Bedrock in the headwaters of Trestle Glen Creek

13 November 2017

Surprisingly, the town of Piedmont has its share of woodland trails — well they’re paved sidewalks, but they’re unused, covered with duff and overhung with untended shrubbery. On a weekday afternoon, you can walk quietly on miles of these soft paths and encounter only a handful of property workers. It’s in that spirit that I recommend a geologizing stroll around the highest part of Trestle Glen Creek’s eastern watershed.

The shaded-relief map below shows the creek valley in eastern Piedmont, with Dimond Canyon on the right edge. The area I’m featuring is the triangle just right of center bounded by Crest Road, Pershing and Estates Drives and Hampton Road.

And here’s a closeup just to display the street names. During weekdays, the 33 bus stops at Lexford and Hampton, where the two valleys in this little watershed join.

The geologic map shows that this area is solidly within the block of Franciscan sandstone (Kfn) that underlies most of Piedmont. The hilltop above it consists of Franciscan melange (KJfm) that includes bodies of chert (fc), notably the one on Pershing that I’ve called the best bedrock in Oakland.

The neighborhood is gracious. This view looks up Huntleigh Road, which runs on the valley floor. As I traversed the streets, I used sidewalks that almost never feel a human foot. At times it was easy to imagine being in a Tolkien novel.

Lexford Road, in its own valley, is more secluded and more whimsical architecturally.

For the geologist, these streets are valuable because they aren’t as tightly landscaped as in most of Piedmont, and the Franciscan bedrock can be seen and studied at leisure in several places where the road builders exposed it. That’s unusual for this town.

Plenty of hand specimens are available too, if that rings your chimes.

There are even a few empty lots here. Unlike the existing homesites, these are especially challenging due to the steepness of the terrain and the strength of the rock — not just on the surface, where the weathered sandstone has fractured into rubble, but also deeper down where foundations would need to be dug into the hard, unweathered bedrock. When this lot was cleared recently, it had shed enough rubble to nearly cover the sidewalk.

A large house has been proposed here for many years, and the record of the intricate wrangling needed to invite and address everyone’s concerns is mind-numbing. However, the record does include geotechnical reports that give us a glimpse underground.

What’s on the site now is this set of what are called story poles, which serve to outline the planned building.

Geologists acquire a certain ability to see the ground through everything growing or sitting on it. It’s an ability to visualize the landscape as if it were covered with story poles instead of vegetation and structures. This bit of watershed is a good field site to practice.

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Grotto Rock Park

6 November 2017

In preparation for the 4-mile walk I’m leading on 18 November for the Berkeley Path Wanderers, I’ve been visiting some of the unique and wonderful rock parks in north Berkeley. Grotto Rock Park will not be on the route, so I’ll feature it here.

It’s a little park on Santa Barbara Road at Indian Rock Avenue, just the size of a large lot, that preserves a nice outcrop of the remarkable Northbrae Rhyolite. The first thing you’ll notice about it is its very light color.

This is volcanic lava. Unlike the black, low-silica basaltic lava we know from the Hawaiian volcanoes, rhyolite is light colored and very high in silica. That makes it very stiff, even at the highest temperatures. Rhyolite lavas tend to form domes, like the Inyo Domes just south of Mono Lake or the central peaks of the Sutter Buttes.

After the Northbrae Rhyolite erupted, about 11.5 million years ago, the silica in it permeated the rock and turned it exceptionally hard and solid. It’s just about the best rock there is for climbing. Even the littlest toeholds will bear your weight.

Grotto Rock displays a typical texture of rhyolite — flow banding — that arises as the viscous lava flows like taffy. The name “rhyolite” in fact means “flowing stone” in scientific Greek. It’s also very bare. There’s only a little lichen growing on it because it has few nutrients, being mostly quartz.

Most of Berkeley’s rock parks feature the Northbrae Rhyolite. Yes, the rock is beautiful, but the developers gave the land to the city because the rock is so indestructible the lots couldn’t be built upon. Nevertheless, nature was powerful enough to round the corners off these bodies of lava.

Landslides could have done that, but surely the nearby Hayward fault did its part in rubbing these rocks smooth.

In non-geological news, Grotto Rock is said to offer better views than Indian Rock.

Generations of California geologists, including some quite eminent ones, couldn’t tell that the Northbrae Rhyolite is utterly different from the Leona volcanics of Oakland. Going through the literature on these rocks will teach you humility. It was a re-entry grad student at Cal State Hayward, a climber named Lin Murphy, who straightened everyone out about 15 years ago.

I’m started to get excited about the walk.

Orinda’s 1204 Hill

30 October 2017

As far as I know this hill has no name, but it’s a highly visible part of Orinda. You pass it on Route 24 between the Wilder exit and downtown, as seen in this Google Maps perspective view. The USGS topo maps give it an elevation of 1204 feet, so I’ll call it 1204 Hill.

