Cactus Rock

Walking along Acacia Avenue, you may feast your eyes on the homes and grounds of the core street of the Upper Rockridge neighborhood, but one exception stands out at 6240 Acacia: this rock peeking over the scene. (click it for full size) This appears to be Cactus Rock, attested to in old postcards about the ongoing development of this streetcar suburb (still served by the improbable bus route 59A). I can’t get a good look at it, but it appears to be a standard Franciscan knocker that is much smaller than the mysterious Rockridge Rock. To judge from the prospect at Alpine Terrace, the next street uphill from here, the views from the rock are fantastic.

Siesta Valley

Last week I took a nine-mile ramble up Claremont Canyon, then along the East Bay Skyline Trail as far as Lomas Cantadas, then down to Orinda and the BART station. The trail goes across the head of Siesta Valley, an interesting geologic feature and a wonderful view (click full size). Route 24 cuts across the valley right in front of the construction, which will be Orinda’s newest neighborhood some day.

This valley is not a streamcut valley, but rather is formed by the folding of the rocks beneath it in a syncline. That’s a shape with a trough in the middle and upturned sides. (The opposite is an anticline, a ridge with downturned sides.) The notch that route 24 goes through is cut by a stream. My guess is that it’s a water gap, cut by the stream at the same time as the rocks were being folded. The rocks of the Siesta Valley are sandstones and mudstones belonging to the Siesta Formation, the next youngest set of rocks after the basalt flows of the Moraga Formation. Speaking of which, I also took this shot of Round Top from the north, with the old basalt quarry grounds in front of it.

Huckleberry

Huckleberry Botanic Regional Preserve is a little-visited piece of wildland just over the Oakland Hills crest south of Sibley Volcanic Regional Preserve. It owes its existence to this rock, the Claremont Chert. The brochure lavishes attention, and rightly so, on the plants of the botanic preserve, but it’s also a good place to see the chert in many settings. Cliffy here or buried there, shaly or rugged, the chert asserts itself amid the soils and growth like the bass player in a jazz combo.

Owing to its history, the Claremont Chert is high in silica and low in nutrients. It drains quickly and breaks down slowly, and for the purposes of today’s vegetation it slows down the natural process of faunal succession—the series of plants that goes from pioneer species to climax forest. Thus where much of the hills is a uniform oak/madrone woodland, the Huckleberry Preserve is a variegated assemblage of everything from gravelly manzanita balds to soft seeps populated with irises, plus huckleberry thickets of course. Hike the nature trail and meet some of the natives. The self-guiding brochure carefully states the role of fire in maintaining the hill ecosystems, mainly to show how the Huckleberry is an exception. But these days, everywhere you look in the hills is a fire long overdue. Some day we will have to catch up with the Ohlone tribes, who managed these lands with regular burnings.

The trail provides several fine views eastward. You can pretend that white settlement never happened and imagine Mount Diablo pristine, as it was when Cabrillo forced his men through this land 250 years ago. (click for larger version) And you can enjoy Round Top’s symmetry from the rare southern vantage:

The big set of knockers, Mountain View Cemetery

Just below the highest hill in the cemetery, across a flat space north of the utility yard, is the best bedrock outcrop in the whole Mountain View property. This shot is from the south end looking toward the utility yard; red chert is in the foreground and other Franciscan rocks lie behind. I suspect that it was once a free-standing ridge that has been filled in on the east (uphill) side. The west side is a wall of trees, some of them growing right out of the rock, that hides everything pretty well.

Rocks of the Franciscan Complex, to remind everyone, include red chert, light-gray coarse-grained sandstone, dark shale, and dark volcanic rocks with various degrees of metamorphism. The volcanic rocks came first, formed at a deep-sea spreading ridge. Deep-sea ooze made of siliceous microfossils settled on the volcanics and became the chert. As the whole seafloor assemblage approached North America, sediment from the continent cascaded down submarine canyons and later turned to sandstone and shale.

All of these entered a tectonic subduction zone, marked by a deep-sea trench like those off Japan today, and the whole assemblage was squeezed, heat-treated, crumpled and plastered against the prow of the North American continent. The different rock types, ranging in age between about 150 million and 60 million years (Jurassic to Paleogene), were churned into an intricate mixture called mélange. Chunks of the harder rocks float in a scaly matrix of soft shale and tend to emerge above ground as the rocks erode into soil. Those chunks, not quite bedrock and not quite boulders, are what generations of California geologists have called “knockers.”

Later, sideways movements along the wide San Andreas fault complex tore up and rearranged this complex of rocks even further. Today Franciscan mélange is found in the Coast Range from south of San Luis Obispo all the way up to Cape Mendocino.

Panoramic Way in (yes) Oakland

Panoramic Way is one amazing street. It starts in Berkeley right at the Cal Stadium, crosses the Hayward fault immediately, and winds upward into the hills past one fine house after another. Then it enters Oakland, forming the spine of the city’s northernmost neighborhood. On a day like today, the view is as good as it is from anywhere in the bay area. Some day I’ll climb Panoramic Way all the way to its top, but today I’m beat.

Bedrock is scarce, but what I saw in one spot looked mighty like the basalt of the Rockridge Shopping Center quarry:

But the area is mapped as Great Valley Complex. Go figure.

Dracena Park, Piedmont

Dracena Park is a repurposed quarry pit in lower Piedmont. The quarry was started in 1852 by Walter Blair, at the head of Pleasant Valley Creek (now culverted under east Grand Avenue), and supplied stone for Oakland’s first streets. After it was closed, the City of Piedmont used it as a maintenance yard, then converted it to a splendid park with an enchanting lawn surrounded by a walkway. At the entrance is a sturdy, attractive new play complex; to the north is a narrow ravine where the headwaters of the creek once flowed.

The stone appears to be a faulted mix of sandstone and basalt of the Novato Quarry Terrane of the Franciscan complex, like the rest of Piedmont and its surroundings. Here the two rock types are juxtaposed. I’m eager to come back and poke around these rocks some more — with my hammer left at home, of course.