The other week I noticed a big outcrop of basalt of Moraga Formation right next to Grizzly Peak and remarked that it would be worth a visit. And so I did, while preparing my post for KQED Quest Science Blogs last week on the Moraga Formation. It’s easy to reach from a couple of different pullouts on Grizzly Peak Boulevard. Here’s a view looking south.
The rock is thoroughly covered with lichens, so you can’t tell much about it, but it appears to be a massive flow of lava that, like all the rocks in the Berkeley/Oakland Hills, has been tilted nearly upright with the top toward the northeast. Stratigraphically above it, beneath the road, it’s mapped as sedimentary rocks associated with the Moraga Formation. The area was a lake or wetland at the time, about 9 million years ago, with high hills on the west where the Bay sits today. And on the other side of the road, including Grizzly Peak itself, the bedrock is the Bald Peak Basalt, a slightly younger volcanic unit.
Here’s a better view of the west face.
There’s quite a large rock face exposed on that side of the lava bed, but almost nothing exposed on the uphill side. The rock has a lot of paint on it here and there, including some marks that look like rock-climbing instructions. The rock is pretty sound for climbing, but be careful anyway because its strong exposure to sun and weather can weaken this basaltic rock without showing. (The rhyolitic rocks in Berkeley proper are better that way.)
It’s a well-used outcrop, but still, please don’t take a hammer to it as it’s located on UC Berkeley land. It’s the headwaters of Strawberry Creek.
Here’s the view back to the place where I first spotted this feature: in the row of trees just to the left of the rock.