14 February 2011
A year and a half ago, on my way back to Modesto from Yosemite, I pulled over at the side of the road and found a piece of urbanite full of whoppers. But that curiosity wasn’t what caught my eye and made me pull over in the first place. Instead, what I noticed was this:
It appears to be some kind of volcanic breccia, based on the relatively angular clasts and the presence of vesicles (amygdules). I reckon it could be a survivor of the extrusions of the Sierra Nevada magmatic arc when it was actively erupting in the late Mesozoic. The frozen roots of those volcanoes are today exposed as the granitic plutons of the Sierra Nevada batholith.
But at some point, those volcanoes were burping lava out onto the surface, and potentially triggering lahars and ashflows and those extrusive rocks may be preserved in a few low-lying places were erosion hasn’t yet dispatched them. Is this one?