Archive for the ‘Oakland geology views’ Category

Marine terrace in downtown Oakland

28 December 2015

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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.

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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.

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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

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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.

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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.

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

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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.

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

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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.

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

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

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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.

Basalt at the foot of Frowning Ridge

16 November 2015

I took this photo last Tuesday, the day after our nice good rain. If you weren’t outdoors last week, you missed a brief moment in the Oakland year that lasts just a few days.

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It’s the period between the first significant rain and the sprouting of the grasses.

(Before I continue, this is the last week of the scientific blog survey, to which you’re invited to respond at bit.ly/mysciblogreaders. There are prizes, plus the good feeling of helping research. More than 100 of you have already taken part.)

The first rain drenches the ground and changes the dry, gold-brown hillsides to a rare saturated dun color. Soon afterward the hills flush green, and we’re off to a new year in the Mediterranean climate cycle that governs the Bay area. Think of it like the week between Christmas and New Years, only it’s in the calendar that plants use.

This is a special hill at the southern end of Frowning Ridge, the highland that includes Grizzly Peak and its lower, gentler neighbor Chaparral Peak. Old topo maps mark it with its elevation of 1684 feet. To the right of this photo, shot from Skyline Boulevard near Radio Tower Hill, the ground plunges to the water gap and roadcut of Route 24. On the other side of 24 the ridge resumes, under the name Gudde Ridge, and rises to the peak of Round Top. The following shots from 1684 Hill are from a visit in July 2013, during the gold season of the plant calendar.

1684-view-south

Frowning Ridge is held up by the thick lava flows of the Moraga Formation. Like most of the rocks in the Oakland/Berkeley Hills, the Moraga Formation is tilted up to nearly vertical. You can reach 1684 Hill by an informal path off the Skyline Trail. The lower western slope of the ridge is underlain by Orinda Formation conglomerate, but basalt makes up its bulk.

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Let’s look back west toward Radio Tower Hill. Last week’s photo was taken from the little saddle at the left edge.

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The view north takes in the upper part of Siesta Valley. That’s Vollmer Peak in the middle, highest point in the Berkeley Hills. Grizzly Peak is just out of sight at the left, but the tip of its radio tower shows.

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The view east overlooks lower Siesta Valley and Mount Diablo.

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On a clearer day I imagine the Sierra Nevada is visible along the left horizon.


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