirs that date from Oakland’s water wars, when private water companies struggled to supply the fast-growing region from local sources while they vied with each other in deadly capitalist strife. (A ruinous series of bankruptcies and mergers ended in the 1920s with the formation of EBMUD.)

The Dingee and Estates reservoirs were constructed, fast and furiously, at the highest point of the Piedmont hills to provide good water pressure. EBMUD is upgrading these old concrete bathtubs to proper steel tanks, built to withstand big earthquakes on the Hayward fault just a few hundred yards east.

From here you head down again to La Salle Avenue, which takes advantage of yet another saddle in the ridge. If you’re ready to quit at this point, go left on La Salle, left again on Bruns Court and cross that high pedestrian bridge over the Warren Freeway to Montclair Park (because you can!) and catch the 33 bus at La Salle and Moraga. Otherwise, cross La Salle and stay on Estates, which is a little to the right.

Estates climbs again, not so far this time. Because this walk hits the highest spots, take Dawes Street up the hill and over, where you simply must visit the south end of Pershing Drive and admire Oakland’s best outcrop of Franciscan chert.

From here on it’s all downhill. Dawes rejoins Estates Drive here, and as you start down Estates you can see across Dimond Canyon.

Geologically and geomorphically speaking, the other side is also part of Piedmont Ridge, but the large water gap of Dimond Canyon is impassable without a long detour. So, down you go to the bus stop at the Leimert Bridge.

Along the way are two more highlights. First, at the Piedmont line the road passes the head of the former Diamond Cañon Quarry, which today houses the Zion Lutheran Church. Recent foundation work here has exposed fresh rock; maybe you’ll see some too.

And second, enjoy this wonderful volcanic breccia used for the landscaping at 170 Estates Drive. There are whole walls of it.

The house itself is something to see, too.

And as promised, here’s the detailed route map (1126 X 1126 pixels), followed by some announcements.

The excellent, out-of-print book Stories in Stone: Travels Through Urban Geology by David B. Williams is being reissued in paperback by the University of Washington Press. If I didn’t have the hardcover already I’d buy this classic. More information on David’s website.

My own book manuscript is making the rounds of a publisher, and while I await a yea or nay I’m trying to get my arms around a whole lot of scientific literature pertaining to Oakland’s rocks. It’s a ridge walk of the intellect, but I want to make Chapter 5 as good as humanly possible. I hope to buttonhole some of the real experts next month at the Geological Society of America’s annual meeting in Phoenix.

But first I’ll be giving a talk to the Friends of Sausal Creek, on 18 September at the Dimond Library, titled “Sausal Creek: The Last Million Years or So.” You read it here first (unless you follow me on Twitter, @aboutgeology), and I’ll repeat the announcement in the Q&A thread soon.

Oakland’s wild rail path

5 August 2019

The seasons are changing now, if you follow the pagan calendar. This weekend marks the turning point between astronomical pagan summer (6 May to 6 August) and pagan autumn (6 August to 6 November), or as I think of them, High Season and Waning. They are offset exactly half a season from the conventional astronomical seasons. High Season consists of long days, and Waning consists of shortening days. (Likewise, Low Season consists of short days, and Quickening consists of lengthening days.)

Nature is acutely aware of these seasons. The belladonna lily (Amaryllis belladonna) sends up its naked-lady flowers at this time. The strawberry tree (Arbutus unedo) ripens its rich little fruits (I can understand why Pliny the Elder named them “eat-only-one” because they’re so satisfying).

And of course the blackberries are in full swing.

I returned last week after eight years to the “secret street” at the south end of Florence Avenue, where it meets the old railbed of the Sacramento Northern Railway (also known as the Oakland, Antioch, and Eastern Railway). Unlike that first visit, when I was busy and could only gaze up the path, this time I had the leisure to walk its whole length, up to Broadway Terrace where it’s fenced off.

The path has geology up at its north end, but it’s worthy just as woods. Even right next to the Warren Freeway, it’s as secluded as any place in Oakland.

It’s shown as the dashed route on this map. You can see that Florence Avenue, heading over a saddle in the ridge above Piedmont, used to connect with Florence Terrace once upon a time. That’s the Lake Temescal park at the top.

There are lots of blackberries growing here, so don’t wait. The first ones are the best. Near the north end is a landslide scar that was repaired with much labor to protect some homes on Sheridan Road. The work was finished with dark shotcrete, but it doesn’t really blend in.

If you look close you’ll see little splotches of white. Those mark cracks where lime-bearing groundwater has seeped through and deposited calcite as it evaporates.

I can foresee these growing into falls of travertine in a few years. Beyond the landslide is a high cut into the hillside, made decades ago when the rail line was first pushed through. And the bedrock here is mapped as classic Franciscan melange, the big blue field on the geologic map — the edge of which happens to correspond to the Hayward fault.

I half expected the rock exposed here to be fault gouge, the fine-ground, mealy stuff that fills many of California’s active faults (for instance at the London Road slide). It’s real close to it: highly weathered mudstone that’s likely to come down hard in our next big quake. Whether the railbed will be cleared again afterward can only be conjectured. I’ll look at this cut again more thoroughly next time I’m here, whenever that might be.

On your way back, look again for blackberries. I know I didn’t get them all.

