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.

1684hill

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.

1684-moraga-basalt

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

1684-view-west

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.

1684-view-north

The view east overlooks lower Siesta Valley and Mount Diablo. On a clearer day I imagine the Sierra Nevada is visible along the left horizon.

1684-view-east

(Again, it would be really nice if you participate in the survey of science blog readers at bit.ly/mysciblogreaders. There are prizes, and November 20 is the last day. Thanks.

(This will be the last time I promote this study, so next week we’ll be back to normal. I’ll follow up in the Q&A/Announcements thread. Here’s the full announcement, one more time:

(Help us do science. I’ve teamed up with researcher Paige Brown Jarreau to create a survey of Oakland Geology’s readers. By participating, you’ll be helping me improve this blog and contributing to SCIENCE on blog readership. You will also get FREE science art from Paige’s Photography for participating, as well as a chance to win a $50 gift certificate, t-shirts and other perks. The survey should only take 10-15 minutes to complete.)

Read Oakland’s landscape on public transit

9 November 2015

busbart1

Last Thursday I finished a long project in which I walked every block of every street in Oakland, both sides. I did this largely using public transit — our stalwart AC Transit buses and our intrepid BART cars. Both of these systems offer good vantage points to appreciate Oakland geology.

The buses give riders nice views of the hills and the bay, if you keep your eyes open and make the most of glimpses. A day pass is five bucks, and if you have a Clipper card it’s less, and if you’re of senior age it’s even less. Usually the windows are clean, and the landmarks visible en route will keep you with a sense of where you are.

busbart2

The backbone routes, good for sightseeing, are the 1, the 51A/B and the 57/NL/58L. From them you can get up to the hills via several rib routes — the 11, 18, 39/339, 54, 45 and 46/46L. And that’s the real beauty of public transit, the fact that it can take you out to the land, not just in to town, work or school. And with various amounts of walking or cycling, you can then wander your fill among our rocks and scenic neighborhoods.

Then there’s BART. The windows aren’t clean often enough, but when they are the trains offer extended views of the hills with the rare advantage of continuous motion, which helps in sorting out the hills. I never tire of that.

The BART stations, conveniently, connect with the bus lines. But if you’re early for the bus, or early for the train, they’re also the best places to photograph the hills, or study them with binoculars. And here I want to give the Fruitvale station parking structure, where this shot was taken, an honorable mention.

busbart3

The BART stations are the best public places to capture picture-postcard photos of the Oakland hills. The Rockridge, MacArthur, West Oakland, Fruitvale, Coliseum and San Leandro stations each have their own charms.

busbart4

This winter ought to offer views that are truly more picturesque. A sky with clouds, if you get the light right, is lovelier than a big blank blue sky. And misty days can bring out the true depth of our hills, ridge upon ridge.

busbart5

Urban geology, just like geology out in the country, depends on deep knowledge of the land. That comes from extended visualization, seeing the land from all angles; and from what I might call extended visceralization, walking on the land in all directions and feeling it in your bones.

Knowland Park knockers II: Rocks other than chert

2 November 2015

The distinctive landscape of Knowland Park owes much to its large exposure of Franciscan melange, in which lumps of various rock types stick out of the ground like raisins in pudding (or whatever culinary simile you prefer). A few weeks back I featured the chert knockers, because there are so many, and this week’s subject is the ones that aren’t chert.

Before I go further, a reminder of the scientific blog survey. The deadline for participating has been extended until November 20. Here’s the info.

Help us do science. I’ve teamed up with researcher Paige Brown Jarreau to create a survey of Oakland Geology’s readers. By participating, you’ll be helping me improve this blog and contributing to SCIENCE on blog readership. You will also get FREE science art from Paige’s Photography for participating, as well as a chance to win a $50 gift certificate, t-shirts and other perks. The survey should only take 10-15 minutes to complete. You can find the survey here: http://bit.ly/mysciblogreaders.

OK. Here’s the geologic map showing the Franciscan area, labeled KJfm (for “Cretaceous/Jurassic Franciscan melange”). The places featured in this post are numbered 1 through 8 from north to south.

knowlandKJfm-knockermap

Knocker 1 isn’t really a knocker, but an exposure in the fire road, of greenish serpentinite.

Knowl-JKfm-knocker1

I include it because there are relatively few in this piece of melange. Other melange areas, like those in San Francisco or Marin County, may be mostly serpentinite, but not here.

Knocker 2 is at the edge of a cul-de-sac overlooking the gorge of Arroyo Viejo. It’s a lovely greenstone.

Knowl-JKfm-knocker2

A closeup shows the greenish rock, which is a metamorphosed lava, along with its iron-rich weathering rind and the carbonate veins that are evidence of its deep-sea origin (more here).

Knowl-JKfm-knocker3

Knocker 3 is exposed along the road just above here, a nice graywacke, or dirty sandstone.

Knowl-JKfm-knocker4

The Piedmont block, Oakland’s other body of Franciscan melange, is largely graywacke.

Knocker 4 is the big one, which caught my eye the first time I set foot in the park.

Knowl-JKfm-knocker5

Its bluish color stands up to close inspection. This is a classic high-grade block, a body of rock that was carried deep into the Earth and returned to the surface quickly enough that the high-pressure blueschist minerals it turned into were preserved.

Knowl-JKfm-knocker6

You have to look closely at these rocks to see past the lichens that tend to cover every exposed surface. Geologists carry hammers to ensure fresh exposures, but rocks in the park should not be hammered.

Knocker 5 is just up the hill. I haven’t given it a good look yet, but my initial impression is that it’s lava.

Knowl-JKfm-knocker7

Knocker 6, across a small gully from knocker 4, is populated by a clump of trees. I think there’s a reason for that because the rock fractures nicely enough for the roots to reach deep.

Knowl-JKfm-knocker8

I interpret it as metamorphosed lava, from its greenish color, extremely fine grained (aphanitic) character and massive fabric.

Knowl-JKfm-knocker9

The next two knockers are outside the park — the Franciscan doesn’t honor property lines, and the Chabot Park neighborhood once looked just like Knowland Park.

Knocker 7 is on posted land at the end of the public part of Kerrigan Drive. I think it’s serpentinite . . .

Knowl-JKfm-knocker10

. . . because that’s what’s underfoot here.

Knowl-JKfm-knocker11

Knocker 8 is exposed along lower Lochard Drive and is too large to photograph easily. Looming over the road, it looks like basalt lava.

Knowl-JKfm-knocker12

But a fresh exposure shows some cryptic internal features, plus extensive deposition of iron oxides from weathering below ground.

Knowl-JKfm-knocker13

I’ve visited this site twice and am still not sure what to call it.

There are more knockers to be found in Knowland Park and south of the park. I plan to keep up my search to the south end of the Franciscan, along Chabot Reservoir.

Once again, I hope you’ll take part in the blog survey between now and November 20. It has prizes.


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