Archive for the ‘Franciscan rocks’ Category

Avalon Hill

18 July 2022

There’s a little hill south of the UC Berkeley campus, near the Claremont Resort and the Hayward fault, that cried out for a visit. It stands out on Google Maps (with the “terrain” setting, naturally).

On the geologic map, it appears as a blob of Franciscan sandstone.

I thought I’d give it the same treatment I gave Easter Hill in Richmond: explore the rock and comb through old maps and documents.

The road running by the south side of the hill is Avalon Avenue, so I’ll call it Avalon Hill. It was never exactly a landmark, but it has a past and retains a certain presence.

The hill was owned by John Kelsey in the late 1870s, and Kelsey Street commemorates his name. Today’s Claremont Avenue was called Telegraph Avenue at the time, as shown in the 1878 Thompson map.

In 1884 the Berkeley Water Works excavated a large reservoir on the hill, and the following year the Alameda Water Company took it over. The Garber Reservoir held a million gallons and was lined with concrete. Here it is on the 1894 Wagner map.

It is not to be confused with today’s Garber Reservoir, the flat-roofed structure 1500 feet south on the north side of Claremont Avenue.

Development began to surround the hill in the 1890s and 1900s, especially after the 1906 earthquake. From then until the 1930s, the struggling private water companies of the northern East Bay merged, the East Bay Municipal Utility District swallowed them whole, and the water delivery system began a slow and expensive transition to the rational and robust setup we enjoy today. The 1912 street map shows the property still in the hands of the Peoples Water Company, the next-to-last of the private firms.

I don’t know when the reservoir was decommissioned and removed, but the homes adjoining it were built starting in the 1910s. The large home on the hilltop was apparently built in 1960.

The best picture of the hill itself is the digital elevation model made from a special survey of the Hayward fault. Its sides are too steep for streets, which has helped keep it quiet and isolated.

On the north side, Garber Street has a rustic interlude where a narrow road sashays down the hillside like a mini-Lombard Street. Avalon Avenue, on the south side, is blessed with three stairways, one at the end and two going down to Russell Street.

All those old houses with their mature landscaping cover up the rocks very effectively, I can testify. But there’s an excellent exposure at the end of Avalon Avenue underneath a private driveway.

What few flat surfaces there are on it appear to reflect fracture planes, imposed by the tectonic stresses on the rocks over the years, rather than any original bedding.

Up close, the rock is a hard siltstone much like the rocks in the quarries of the Piedmont crustal block: the Bilger, Blair and Davie tennis stadium quarries.

But just looking at it isn’t definitive. The geologists who’ve mapped it have left it unclassified (KJfs) rather than lumping it with the sandstone of the Novato Quarry Terrane (Kfn) exposed in the Piedmont block to its south. Here’s a larger piece of the geologic map showing what I mean: it might be the northern tip of the block or it might be the first of a string of rock bodies to its north. The thrust fault leading up to it, the dotted line with the teeth on the upthrown side, is reasonable but conjectural.

Avalon Hill is a cool little bit of Berkeley. Stick your nose in and poke around some time.

Yes, “Avalon Hill” is a private joke. My siblings and I warped our personalities in the 1960s by moving armies and fighting over the rules of the board-based wargames produced by the Avalon Hill company, starting with Tactics II and Gettysburg.

Murieta Rock, El Cerrito

14 February 2022

In Gold Rush days, the Bay area was as wild as the rest of California: depopulated of Indigenous people and a free-for-all of frontier characters. One of those characters was the legendary outlaw Joaquin Murieta. His story, at least the version we have today, had all the makings of legend — a handsome, peaceable Mexican, viciously victimized along with his wife and family at the hands of Americans, who turned desperado and came to a bad end. As befits a good legend, every crime in California was added to his name — and this fine outcrop too in the hills of northern El Cerrito.

The rock stands out in early photos of El Cerrito, back when the hills were still bare, but today it’s unobtrusive in surroundings of trees and homes at the intersection of Cutting and Arlington Boulevards. It’s also smaller than it used to be; a rectangular quarry pit has been carved into its southwestern side.

Supposedly Murieta’s gang would watch the main road from up here and swoop down on victims. Or this would be their lookout when they hid out in Wildcat Canyon. That may have been. I think the name stuck because it looks like a broken-down haunted house made of a rare, unearthly-looking blueschist.

The area is geologically interesting. The rock is just south of Cutting, below the large “L” at the center (part of the name of the old San Pablo rancho).

All the bluish rocks are Franciscan, the orange (Tor) is the much younger Orinda Formation, and between them is the Hayward fault zone. “KJfy” is a metamorphosed sandstone and “spm” is the melange. Regular readers may recognize “Jsv” as the Leona volcanics, but this little pod is actually the northernmost occurrence of Northbrae rhyolite, the stuff of Berkeley’s rock parks. (Thanks to Karl in the comments for flagging my oversight.)

