(Starting now, I will be putting up images 600 pixels wide instead of 450. Hope you like that as much as I do.)
A big chunk of the high hills consists of the “Leona rhyolite,” shown as the pink unit labeled Jsv on the geologic map below.
To help you orient yourself, here’s the same area in Google Maps. The area I’m featuring is around the asterisk near the bottom of the geologic map.
“Jsv” stands for Jurassic silicic volcanics (or keratophyre), the kind of sticky, explosive lava and ash that island arcs are made of. Similar examples today would probably be the Greek islands. The original rocks have been rather thoroughly altered since their birth around 165 million years ago, but they still stand out among Oakland’s rocks. I described them previously here in the former Leona Quarry and here in Leona Canyon. The experts are still arguing over these rocks, and for now I will spare you the details. But basically, they aren’t really rhyolite so the name isn’t used officially any more.
This is the view north across I-580 to the northern side of the quarry scar. The high valley is the headwaters of Chimes Creek, and I continue to be fascinated by the idea of standing up there in that perched catchment.
A property on Sunnymere Avenue has a nice boulder of this stone.
And a yard on Columbian Drive uses the stone for landscaping. You may already notice how consistent the color of this stuff is. I think that may be the best way to identify it around town. Some of the Tertiary sandstone has a similar honey color, but it weathers into evenly colored, rounded forms whereas the Leona turns craggy and mottled.
And here’s a hand specimen found at the end of Field Street.
This appears to be Oakland’s oldest rock.