Archive for the ‘Oakland rocks’ Category

The Eocene mudstone, part 1: East Ridge

1 April 2019

I’ve spent an inordinate amount of time lately in the obscure part of the Oakland Hills between Piedmont Pines and Canyon. Much of it leaves me puzzled, and some of it leaves me dazzled, but I did manage to slow down and get a decent look at the rock along the East Ridge Trail in Redwood Regional Park. You’ve walked on it if that trail is your favorite hike, starting from the Skyline Staging Area.

The first mile of the trail exposes this rock in the roadbed. Stay on the trail, because actual outcrops are hard to find, and soon the poison oak will seal the woods off for the year. It presents colors of greenish-brown, buff, tan, dun and gray but the overall impression is a light brown.

The Eocene mudstone doesn’t have a formal name. It’s just a strip of fine-grained, mostly silty rock with a bit of sandstone here and there, that’s mapped across the midsection of both Thornhill and Shepherd Canyons and peters out along East Ridge (which has no formal name either; elsewhere I’ve called it Pinehurst Ridge). Here it is as shown on the geologic map labeled “Tes”, a dagger of cyan-ish color with the blade pointing east. This post is about that east-pointing blade, where there are no homes to spoil the ground. (Part 2, when I get around to it, will gather notable outcrops in the residential neighborhoods of the handle.)

What do we know about it? James Case gave it a searching look for his 1963 PhD at UC Berkeley and assigned it an age, based on fossil shells and foraminifers — one-celled “animals” with carbonate skeletons — in the early to middle Eocene, somewhere around 50 to 40 million years old, maybe a bit older. The specimens came from “thin beds of fossiliferous limestone” that Case noted on East Ridge.

I found some in the trail. It fizzed very nicely in a drop of acid, as you’d expect. The shells were small and mostly fragmentary. Naturally I left it there, under the East Bay Regional Park District’s protection.

Dorothy Radbruch of the U.S. Geological Survey looked at this rock unit again in the late 1960s and called it “sandstone and shale,” primarily fine-grained sandstone. She noted that it was fairly strong, holding up 1:1 slopes, which is reassuring for homeowners in that part of the canyons.

Most of what I saw in the roadway was siltstone — usually massive, or featureless, but occasionally laminated like this.

The USGS’s Russ Graymer, in the 1990s, characterized it more simply as green and maroon mudstone with occasional sandstone. He stated confidently that it was faulted on the top and bottom — just another small card in the well-shuffled deck of Coast Range rocks in the greater San Andreas fault zone.

In brief, it’s an isolated body of pretty clean mixed fine sediment that must have formed off the seacoast, not too near. It got lost in the shuffle as California was sliced, diced and rearranged between the middle Eocene and now.

Here’s a detail of the geologic map, plus the equivalent area in Google Earth, in case you feel like poking around. But note that just north of the East Ridge Trail, it’s East Bay MUD watershed land.

Top to bottom: Tor, Orinda Formation; Tcc, Claremont chert; Tsm, Sobrante Formation; Tes; Kr, Redwood Canyon Formation. The line with the teeth is a thrust fault, south side up.

The woods are rapidly closing in from their winter openness, and the slopes are in that brief interval between slippery-wet and crumbly-dry. I’m itching to return while I can, and it’s not from the poison oak, yet. Already I’ve missed the manzanita blooming season, except for a rare straggler . . .

and the land beckons.

Claremont Canyon conglomerate

18 March 2019

I haven’t mentioned it lately, but I’m writing a book on the geology of Oakland. One chapter is about a road trip that takes you to nearly every different rock unit in town — actually it’s pretty much the one I posted in 2017. One problem with that posted route is that it doesn’t pass an exposure of the Orinda Formation with its beautiful conglomerate.

The Orinda underlies the four-way intersection at the top of Claremont Canyon, but you can’t see any of it there. The intersection is in a topographic saddle, and it’s a saddle because the rock erodes more easily than the lava flows and chert beds that flank it on either side. That’s right, conglomerate — that rugged-looking stone studded with cobbles and boulders — is crummy rock.

So my road tour has to detour at this intersection, going down Claremont Boulevard a little ways to a spot where you can study the conglomerate at leisure. You’ll pass excellent exposures of it on the way, but there is literally no space to stand there.

The closeup of that rock that I posted here back in 2008 (still a favorite shot) was acquired at some peril. No, instead of stopping, you go around the hairpin turn and pull over at the entrance to UC Berkeley’s open space reserve. If you’ve followed along over the years, this is a familiar stop, the type locality of the Claremont Shale. Just uphill from that classic spot, the chert beds give way to Orinda conglomerate.

Look back across the road toward the head of Claremont Canyon, and that fire road (the Summit House Trail) is what you’ll take, just to where it enters the trees.

If you hurry, these daffodils will still be there to check your credentials and wave you through.

The uphill slope is dotted with big clean boulders. Some are showoffs . . .

and others are shy.

But all are worth a close look.

I ranged up the slope here, looking for proper outcrops of the conglomerate, but there are none to be found. It seems these boulders rolled down here the last time Claremont Boulevard was upgraded, and it’s pure serendipity they’re so nicely on display for you now.

