As you drive up Route 24 toward the tunnel, look to the right, and you’ll see a rocky ridge facing the Hiller Highlands neighborhood with a power line on it. One set of towers is right above the intersection of routes 13 and 24. This spot is at the next set of towers; you reach it via the fire road from the North Oakland Regional Sports Center on uppermost Broadway. Few people visit here except the usual partiers, but the view is great. The rock here is mapped as undifferentiated Late Cretaceous rocks of the Great Valley Complex. It seems to be mostly shale, but the visitors have helpfully gathered the lumps of harder, flinty rock in it to make this fire ring.
This broken stone shows a dark, siliceous surface that reminds me of the flint nodules that are common in many limestones. On the other hand, it’s just as likely to be a concretion. What differentiates a nodule from a concretion is that the material in a nodule is cleanly separated from and often different material from the surrounding rock, whereas a concretion is merely a strongly cemented portion of the material around it, in which sedimentary structures are uninterrupted. But if there is no structure to be uninterrupted, then I need to check these more carefully before giving them a name (a good reason to come back). Also, nodules tend to have a lumpy surface whereas concretions are smooth.