The Oakland Conglomerate extends from Montclair, a little north of Snake Road, south along the whole southern end of Skyline Boulevard and beyond Lake Chabot to the northern outskirts of Castro Valley. It’s part of the Great Valley Complex, a huge sedimentary pile spilled off the ancient Sierra/Klamath ranges in Late Cretaceous time, specifically the Turonian and/or Cenomanian ages (about 90 to 100 million years ago). This may be its northernmost exposure, on a fire road across from the Shepherd Canyon fire station where a basketball court has been cut out of the hillside. I brought home some samples and hope to have fun with them soon.
What’s cool about this rock unit is that the big clasts are almost all cracked or shattered or dinged up. That doesn’t happen to these potato-sized chunks of quartzite and granite in riverbeds or the seashore, where the stones were originally shaped. Researchers at Cal State East Bay have argued that they were damaged by thousands of earthquakes on the Hayward fault as they lay buried some 5 kilometers down.
This rock unit is actually mostly sandstone, especially the farther south you go. It crops out all the way down to the Alum Rock area.