The nearest thing to an outcrop is a spot where the fire road scrapes down to bare rock. It’s a clean fine-to-medium sandstone, also part of the Orinda Formation.

This finer-grained rock is where fossils are occasionally found, but I have to tell you I’ve never seen one myself. When the Caldecott tunnels were being bored through the hills, paleontologists were hired to collect fossils from the spoil piles. (See some of the Orinda Formation fossils here and here.) The fossils testify to a warm land of woods and year-round moisture, not much like today.

These boulders look like they’ll be around for a while. Pay them a visit some time in the next few decades.

But ultimately, rocks are perishable. Sooner or later they’ll make their way to the sea and be recycled in new rocks. The cobbles preserved in the Orinda conglomerate are all that remain of a whole landscape of rocky uplands that once existed here in Miocene times, some 11 million years ago. Not even mountains last forever.

Geology of King Estate Open Space

4 March 2019

After tramping all over Oakland, I still find its landscape full of uncertainty and mystery. The alluring hills of King Estate Open Space Park have brought me here time and again, sometimes to lead walks, sometimes to just stop and smell the flowers. Last month I came back yet again, this time to look harder at it.

The best resource on the park’s history and vision is on the Oak Knoll Neighborhood Improvement Association site at oknia.org. The aspirational Site Plan has the following concept for the park: “The winds sweep my imagination across the horizon. We move over the hills exploring the wilds and oaks embrace us. Here, in this place for everyday we cultivate community.”

King Estate Park is a grassy ridge at the south end of the Millsmont-Eastmont hills, an apparent pressure ridge that stretches along the Hayward fault from Mills College to the Oakland Zoo area, between Seminary Creek and Arroyo Viejo. What drew me here as a geologist was the geologic map (USGS MF-2342) that depicts the area as a peculiar ancient gravel (Qpoaf on the map), the largest piece of this material in Oakland.

The attractive thing is that according to the map key, these deposits “locally contain freshwater mollusks and extinct Pleistocene vertebrate fossils.” The odd thing is that they don’t match another criterion: that this dense gravel “can be related to modern stream courses.”

I explored the portion of the park north of Fontaine Street. Here’s the street map, marked with the three locations I’ll be showing photos of.

And here’s the aerial view, from Google Earth.

And just for fun, here’s the digital elevation model, with buildings and trees removed.

OK! Location 1 is on the steep western slope at the north end of the park, which I climbed. Halfway up is a sizable area of rock rubble consisting entirely of Leona volcanics, the same bedrock shown in pink labeled “Jsv” on the geologic map. The near-outcrop is in the lower left corner of this shot.

And the rock looks like this.

Down at location 2, there’s a spiral labyrinth that people have made in the last few years; I don’t remember seeing it before. But on the assumption that it’s made of stones from the immediate surroundings, I infer that it’s Leona volcanics over here too.

The Leona is generally light-colored with some greenish bits and some red-orange coatings where it’s weathered, hard to describe in detail but distinctive once you’re familiar with it. Once a range of underwater volcanoes that subsequently underwent a lot of alteration, it offers up a variety of intriguing bits that lacking a petrographic lab I can only scratch my head at.

Anyway I’m looking all over, and every bit of gravel on this hill is Leona volcanics. Now a river gravel, which is what the map describes and what I expected (indeed, what I actually perceived on previous visits!), consists of rounded clasts and a variety of rock types from the stream’s catchment. Other gravels in this town are just that way, but not this. The whole time I’m there I’m muttering to myself, “this stuff is colluvium” — raw rock rubble, mixed with soil, that hasn’t moved from its birthplace except maybe in landslides.

In location 3, we have proper bedrock. It show up where the soil has been scraped bare . . .

. . . and farther down the slope in genuine outcrops, which I always cherish.

So in sum, the whole north half of the park, far as I can tell, is either bedrock or colluvium of the same stuff: Leona volcanics. How did it get to be mapped as Pleistocene river gravel? The MF-2342 geologic map was published in 2000. There are two previous serious maps of this area. Dorothy Radbruch mapped it in the 1960s for map GQ-769, and there the area looks like this.

She called it “Qg”, “gravel, sand and clay” and noted that it contains pebbles of Leona Rhyolite (what I call Leona volcanics). She also said this: “Contains molluscs of probable early Pleistocene (Irvingtonian) age.” They were at the locality marked by the triangle with “22133” next to it; that number refers to a “report filed at Washington, D.C.” which is probably gathering dust deep in a back room. She also referred to reports on two boreholes, numbered 95 and 96 on the map, which recorded various kinds of gravel down as deep as 45 feet. That could’ve been deeply weathered Leona as easily as anything else.

But you know what? I’m going to go with Andrew Lawson’s original map of the area from 1914 (Folio 193), in which everything is just straight Leona.

Even though he thought the Leona was very young (hence the name “Tln” meaning Tertiary Leona), he could tell what the ground was saying. At least, he and I agree. I’m sure, though, that he scratched his head as much as Dorothy Radbruch and I did. And they must have enjoyed the view as much as me.

I’ll just have to poke around here some more.

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.