Murieta Rock is a high-grade block in a melange of serpentinite — a rare outcrop within a rare setting — and for background I refer you to this post from the last time I was up this way. Notice the large areas of the map labeled “Qls”; these are gigantic, slow landslides all of which originate in that melange. More of them are in north Berkeley (see my 2017 walking guide to the area, from the Berkeley Path Wanderers Association site).

Enough of that. You can reach Murieta Rock on the 7 bus line, from either the El Cerrito del Norte or Downtown Berkeley BART stations, or drive there yourself of course, but I enjoyed walking there through Canyon Trail Park — if I were Murieta, I’d swoop that way to carry out my robberies. The view from the top of the rock over San Pablo Bay is superb.

And since this is Valentines Day, why not consider the rock for a romantic geo-outing?

Lake Temescal, the west side

12 April 2021

For one of Oakland’s most rugged places, the west side of Lake Temescal doesn’t expose a lot of rock. But what’s there is unusual for Oakland, and interesting.

I’m talking about this ridge — tectonically, a shutter ridge — across the lake from the swimming beach.

The map of Lake Temescal Regional Park shows two trails there, the low one along the water and the high one up in the woods. An even higher trail, not marked on the map, is off limits and doesn’t expose much rock anyway.

I often wish I’d lived here in the 19th century when Oakland was new. It was in 1868 when Anthony Chabot acquired a steep little canyon back of the hills and built a dam to supply the young city with dependable water service. His technique, perfected in the gold fields of the Sierra, was to take a high-pressure water hose and wash down the sand and gravel from the hillsides to build the dam. I wish I could have inspected the scrubbed slopes at that time, but there were almost no trained geologists in the whole state, let alone me.

The canyon has a flat floor now, after decades of sedimentation, but you can see from the high trail that it’s still steep and narrow.

And the action didn’t end when Chabot finished the dam. He built a control tower in the new reservoir, but a landslide soon took it out. I’m guessing that was probably near the sluicegate where the beach house is today, and I’m guessing that the rainy winters of 1868-69 and 1869-70 plus afterslip and aftershocks of the big 1868 earthquake on the Hayward fault had something to do with it.

Speaking of which, two major strands of the Hayward fault run right through the reservoir. They’re helpfully shown on the map above. This is where the main strand crosses the dam. (Don’t worry, the massive dam will not fail even under the largest possible quake on this fault.)

The dam itself was raised and then lowered during the next few decades, and presumably the lake rose and fell too. Meanwhile trees and brush moved in upon the slopes where the Ohlone had previously maintained grassy meadows, and the rocks decayed and soil built up.

And the rocks themselves embody the complicated history of starting out in a vigorous subduction zone, being deeply buried and exhumed probably more than once, then being torn up and shoved around by the San Andreas fault system of which the Hayward fault is part.

All that is to say that Lake Temescal is a dynamic area at all time scales. It’s more complex than the small-scale geologic maps can show, even though it’s complex enough on that map.

KJfm, Franciscan melange; ch, chert block; af, artificial fill; sc, silica-carbonate rock; sp, serpentinite; Jsv, Leona volcanics; fs, Franciscan sandstone; KJkm, Knoxville Formation

The west side is mapped as melange, which is basically a mess of mashed-up sandstone with big blocks of other rocks, each with its own separate history, suspended in it. The little blip labeled “ch” is this block of chert at the top of Hill Road. So when I visited the west side trails last week, I expected to find things I didn’t expect. And most of the rock along the trails appears to be nondescript sandstone — I say appears because since hammering is forbidden, it’s hard to find a fresh surface. But lo and behold, along the high path coming down from Broadway Terrace, there’s the telltale gleam of blueschist in the exposed subsoil.

The color really comes out when you wet a piece.

Other apparently high-grade metamorphic rocks include this hard, glittering schist. Not having access to thin sections and petrographic microscopes, I can’t check for the presence of jadeite, which has been reported in blocks from this melange.

And over at the north end of the train is a distinctive outcrop of another schist. Hard rock supports slow-growing lichens, and the species differ depending on the rock’s chemistry.

Underneath the lichens, the rock is a bluish-gray mixture with a strongly folded texture, both signs of a rock that’s been through a lot of distortion at high pressure and temperature. These photos are from exposures by the lawn. The first shows the folding and the second shows fault-related crushing on the left side. The crushed material is called gouge, and bits of it are common in this sector.

My authority on Oakland’s Franciscan rocks, John Wakabayashi, holds that the west side of the lake hides the same ancient major thrust fault — a megathrust, in fact — that’s famously exposed in El Cerrito above the recycling center, where high-grade rocks have been pushed above lower-grade rocks. Unfortunately the fault itself appears to be in “a brush-filled gully with no exposure,” so it may be a while before we ever learn more.

But I did learn something more last week, about the beach house.

It is made with the local rock, namely the Leona volcanics. Whether the stones came from the hill just upslope to the east, the one that collapsed in the 1870s landslide, or from across the freeway in the great Tunnel Road cut during the 1930s, I do not know. I’m guessing the former, but I would be happy to be wrong if it means I can be certain. Putting the building and the roadcut in the same photo, there is a resemblance.

The fun thing is, both locations can be closely inspected. That sounds like a good afternoon project.