On Pryal’s quarry

21 January 2019

As I find the time (or as the subject hijacks me, in this case), I sniff around for details of Oakland’s rock quarries. There are a good two dozen of them. One I’ve always been curious about first appears in 1868 in The Natural Wealth of California, by Titus Fey Cronise: “The quarry from which the stone used in erecting the Deaf and Dumb and Blind Asylum was obtained, is situated on Pryal’s ranch, about four miles from Oakland. The supply of this stone is exhaustless.”

First thing was to find Pryal’s ranch. Andrew Dewitt Pryal (1832–1907), universally known as “A. D.,” had a spread in Chabot Canyon, the valley of Temescal Creek below the Lake Temescal dam. It was a thriving nursery that Pryal had started back in 1853, on land just upstream from Vicente Peralta’s reserve. Here it is on the Henkenius map of 1888, which has the arc running across the middle representing four miles’ distance from City Hall. It also labels the streams; Harwood Creek is called Claremont Creek today.

That map isn’t lined up with true north, so let’s be more systematic. The next set of maps all cover the same area. Here it is on the Dingee map of 1884:

and in Google Maps today:

So the Pryal ranch was on First Street, now Chabot Road, in the bottomland now occupied by Clover Drive, Chabot Court, Patton Street and part of the Chabot Elementary School grounds. Can you see what caught my eye? It’s that excavation on the north side of Chabot Road east of the hill, or what looks like one. There are so many quarry pits around.

Here’s a photo of the old ranch from the 1897 book Athens of the Pacific:

and roughly the same shot today, from farther east and higher up at the end of Margarido Drive:

Pagoda Hill got its name from the eccentric mansion built on its crown by J. Ross Browne. The young eucalyptus grove was typical of the time; Californians had been planting various Australian species for many years. Later a subdivision of Browne’s land was named Eucalyptus Hill, and Eucalyptus Road runs through it. There even seem to be a few trees left from that grove.

Anyway, back to the quarry. The original Deaf Dumb and Blind academy was a gorgeous thing built of an excellent “blue granite” that unfortunately was all discarded after the school burned down in 1875 and was rebuilt with a different plan. All “blue granite” means in this context is a hard stone with visible grains and no lime. And now we can look at the geologic map of the Pagoda Hill area.

The hills are made of Franciscan melange, a body of mostly metamorphosed sandstone and shale (argillite) with various-sized lumps, or knockers, of things like basaltic lava (fg, for greenstone), chert (red blobs) and serpentine rock (blue).

Here’s what’s over there. At the top of Roslyn Court, right under the big “J”, is greenstone. It’s shot with calcite veins and would never be picked to build a structure.

On Roanoke Road, the street between the “m” and the red blob, there’s mostly hard sandstone of the type customarily called “blue rock” by local quarriers.

I concluded, from a close-up look at the contours of the land and the general lack of decent rock, that there was no quarry here. It would have been one of the largest quarries in the county, supporting decades of production, but there’s no record of such a thing. This was just your usual digging and grading for a housing development.

So where was Pryal’s quarry? On the south side of his property. I remembered a photo displayed during an Oakland Heritage Alliance walk in Chabot Canyon: a shot looking across the valley along the old train trestle, and on the bare hills opposite was the high, rugged face of the abandoned Berkeley Rock Company quarry. I did my best to reproduce that photo by standing on the old grade, next to the top of Reata Place, and looking southwest. The quarry scar, as I recall the old photo, was at upper center about where the heavy cable passes in front of a house and lamppost.

That’s the rock face on Broadway, between Brookside Avenue and the Margarido Stairs, where three new houses were inserted about 10 years ago.

California State Mining Bureau Bulletin 38, from 1906, says about Berkeley Rock’s operation: “The deposit is a much altered trap-rock, and is used for concrete, macadam, and gutter rock. The company produces about 250 yards a day.”

I conclude that Berkeley Rock was working a cut that Pryal had opened 30 years earlier. That Pryal’s quarry produced enough good rock for a large stone building was a lucky accident, because the melange zone is a plum-pudding of mixed rock types.

The Berkeley Rock quarry made news during its years of operation, which started in 1902. The quarry’s 10-acre site was in the way of the Broadway extension, and a lawsuit in 1905 established that the road would go through. On 18 July 1906, an unknown dastard booby-trapped the quarry’s main engine with a package of dynamite, gravely injuring Frederck Hoffman, the superintendent. They used to call such criminals dynamitards. Another dispute over the quality of the company’s stone led to gunfire later that year. The company continued in business, however, for a few more years until the Oakland and Antioch Railway established its right-of-way through the property in 1911 and the Broadway extension was finally pushed through in 1915.

The homes in the old quarry have some rocks lying around. More Franciscan “blue rock.”

This tale still leaves a mystery. Cronise’s book also contains this interesting passage: “In 1864, Mr. A. D. Pryal, owner of a large ranch about four miles east from Oakland, discovered a vein of auriferous quartz in the Contra Costa hills, which cross his lands. Some of the specimens from this vein were rich in free gold, and the mine opened under the name Temescal, paid well for a short time, but the dislocation of the strata, a little below the surface, rendered its further working unprofitable.”

Gold is otherwise absent in the East Bay, as far as I know.