Archive for the ‘Oakland rock types’ Category

In search of McAdam’s quarry

3 August 2020

Alexander McAdam (1854?-1920s) was a minor character in Oakland’s history who left a highly visible mark in our cityscape. A Canadian farmboy who was orphaned at a young age, he came to California after apprenticing as a wheelwright, and after eight years he saved enough money to buy a farm “at the head of Thirteenth avenue,” according to a short biography by James Guinn in 1907. “He was successful in this occupation, but in the meantime had discovered a sandstone quarry on his property. Upon the sale of his farm he acquired considerable financial returns. Stone from it has been used in many of the largest buildings of Oakland, among them being the Unitarian Church, the last buildings of the deaf and dumb asylum, numerous retaining walls, and for many other purposes.”

This caught my eye because I have long thought that Oakland’s rocks were exclusively used as crushed stone. Yet here in the First Unitarian Church, ashlar blocks of genuine Oakland sandstone form the dignified cladding of this important cultural monument and civic institution, built in the early 1890s under the energetic leadership of a leading Progressive of his time, Rev. Charles Wendte.

Rev. Wendte oversaw the building project from his home across the street. The stone cladding was the costliest item in the project, and he singled it out in his memoirs: “Our employment of stone led to vexatious complications. Quarrymen were unable to deliver this material in sufficient quantities, workmen struck for higher pay in handling it. Contracts were broken or remade.”

I had to track down this stone somehow. The documentary clues are slim, and any signs of the quarry appear to be lost. But first, there is the stone itself.

It’s a fairly sound stone of an even consistency with a warm grayish-brown color and massive (i.e., absent) bedding. The block serving as a lintel over the doorway probably broke during the 1906 earthquake, when most of the cladding along Castro Street and the top of the tower collapsed. (The tower was rebuilt without any stone, a smart move.)

A closer look shows that the stone actually varies (although some of that may be substitute stone from another source, as Wendte’s wording suggests), and that a century of exposure has caused a fair amount of spalling. No wonder there were quality problems during construction.

A still closer look reveals it as a medium-grained wacke (“wacky”): a sandstone with grains no larger than a millimeter and a large component of minerals that are not quartz. The black grains are mostly biotite mica; without a microscope I’m limited in what more I can say.

It’s familiar to me. It’s not the Franciscan sandstone produced by the dozen or so quarries in and around Piedmont. I can rule that out categorically. It’s from the high hills on the far side of the Hayward fault.

All of this is consistent with the documentary evidence placing the source in Montclair. The “head of Thirteenth avenue” is where Park Boulevard, the former 13th Avenue in Brooklyn Township, meets Mountain Boulevard. It’s the intersection at the bottom of this excerpt from the 1897 topo map.

To orient (or disorient) you, here’s the same area today.

The “XII Report of the State Mineralogist,” published in 1894, said the following about McAdam’s quarry: “It is in Medos Cañon, back of Piedmont, and is a small quarry, producing sandstone for rubble and ashler [sic]. It is not worked regularly.” The official who wrote that description, a busy guy on a quick visit to cover the whole county, wrote down “Medos Cañon” when someone said “Medau’s canyon,” meaning the valley of present-day central Montclair where the dairy farm of John H. Medau once lay. I believe that if the site had been in Shepherd Canyon, his informant would have said so as that name was in wide use at the time.

All of this means that the quarry could have been a good exposure of the Redwood Canyon Formation, a wacke of Late Cretaceous age, that forms part of the east side of Montclair’s valley along the Hayward Fault. It’s the unit marked “Kr” on the geologic map, below. The lithological description of the unit, and the composition data from Jim Case’s 1963 Ph.D. thesis, are close enough to the stone in the church.

But also likely is the Shepard Creek Formation (Ksc) and even the Oakland Conglomerate (Ko), when you consider that the units are only subtly different, variable in composition and not well mapped despite the best efforts of competent geologists.

In any case, I had a good time visiting these rock units along the Montclair Railroad Trail the other day. There’s a lovely outcrop of the Redwood Canyon Formation above the trail along the route of the recently upgraded powerline, southwest of the word “grade” on the map. That warty weathered surface, reminiscent of the Incredible Hulk’s hide, is one of this unit’s distinctive features.

But the rock there’s not a good match.

Neither is the rock in the landslide at the upper end of the trail.

And just for good measure, here’s a chunk of sandstone from the Oakland Conglomerate. The material is coarser and wacke-er, but again under the 10X hand lens it’s not like the church’s stone.

Nowhere in this area, in many years of visits, have I seen a body of rock big enough and sound enough to support a quarry capable of producing ashlars — not on this side of the Hayward fault. The nearest quarry site is down Park Boulevard where the Zion Lutheran Church sits today, the former Heyland/Diamond Cañon/Bates & Borland quarry on the side of Dimond Canyon. But that produced crushed Franciscan sandstone, something quite unlike McAdam’s stone.

I can only conclude that McAdam found a lucky hillock on his farm and made the most of it, one that’s been obliterated during the waves of development since 1890. And the site of his farm is, as we say, poorly constrained. Even his life dates are fuzzy. But his accomplishments include making a profit from farming, acquiring a large home in Temescal, serving two terms on the City Council in the nineteen-oughts, and equipping an important building with a handsome exterior (despite the vexation he caused Rev. Wendte). I can’t confirm when he died or where he’s buried, so this building surely is his monument.

While I was researching this post, the papers covered a lovely story about how archeologists used advanced geochemistry to pin down the source of Stonehenge’s biggest stones, a peculiar sandstone known in Britain as sarsen. The New York Times version was my favorite writeup, and the hardcore details are in Science Advances in an open-access paper.

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.

Decorative blueschist

11 May 2020

Since the shutdown I’ve been scrupulous about going out only on essential business. Yes, my idea of essential is wandering around with one eye on the ground, the other on the landscape and my mind in the clouds, but it’s not really essential. Basic, baseline exercise is essential, and I can get that in with a walk of a mile or two.

That said, a walk of a mile or two can take you to lots of interesting places no matter where you live. The upper part of Fairmount Avenue is within that perimeter, and that’s where this stone wall caught my eye. Oakland’s houses of a certain age occasionally feature our local blueschist, and this is a particularly striking example.

Just a few feet farther, the scope of the work emerges.

The entire front of the property, plus the long stairway to the entrance including the risers, employs this stone to dignified effect.

Blueschist can be garish, but here it complements both the concrete and the Craftsman house in color and texture. I took a close look at it to compare with its appearance at various Oakland locations. The stone is unusually sound and was apparently quarried for this purpose. I don’t think rock yards carry this kind of unfashionable stone today — it’s hard to source large, consistent quantities, and artisanal stonemasons are no longer as plentiful as they were a century ago.

I don’t know any commercial sources of rock like this, although it’s conceivable the old Hutchinson quarry in El Cerrito, where the recycling center is today, used to yield some from its upper level. I suspect it came from a pit in the Crestmont/Serpentine Prairie area. This big exposure on Old Redwood Road shows the general quality of the rock up there.

Some of the rock in Crestmont is a pretty close match, but in general it’s more fractured and pocked with carbonates.

I suspect, but have no way of proving at present, that this old pit at the back edge of Serpentine Prairie was the source. Our serpentinite/blueschist isn’t useful for crushed stone, and although “a few tons” of poor-quality asbestos was once mined up here, this particular rock has no sign of containing tempting amounts of chrysotile. So that leaves decorative stone as the most likely product.

And that reminds me that I need to visit the meadow up there soon, for the exercise.

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.