Archive for the ‘Oakland blueschist’ Category

The Mills College high-grade blueschist block

14 January 2015

Longtime commenter artisancrafts reminded me, in a comment to my last post, that there’s a nice exposure of blueschist at the north edge of Mills College. Yesterday I easily located it by following his/her directions. First there’s the old railbed that once ran to Laundry Canyon. This stretch of it, which once continued down through the Fan parallel to High Street, used to be part of Courtland Street.

mills-railbed

At the spot in the distance where the sun shines across the roadbed is this lovely exposure, about 2 by 3 meters.

mills-bluschist1

It’s clearly worn by water, and a little concrete-lined ditch running along the uphill side of the roadbed feeds it. Following it upstream takes you right up to the 580 freeway abutment, where it veers north in a culvert. The stream is a mystery that I won’t try to solve today.

Back to the rock. Getting closer to it, you may not believe your camera. Clear blue skylight can do that.

mills-bluschist2

This closeup, showing tightly folded lamination in the cleft on the right edge of the first shot, is a truer indication of its color thanks to my camera’s flash.

mills-bluschist3

It’s classic blueschist, the largest outcrop of it I’ve seen in Oakland. Let’s call it blueschist-grade melange, what’s usually referred to in Franciscan circles as a high-grade block, and I’m very pleased to know that we have one in town.

Lower Piedmont Park walk (#28)

10 August 2012

Walk number 28 in Charles Fleming’s Secret Stairs East Bay winds around the fine homes and hills of Piedmont along the valleys of Wildwood and Bushy Dell creeks. Here’s the route, shown on the Google Maps topo base.

walk 28 topo

The first and last part of the loop is in the watershed of Wildwood Creek while the rest is in the Bushy Dell Creek watershed. (They run down Lakeshore and Grand Avenues respectively, separated by Warfield ridge, and combine down at Lake Merritt where their names are sunk in bronze by the pergola.) Geologically, the walk covers the uppermost part of the big Pleistocene alluvial fan and the edge of the Franciscan bedrock block that underlies most of Piedmont.

geologic map

We start at the Lakeshore-Winsor split in the stream valley and make our way toward the divide. On Portsmouth Road the high ground of the bedrock zone stands out ahead.

At the far end is a steep climb to Wildwood Avenue, where we can look back across the stream valley to the ridge topped by Calmar Avenue, on the Oakland side of the city line.

Turning the other way, we look over the valley of Bushy Dell Creek. Once a large formal garden, this part of the valley was filled and leveled for its current use as a sports complex. It appears never to have been a quarry, unlike Dracena Park to the north or Davie Tennis Stadium to the south.

We turn upstream along the creek, where the land is relatively untouched. Just above this spot is the site of what was reputed as a sulfur spring.

The geologic setting doesn’t really give much support for the presence of a proper sulfur spring like the one in Walnut Creek, but after all this time the question is moot. Certainly I didn’t notice any odor. The grotto was very pleasant anyway, and there’s real bedrock all around. It’s mapped as Franciscan sandstone of the Novato Quarry terrane.

The route goes farther up and takes a loop past a pair of boulders.

Take a close look at these: they’re genuine Oakland-style blueschist, globs of old ocean crust that have been carried tens of kilometers down into the earth along a subduction zone, then spat back out, possibly more than once. (The details are at the bleeding edge of California geology.) The one boulder displays good color and mineralogy:

The other has some nice slickensides to show us.

Coming back downstream and past the baseball diamond, we pass the entrance to the football field. The view looks down the valley toward the lake and downtown.

Near here we can see more exposures of the sandstone bedrock, but soon afterward the route returns to the alluvial fan. The two substrates make subtly different topography, but that can be hard to see given the heavily landscaped landscape.

Palm Drive offers a picturesque farewell view of the Bushy Dell Creek valley.

Again we cross the divide between the two watersheds at Wildwood Avenue. The near valley is accentuated by glimpses of the higher hills.

I never get tired of this stuff.

Here’s the route in more detail.

route map

Rocks of Upper Rockridge III: Blueschist

24 October 2010

At the uppermost top of Upper Rockridge is Contra Costa Road, where amid the fine homes and gardens is this knocker of real blueschist.

blueschist knocker

Knockers are the California geologist’s nickname for blocks of resistant rock in Franciscan mélange, the smorgasbord of rock types (chert, basalt, greenstone, serpentinite, etc.) bound by shaly matrix that is common throughout the Coast Ranges. Knockers are too big to be called boulders but too small to be mapped. Anyway, this knocker is a tough stone of a deep indigo color from the high-pressure mineral glaucophane, which if you remember your Greek simply means “blue in appearance.” I couldn’t resist taking home a chip.

rockridge blueschist

Unlike the garnet-mica blueschist of Joaquin Miller Park, this outcrop is almost monomineralic except for some white veins, probably quartz. It gleams like leather in the magnifier, with intricate crenulations and understated foliation—not a real schisty schist, but layered enough to qualify. I’m in love with it.

Serpentinite of Visionary ridge, Joaquin Miller Park

6 May 2010

The ridge behind Woodminster Theater, in Joaquin Miller Park, is mapped as serpentinite. You start to notice this around the Moses pyramid, but as you explore you’ll find a lot more of it plus various associated metamorphic rocks.

joaquin miller serpentinite

For instance, on the Sinawik Loop Trail I spotted a fine chunk of actinolite schist, detailed below.

actinolite

The ridge doesn’t have a name on the map, so I’ll give it one. It has the Moses pyramid, the Elizabeth Barrett Browning monument, the fireplace that Joaquin Miller built for his own cremation, and Lookout Point where Miller loved to contemplate Oakland and the bay. So by my decree, this is Visionary ridge.

At Lookout Point, the large boulders surrounding the viewing patio caught my eye. High-grade blocks!

blueschist

A closer look showed me the garnet-muscovite-glaucophane association that marks blueschist. There was also some green (chromian) mica. Bring your magnifier, not your hammer, if you come this way.

blueschist