As I review my discoveries at Knowland Park since that day in July when I first set foot there, it makes me want to linger over its rocks. The large exposure of the Franciscan Complex is probably the park’s most significant geologic feature.
Oakland has two exposures of Franciscan rocks. The big one underlies Piedmont and the adjoining neighborhoods of Oakland, which means it’s covered with houses, roads and landscaping. The small one underlies Knowland Park and the uplands of the Chabot Park and Chabot Park Highlands neighborhoods. The portion in Knowland Park is the largest open exposure of this rock unit in the city and perhaps the whole East Bay.
It’s Franciscan melange, but distinctly different from the Piedmont block. Melange is a mixture of rock types, largely mudstone of various types, with big chunks of harder stuff floating in it. In the landscape, the chunks emerge as the surroundings erode away, and generations of California geologists have called them knockers.
Knowland Park’s knockers include several different rock types, and the largest number of them are high-grade chert, or metachert. Unlike the red chert that’s typical of the Piedmont block (and San Francisco and the Marin Headlands’ great exposures), Knowland’s chert is green and hard and recrystallized. It has undergone deep burial, perhaps more than once, yet it appears to retain its original layering.
Here’s one of a number of very large boulders exposed down by Arroyo Viejo in the woods. (All images are large and should be clicked for the best detail.)
Most of the chert knockers aren’t this well organized. Here’s one that I showed to the group on the October 4 geology walk.
And here’s one I didn’t. The chert fabric is pretty much gone. Still other knockers are probably totaly chewed up, and without hammering them to get a fresh exposure, which is forbidden in the park (and against my practice anyway) it may be hard to identify.
This is the park’s most prominent chert knocker. Both the October 4 walkers and the September 20 mappers enjoyed the stone and the views.
Close up, the stone shows tantalizing details.
But elsewhere in the park I found this beautiful fragment that displays the clean color of the stone and the etched surface of its layers. I left it behind for you to find.
Perhaps some amateur with a hammer made this fragment. If people feel free to take pretty things away from the park, after a while there won’t be any more pretty things in it. On the other hand, maybe it was the graders who created this fragment as they maintained the roads in the park. Those are lucky breaks for the rest of us.