The south shore of Lake Chabot—the whole eastern side actually—is a good place to study the Joaquin Miller Formation. Outcrops of this shaly unit are hard to come by, so the shoreline and the roadcuts along the trail are useful. Even so, you realize that Earth isn’t always tidy when you come upon this gulch full of siltstone boulders.
Massive (that is, unbedded) siltstone isn’t part of the Joaquin Miller Formation’s description, which is “thinly bedded shale with minor sandstone,” grading into “thinly bedded, fine-grained sandstone near the top of the formation.” Oakland’s collection of Great Valley complex rocks, which includes the Joaquin Miller Formation, is crisply marked on the geologic map. But unlike your standard street or topo map, a geologic map is an exercise in vision and interpretation and approximation. The lines might be moved by the next geologist, with complete respect paid to the previous mappers. The photo is near the “70” mark on the map below.
Might this belt of stones actually be in the overlying Oakland Conglomerate? I don’t think so, because there are no coarse grains in them. They’re just an unusual element in the Joaquin Miller, to be appreciated but ignored in the larger context. We can’t forget about the hundreds of meters of soft, obscure rocks that surround them.
This valley has its head in a recently populated part of northernmost Castro Valley, which means that it poses a risk of allowing urban runoff into the reservoir. Clearly it carries a lot of water at certain times, even if those occasions are rare. I hope to revisit and get to know it better, if only because living bedrock appears to crop out up there.