mystery question I asked the other day suggested that the oldest rocks in California lie within a few miles of the Santa Clarita Valley. I kind of knew I would be opening a can of worms by pointing this out, and I will try to explain why this idea might be ambiguous at best, and highly controversial at worst. In essence, yes, some of the oldest rocks found in the state are found there, but defining how 'old' a rock is can be a real conceptual problem at times.
A side note: Carbon dating is another form of radioactive age dating that is recognized by many people, if not well understood. It is rarely used in geology because it only useful in materials that are less than 100,000 years old. Since it is carbon-based materials like bone or wood that can be dated, the method is favored by archaeologists who are working with artifacts.
So there you go...it turns out that the "oldest rocks" in California can be found over a rather wide area. There are rocks that date to 1.7 or 1.8 billion years ago in Death Valley National Park, in the eastern Mojave Desert, in Joshua Tree National Park, and the San Gabriel Mountains, which form the eastern edge of the Santa Clarita Valley. Most of the rocks were part of a vast mountain system that developed in a series of terrane collisions as the supercontinent that we call Rodinia came together (here is an excellent overview if you want the gory details of the story). As I walked through Placerita Canyon, I was very close to exposures of the Mendenhall gneiss, which is dated, like the others, at 1.8 billion years.
One really big change, though: in the 1970s, when I was a budding geologist, the rocks of the San Gabriel Mountainswere mapped as "metamorphic-undivided" and were poorly known. Today there is a richness of data that allows us to see the role of the rocks in reconstructing the American West of nearly 2 billion years ago.
The Other California is my continuing series highlighting the geology of the fascinating places in my fair state that don't often show up on the postcards that tourists buy. The state is a big place and I have yet to see it all, so if you have a favorite corner of the state and would like to put together a blog entry, I would gladly add it to the series.