I paid a visit to Alum Rock Park today, in San Jose just past the far end of the Hayward fault where it joins the Calaveras fault. It’s a lovely place. One thing I found especially pleasing was the historic warm mineral springs, which happen to be confined to the Claremont Chert.
Down here the Claremont is dark, perhaps because of a high carbon content. I don’t think I’ve mentioned it here before, but the Claremont is actually a small part of the Monterey Formation, a very widespread unit of siliceous sedimentary rock that accounts for a large share of California’s petroleum. The carbonaceous material, in combination with its high permeability and the presence of hot fluids in this highly faulted area, probably accounts for the concentration of stinky sulfur springs here. It’s a fascinating place that I’ll be describing in more detail Thursday on the KQED Quest Science blog. But I knew you’d like to know that I raised a little mental fist as I saw one of Oakland’s distinctive rocks here.