It’s part of San Pablo Ridge, but the west branch of San Pablo Creek cut a deep gorge through the ridge while it was being uplifted, forming a classic water gap. That’s clear to see in the 1915 topo map, made before anything significant was developed there.

During the mid-20th century, at least four quarry pits ate their way into this hill, one on the north side and the others on the south. Now it provides a bit of seclusion to the exclusive luxury Orinda Wilder development. I paid the hill two visits this month, and to me it offers seclusion from Orinda Wilder. Here it is, as seen across Wilder Valley last summer.

A fire road encircles the hill for your visiting convenience. It offers good views all around. This is looking north across 24 to grassy Eureka Peak along with wooded Vollmer Peak behind it. Both are also part of San Pablo Ridge.

And in the other direction is the last, southernmost lump of San Pablo Ridge, separated from 1204 Hill by another branch of San Pablo Creek.

Looking west, Route 24 approaches the wall of the Oakland Hills. Eureka Peak, at the right edge, and 1684 Hill on the left, part of Gudde Ridge, form the sides of Siesta Valley.

Both peaks are made of the same lava of the Moraga Formation. The formation is folded like a taco shell under Siesta Valley, a geological feature called a syncline, and hey why don’t we look at the geologic map now. This is the same area shown in the topo map.

“Tmb” is the basalt lava of the Moraga Formation — the taco shell — and “Tst,” the Siesta Formation, is the filling. That red line with the little arrows is the axis of the syncline. The light-yellow unit “Tms” is sedimentary rocks, also in the Moraga Formation, that were laid down between eruptions of lava. To the right of the black toothed line — the Moraga fault — the rocks are much younger and I won’t mention them again.

The quarries were excavating freshwater limestone from the Siesta Formation, used to produce cement and soil amendments, and basalt from the Moraga Formation, used for crushed rock. Here’s the lava.

Where lava flowed onto wet ground, the underlying clay got baked and the steam oxidized the lava, turning both materials red. It’s not always easy to tell what’s what.

There are bits of light-gray limestone here and there. You can tell by the way a drop of acid fizzes vigorously on it. I didn’t take a picture, but I did stop to shoot examples of mineralization in the lava. They might be copper compounds, or phosphates, or several other possibilities that aren’t easy to identify just by eyeballing.

There’s also a little conglomerate that looks for all the world like the Orinda Formation. It goes to show that a geologic map is a simplification of a wacky, complicated Earth. However, I don’t quite trust the stone because it’s loose, not a proper outcrop.

Climbing the hill from the fire road is a scramble. There are only subtle trails, the kind that deer make. But on top of the hill there’s the stubs of a former water tower, and a stone spiral with a little cairn at the center, the kind that locals make. And outcrops.

The last thing to mention is the quarry scar. It displays the structure of the Moraga Formation nicely.

But the exposures are unapproachable and dangerous.

I’m not sure what the developers plan to do with this hill. Probably nothing except watch it closely. The closest street to the quarry scar has home sites only on the far side of the road, and there’s a wide ditch between the road and the quarry that will catch falling rocks.

I’m gonna keep my eye on this interesting spot.

Serpentine and pebbledash on Broadway

22 October 2017

The intersection of Broadway and 20th Street features strong buildings on all four corners. We all know the I. Magnin building (built 1930) and the Capwell (Sears/Uptown Station) building (built 1929) facing it. Across Broadway, we have the metal-clad urban spaceship of the Kaiser Permanente Division of Research building (built 1982) and finally the dark cube of the Golden West Tower Building (built 1968).

One of the nice things about taking the bus is that while you wait, you can look around like architects and urban planners do, as if the cityscape were a stage set. And so, waiting for the 33, I finally took notice of the building wall that I’ve walked past hundreds of times.

It’s chips of dark serpentine, embedded in cement and polished. People in the building trade must know exactly what this is called, but I can’t crack their code. Here’s a closeup.

So all this time, the Golden West Tower Building has been giving the serpentine-clad I. Magnin building a nod, a salute, a shoutout, a heart-tap. I thought that was cool.

That day the 33 took me to Piedmont, where I recognized the same material in Bufano’s “Bear and Cubs” sculpture in Crocker Park, featured here previously.

The other element in that Broadway streetscape is the sidewalk. You’ve all seen it.

This is what’s called a pebbledash finish. Concrete is laid down, then pebbles are pressed into it. When I look at it I think, “What a lovely Franciscan color scheme,” because the reddish and greenish mixture of metamorphosed argillite and chert is so typical of our coastal Northern California rocks. I also think fondly of the red-and-green gravel of Rodeo Beach.