Archive for the ‘Oakland geology views’ Category

Chimes Creek headwaters

1 February 2016

I’ve mentioned how tempting the uppermost catchment of Chimes Creek looks, perched above the Leona Quarry scar:

chimesCktop

Finally got up there last week. Access is difficult and not for casual visits.

Here’s the valley in Google Earth, looking obliquely at it. At first glance it looks natural, but it’s heavily engineered.

Chimeshead-view

I’ll show photos going from top to bottom, between the two dots on the above image. The valley above the upper dot was filled in by the Ridgemont developers, using material cut from the ridge to its left. This is the view downhill from the position of the upper dot.

Chimeshead-1

The floor of the valley here is crisscrossed with concrete ditches, which converge about a third of the way down the transect at a culvert. Along the way you pass a large outcrop. All the rock here consists of the Leona “rhyolite.” The outcrops are tempting, but the slope is steep and treacherous.

Chimeshead-2

Here’s the culvert, possibly the only one in Oakland without graffiti, joined from the right side by the concrete ditch. I think it must carry runoff from the Ridgemont streets. But what’s that ugly orange?

Chimeshead-outlet

Why, it’s acid drainage from one particular part of the subsurface here. I would love to see the geotechnical reports from the time they built this development.

Chimeshead-pipes

This is one of many places where I’ve seen “yellowboy” in the Leona; the most notorious is the old sulfur mine, of course, and I noted another last week. By now I think that every excavation in this rock unit, old or new, should be treated as a potential hazard.

The next couple hundred feet downhill from here is a lovely tree-shaded, undisturbed steep cascade over large boulders. Here are just two of them.

Chimeshead-3

This beautiful rock is extensively slickensided (polished by underground movements) and coated with the iron hydroxide minerals that result from natural weathering. It is not stained orange by the pollution from upstream; in fact the water at this point is only slightly milky. It’s very much like the rocks in the Redwood Road boulder pile.

Chimeshead-4

And what to say about this one? It’s an unusual lithology within the Leona. My impression just from eyeballing it was that it’s an autobreccia — a ground-up body of lava or tuff consisting of lumps of the original rock in a matrix of pulverized (and relithified) rock. But that’s not the only possibility.

Another outcrop up on the valley wall exposes a slickenside that covers a good square meter.

Chimeshead-5

At the base of the cascade is more engineered land, a small catchment housing a screened culvert entrance. Chimes Creek is trapped here and conveyed beneath the old quarry and across the freeway to emerge in the Millsmont neighborhood. As I say, it’s engineered land, but it’s planted with trees and rather pleasant. It also catches runaway rocks before they can take out a townhome down below.

Chimeshead-6

The view up from the lip of the catchment shows the two outcrops and the shape of the land.

Chimeshead-7

I can’t wait to return for a closer look, though with so many other places to see it might be a while.

Marine terrace in downtown Oakland

28 December 2015

mar-terrace-grand

Oakland’s ice age features are subtle and easily overlooked. I’m here to help you see them.

The photo above looks west on Grand Avenue from the tip of Lake Merritt. As you pass Harrison Street heading toward Broadway, you climb a low rise about 20 feet high. That rise is a deposit of sediment that was laid down when the sea was higher than it is today. It’s shown on the geologic map below as Qmt, for Quaternary marine terrace.

mar-terrace-geomap

“Quaternary” refers to the age; the Quaternary Period started 2.6 million years ago, when a long series of ice ages began that extends to today, and encompasses the Pleistocene and Holocene epochs. “Marine” says that the feature formed in the ocean. “Terrace” means that it was built upward from the base — it’s a pile of sediment, as opposed to a wave-cut platform, another flat-topped coastal landform.

Oakland has three areas of Qmt, the largest being under the old town of Clinton and the second being under Lakeside Park. Those are easier to see than this third area, which I would call the Uptown-Valdez terrace.

As you approach the lake on 21st Street, you can see how its elevation matches the terrace across the water in the park. The terrace is a little over 20 feet in elevation here and is very flat.

mar-terrace-21stSt

The western edge of the terrace is invisible. Here’s my best effort to show it, looking up Telegraph Avenue on a clear day with little traffic. View it full size (1000 px) for the best experience.

mar-terrace-Tele

I was standing at 17th Street. The nearest cars are at 18th Street. Behind them the ground slopes down to 20th Street, where the pavement turns fresh. Beyond that, the crosswalk is at West Grand Avenue, and the new pavement ends at 27th Street. Telegraph takes the slightest turn to the right at 30th Street, under the “Qpaf” mark on the geologic map. The freeway crosses just past 35th Street.

In all this territory, the elevation changes hardly at all, except in the foreground. That downramp between 18th and 20th is the edge of the Merritt Sand, the ancient dunefield that underlies downtown proper. The creators of the geologic map presumably mapped the marine terrace in this area with the help of engineering reports, well records and the like, because the landscape gives no sign of its presence.

Mountain View Cemetery rocks: The back forty

7 December 2015

MVCem-1

Mountain View Cemetery is a never-failing source of interest. If you tire of graves, then why not collect the knockers exposed in this splendid preserve of Franciscan melange. I haven’t featured them here in several years, but recently the weather up there was especially photogenic. The one above, near the north edge west of the Cogswell monument, is my favorite, but they’re hard to choose among. Let’s say that knocker 1, my “secret chert,” is my favorite in the civilized part of the cemetery.

As you climb the hills, it’s natural to look around, away from where you are. Lately the cemetery managers have eliminated the overbearing fringe of eucalyptus along the rear, and the views north are enticing.

MVCem-2

If you look carefully, maybe you can spot Cactus Rock, a leading candidate for the mysterious Rockridge Rock. It’s at the bottom of this shot, in the middle.

Cactus-rock

But the view south hasn’t changed. This is the cemetery’s back forty, looking nearly unchanged after 150 years.

MVCem-3

Here’s a look at it head on, over the uppermost of the three ponds that occupy the headwaters of Glen Echo Creek. It’s not fenced off, so it’s open to exploration.

MVCem-4

Up the course of the creek is another small basin, above which the creek briefly emerges from a culvert. This is its current birthplace.

MVCem-5

The hillside gravel here — what geologists call the float — samples several different rock types that occur in the melange. Red chert, graywacke and some sort of serpentinized thingy is visible just in this small footprint. There’s also green chert, greenstone and basalt around.

MVCem-6

Here’s the graywacke — a dirty sandstone — close up.

MVCem-7

And this is my favorite knocker (one of several) in the UNcivilized part of the cemetery.

MVCem-8

It’s made of the high-grade green chert, whereas the first knocker I showed is the classic red ribbon chert. You could brave the traffic and see a huge expanse of it in the Marin Headlands, which is a nice field trip. Or you could stroll here and have it all to yourself, as long as I’m not hanging around.


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