Here are some of the samples I collected from the Oakland Conglomerate last month.
This rock is utterly and thoroughly brown, deeply brown with iron oxides. You can’t scrub it off—it’s brown all the way down. That reflects two facts: the matrix is iron-rich volcanic material and it’s been pervasively shattered and flushed with fluids under oxidizing conditions to let the iron out. The clasts—the bigger stones in the conglomerate—are beautifully rounded, presumably in a vigorous river or beach. But the setting of Oakland at this time, in the Cretaceous, was way offshore in a shallow sea. It appears that all this coarse material was carried offshore in underwater landslides and laid down in a deep series of sandstones and conglomerate beds.
Long after the conglomerate was laid down and lithified, the plate boundaries changed and the San Andreas fault system (including the Hayward fault) splintered coastal California and wrenched it northward, one earthquake at a time, for tens of millions of years. The forces of that time have crunched nearly every stone in the Oakland Conglomerate. Yet at the time these rocks were still deeply buried, and chemical action and pressure cemented the stones back together. The clasts are delicate, but intact. It’s impressive to imagine the force that shattered these hard stones like so many soda crackers. The next thing I want to do is open some of them with my hammer and see what the original lithologies are.