It’s been a while, but lately I’ve returned to walking in the Broadway Terrace/upper Rockridge neighborhood, where this Franciscan knocker sits. This is a disrupted deep-sea chert that has been heavily altered, bleaching out its color.
(See the first set from upper Rockridge here.)
It’s across Ocean View Drive from the chert cluster I showed almost three years ago, at the top of the stairway called Brookside Lane. (I like that this path and West Lane have “grownup” names instead of “X Steps” or “Y Path” like all the other footpaths up here.)
I may be wrong in calling this a knocker; it is more likely to be a boulder that was moved here. But it’s certainly local. There’s a variety of highly altered rock in parts of the Franciscan called calc-silicate, and it might look like this; I haven’t gotten a handle on it yet. That would have started out as a dirty limestone, though, and this looked like a chert. I didn’t have a magnifier with me, and of course I can’t hammer it. Darn, I’ll just have to come back.
Oakland Geology 
3 November 2010 at 10:20 pm
Oh, thanks so much for this! I will tell the guy who lives there. He was very sad about taking out the gigantic eucalyptus, but at the same time was afraid it would kill someone or crush his house, as it was outgrowing his front yard. It was just huge. Okay! Basalt. All that rock in the walls along there is the same fairly dark color.
25 October 2010 at 5:06 pm
As it happens, I was able to visit the spot on Harrison this afternoon. (Boy, isn’t taking out an old eucalyptus a nasty job!) The stones look like basalt from the old quarry up at Sibley.
23 October 2010 at 6:50 pm
Yes, and these are much more modest places but also craftsman bungalows and a couple of four-unit places from about the same period. My house is 1914 (with its old tired concrete retaining wall) and these must be from around the same time. So I guess the builders or developers just liked that rock, and it was available.
23 October 2010 at 11:55 am
That area is mapped as alluvium, so there should not be any bedrock, just firm gravel and clay. But also, wall-builders don’t use stone from the site (those New England farmers and their stone fences notwithstanding); they bring it in from a nearby quarry in truckloads. There are several Oakland quarries the material could have come from.
Some of the hill homes built around 1900 used local stones, but that was labor-intensive, intimate landscaping. I’m thinking of the spectacular Thousand Oaks homes in Berkeley and their Oakland siblings—would you call it the Craftsman aesthetic?
23 October 2010 at 12:20 am
So a number of the houses up Harrison Street from Whole Foods, same side of the street, have rock walls. One has been collapsing around a huge eucalyptus that was just removed, so you can see the individual rocks. If you have nothing better to do, when you come up Harrison some time, I’d be curious to know about those rock walls. A little further up, everything is old concrete walls. But these six or so houses all have this rock retaining wall at the sidewalk. Was all this rock moved there, or is it local?