Archive for the ‘Franciscan rocks’ Category

Upper Indian Gulch

26 October 2020

When I last featured Indian Gulch on this blog, it was about the easy part, mostly a stroll up Trestle Glen Road. It ended with this glimpse of the living Indian Gulch Creek, bounding down the rock slopes of the Piedmont crustal block on its way to culverted oblivion beneath the elegant Trestle Glen neighborhood.

Upper Indian Gulch lies within Piedmont and the west fringe of Montclair that looks down upon Piedmont. Nowhere is the creek up there accessible to passers-by; if you want to see it you have to buy a house whose lot includes it, or make friends with someone who owns such a house. You’ll have to imagine it running in the dark underneath the street, as it does along La Salle Avenue just above St. James Drive.

Here’s an overview of the upper creek from Google Maps terrain view. The creek has three branches; the west fork is the main branch. To be a stickler, that fork should properly be called Indian Gulch Creek and the other two are just tributaries. The old property line between the two middle Peralta ranchos ran up this valley, Vicente’s on the left and Antonio’s on the right. Later the same line separated the Oakland and Brooklyn Townships of Alameda County. Today neither the ranchos nor the townships are relevant any more, but the boundary influenced the pattern of land ownership a century ago as Oakland expanded its territory and developers shaped the outskirts.

A stroll here is a workout. Part of my Ramble 4, Uptown to Montclair, goes up the creek’s middle fork but the steepest part is pedestrian-unfriendly. Three years ago I featured an excursion into the valley of the east fork. That pretty much exhausts the possibilities in those two valleys. In the west branch, two dead-end roads will take you to the floor of the valley, though the creek is not accessible. Indian Gulch Road leads down from Glen Alpine Road, just above the word “West”:

And Calvert Court swoops from Blair Avenue down into the creek’s highest watershed, where Oakland’s most isolated properties lie.

Here and there, you can get a look at the bedrock under the watershed: sandstone and mudstone of the Franciscan Complex.

And it’s hard, on Piedmont’s winding streets, to grasp the contours of the land. This view across the middle fork at Hampton Park is about as good as it gets.

Really, the best experience of these headwaters is on the rim roads that encircle the watershed. They aren’t photogenic in ways that my camera have caught many times over the years, but the views glimpsed through the trees and past the homes have always pleased my eye.

Looking east-northeast up Hampton Road at Sea View is a good view of the high rim of the east branch, topped by Pershing Drive.

By all means visit Oakland’s best bedrock there.

And don’t miss Wood Drive, along the north rim, where this excellent outcrop of Franciscan metachert awaits.

Indian Gulch is a good candidate for a circumambulation.

Rocks of the Bilger Quarry

6 July 2020

It’s been twelve years since I wrote about the former quarry where the Rockridge Shopping Center sits today. Except for the pond, the whole space has gotten a makeover and it’s time for a fresh look. But first, some nostalgia. Back in 2008 the 24-hour “Big Long’s” was still there. I used to shop there; it had everything.

And flanking it was the big old Safeway with more businesses between and beyond, and the bank in the far corner. I used to bank there.

All of this is now gone, and the site is under intermittent redevelopment. Decades ago this was a giant rock pit, active for more than 70 years, that for a long time was the largest rock quarry in Alameda County. It was started by the Oakland Paving Company in 1870, and the name of Frank Bilger was associated with the succession of enterprises that produced crushed rock here, so it’s usually referred to as the Bilger Quarry. (Bilger learned the trade from his father, a German immigrant, and he surely pronounced his name the German way with a hard G.)

Now the Long’s site is a big Safeway, and the old Safeway and bank buildings are part of an empty lot. Only the water-filled pit on the east side is unchanged, and the walls show no sign of decay, which is a very good thing because it’s right next to St. Mary Cemetery.

The whole quarry site occupies a small body of quartz diorite, an intrusive (meaning it didn’t erupt) igneous rock that’s unusual but not unheard-of for the Franciscan Complex. It’s mapped as the purplish blob in the center labeled “Kfgm.”

It happens to be excellent rock for industrial and engineering purposes. The Tribune in 1890 wrote, “The material used by the Oakland Paving Company is a crushed blue rock, a trap dyke that is practically indestructible, submitting without injury to the hardest usage for eight or ten years without repairs, and with proper care, such as any pavement requires, lasting three or four times longer.”

The east end of the quarry exposes the bluish stuff. This exposure, right behind the Safeway, also has flaky veins of calcite. It’s very tough — not that I’ve used a hammer on it lately, but after seeing lots of rock you get a feel for this.

The west side consists of a much lighter material, a bit coarser grained and slightly less durable.

Between them is a contact zone that I recall as being black and sheared, with mineralization that was probably iron-manganese oxides. It was covered up when the new Safeway went in and parts of the rock face were fixed up for safety. I took this photo in 2018 from the roof parking lot.

There’s chainlink netting on the rock face, just in case it decides to start crumbling onto the roadway.

You’re always cautious about building inside an abandoned quarry, because rocks don’t last forever — that’s why they’re mostly underground, covered with soil. The experts have assured us it’s OK, and I trust them pretty well. The cemetery will last a good long while, and the former California College of the Arts campus, while it looks precarious perched above the other side, has passed muster too and someday will be apartments with good views.

What continues to impress me, every time I visit, is how different Oakland used to be. Throughout the late 1800s, the cemetery on one side and the CCA site (then it was the Treadwell estate) on the other were cheek-by-jowl with this huge operation that blasted three times a day, starting at six in the morning, and employed hundreds of men in producing crushed rock. But back then, rocks were money.

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.