Archive for June, 2008

Fire on the mountain

13 June 2008

hiller highlands fire

Yesterday there was a fairly small fire in a treacherous place, the Hiller Highlands neighborhood. There was confusion in the media accounts I saw, but here is the correct version, as you can see in this view from across the freeway this morning. The streets, from top to bottom, are Charing Cross Road, Tunnel Road, Caldecott Lane and Route 24. (A typo in the Tribune, “Charring Cross Road,” may give you grim amusement.) The blaze began on Tunnel where street work took place a few months ago, and nearly reached Charing Cross. As I shot this photo, fire crews were still combing the burn area in search of embers.

The hills love fire, and the ecosystem is adapted to it, but civilization here is not. Given that we have irreversibly encroached on the hills by permitting residential construction there, we’re stuck with the price in dollars and lives in perpetuity. Not even the next major earthquake on the Hayward fault, less than a kilometer west, will change this even though the whole neighborhood would likely burn down again, just like 1991, if it happened today.

Yesterday not an hour before the fire started, I was standing on Grizzly Peak Boulevard looking down at this part of town and sensing just how dry everything is. Instead of coming down through here, as I have before, I walked down through the Grandview neighborhood to its north. Upper Grandview is an uncanny place, having been wiped out in the 1991 fire and disneyfied since. Today I was going to visit the fire site, but I got this shot because instead I took the opportunity to try the fire road above Broadway that ends overlooking the North Oakland Regional Sports Center. If you visit the park, have a look at the fire-resistant garden there. The rocks are mapped as undifferentiated Great Valley Sequence and are mostly an undistinguished gray sandstone.


11 June 2008

huckleberry chert

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.

mount diablo

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:

round top

Heights and flats

6 June 2008

oakland heights

In the East Bay, the Hayward fault separates high ground and low, with a few exceptions. Oakland is an exception (so is San Leandro, Berkeley and points north). From Oakland’s southeastern extreme at Lake Chabot up to the Panoramic neighborhood, the fault generally has a few hills on its Bay side. If you ride BART and look up at the hills, the fault is almost entirely hidden. The hill Piedmont sits on is the largest body of rock west of the fault. So Oakland is not like Hayward or Union City, where the fault is quite stark.

But here on upper Dwight Way, at Oakland’s far north end, is a spot where the height/flats dichotomy is laid right out plain. (Click the photo for a 900×750 version.) This little canyon is the one just north of Claremont Canyon, and I don’t know if it has a name. Behind me is little Dwight Canyon and just to its north is Strawberry Canyon, where the Cal stadium sits. High rock hills lie above the fault, and a plain of deep sediment lies below, an area where seismic shaking is liable to cause ground liquefaction. Of course landslides could happen where I stand; the brown patch below looks like a landslide scar . . . pick your poison.

The big set of knockers, Mountain View Cemetery

4 June 2008

biggest knocker

Just below the highest hill in the cemetery, across a flat space south 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.


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