Archive for the ‘Oakland geology puzzles’ Category

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.

The Dimond Canyon water gap

15 May 2017

In a city full of geologic features, Dimond Canyon stands out as a classic example of a water gap. But it can be hard to see, even from the prime viewpoint of Leimert Bridge.

Let’s abstract ourselves by studying the overhead views shown in maps. Google Maps, with the terrain view turned on, is where I like to start.

Compare Dimond Canyon, cutting straight through the bedrock ridge of the Piedmont block, with Indian Gulch (Trestle Glen) on the left, a conventional stream-cut valley that fans out against the ridge.

For a starker view of the topography, I like to consult old USGS maps like the 1897 Concord quadrangle, made before most of the area was built up and dug into.

Here we can see that the ridge reaches the same elevation on either side of the canyon — without the canyon cutting through it, this would be a continuous crestline.

There is no sign, either, that Dimond (“Diamond”) Creek cut its way through by headward erosion. That would have left tributaries fingering off on either side, like those visible in the contours above Indian Gulch. Indeed, the single little tributary in the canyon is actually a hanging valley that has to descend steeply as it meets Dimond Creek — not as spectacular as those in Yosemite Valley, but with the same basic configuration.

Finally, we can look at the bedrock evidence in the geologic map.

The whole area around the canyon is mapped as Franciscan sandstone (Kfn), with no hint of faulting or other structure that might have favored the formation of a canyon here. Consider the well-developed valleys above the canyon, guided into existence by the rock-crushing Hayward fault, or the more subtle topographic features where the southern edge of the Franciscan bedrock meets old alluvium.

What we have here, then, is a genuine water gap — a deep pass in a mountain ridge with a stream flowing through it. Geology textbooks will tell you there are two ways to make one. One is for a river to uncover an ancient ridge as it strips the countryside of its sediment cover. The classic case is the Delaware Water Gap in Pennsylvania. The other is for a river to sit there, doing its thing, as the land rises up around it. The Central Valley has good examples at the foot of Del Puerto Canyon . . .

. . . and the Berryessa water gap west of Winters.

What’s odd about the Dimond Canyon water gap is that it’s being carried along the Hayward fault. Every few hundred thousand years, then, it gets itself a new headwater catchment. Today its catchment is Shephard and Palo Seco Creeks. Once upon a time, though, it must have carried the waters of San Leandro Creek. Coming up: Temescal Creek.

Rubbing rocks

13 March 2017

Rocks interact with animals of all kinds. Obviously, lizards and voles and snakes and woodchucks live on rocks and/or dig under them. Humans paint on rocks and move them around and blow them up. Today, however, I’m going to talk about animals that scratch themselves against rocks — rather, rocks that animals have rubbed for thousands of years.

Last week John Christian, a sharp-eyed and inveterate walker of our hills, showed me this outcrop next to the Little Farm in the Tilden Nature Area.

Not much to look at unless you get up close. When you do, you’ll see that it’s covered with moss and lichens, except for some oddly smooth bare spots on the outermost surfaces.

Some of these are smooth enough to gleam in the sun.

These features are well known in buffalo country, elephant country and other places around the world. Large herbivores deal with the mites and lice and other irritants in their skin by rubbing themselves against anything scritchy they can find, preferably after a nice wallow in high-quality mud or at least a good roll in the dust. These marks, in a word, are sandpapered onto the rocks.

This outcrop appears to have gotten its smooth spots from that cause. But the cover of lichen and moss shows it hasn’t been used in a very long time. Today, deer have plenty of trees to use, but historically — and prehistorically — most of coastal California was treeless because the Indians kept it that way with regular fires. However, deer aren’t tall enough to make most of these marks. Now during the ice ages, though, this was a treeless cold savanna that supported herds of elk and mammoths and ground sloths and horses and bison and camels. Could those extinct animals really have buffed these boulders?

The best case for that is on the Sonoma coast just south of Jenner. I wrote a piece about the “mammoth rocks” there, which you’ll have to pick through on the Internet Archive because the folks who paid for it threw it away. The archaeologist who discovered the site has also written it up. John and I both know that site, but I never thought to look around our own hills whereas he did. When we hiked a little farther down Wildcat Canyon and he showed me a polished boulder of the same blueschist found on the Sonoma coast, I had a shiver of recognition.

Berkeley is justly famed for the rock parks in its boulder-strewn northern hills. They, like the Little Farm outcrop, feature the Northbrae Rhyolite, a particularly tough volcanic rock that you have to climb to appreciate. Even the tiniest fingerholds are as solid as steel.

At Indian Rock, generations of climbers haven’t buffed anything smooth. Nevertheless you’ll see two kinds of smooth spots. There are slickensides, formed where rocks slide against each other.

And then there’s this wide, vertical rock face that looks like it might be very inviting to an itchy mammoth or ground sloth. And it’s polished.

A few other Berkeley rocks display the same kind of evidence, if you look closely. But I had to see if Oakland can boast it too. Thinking like a mammoth, I visited my favorite Oakland blueschist outcrop in Knowland Park to reconnoiter. It looked very mammoth-friendly, including a good site for a wallow in the headwaters of Upper Elmhurst Creek.

But no such luck. Every surface of the outcrop was rough and rugged as can be. The same with this notable serpentinite knocker farther down the stream valley.

I blame Oakland’s rocks. We don’t seem to have anything as tough as the Northbrae Rhyolite, capable of retaining a polish for tens of thousands of years. But I’ll keep my eyes open; you never know.