Ski, you've done it to me again. I'm supposed to be working, but I'm 'net surfing for for information about yet another piece of spectacular California geology instead. And adding to an already too long list of places to see if I can ever get out that way for a field trip.
Your mention of, and my general awareness of, the extensive (!) granites of the Sierra had me assuming the jointed outcrop at Brown Mountain was a jointed granite, and I did not bother to see if there are any basalts or diabase occurences. Of course there are (and I wonder if the place name Brown Mountain has significance here?). We find columnar jointing rather more often in basalts than in other igneous intrusive or extrusive rocks. The Palisades of New Jersey and New York are jointed basalt flows/shallow sills of Triassic age, and we have some Precambrian and Cambrian basalts in the core of the Blue Ridge Mountains in which joint structures and some "pillow" structures have survived regional metamorphism and multiple episodes of folding (Catoctin greenstones and basalts and diabases of the Grandfather Mountain Formation, among other horizons of Late Precambrian/Cambrian age). Your picture of the smaller outcrop with the arcuate columns is even more amazing than the somewhat ordinary jointed flow or sill at Brown Mountain. I really enjoy the curved columns.
Perhaps best of all is the story behind the discovery of, and the "hiding" of, the Machado Postpile. What a treat it would have been to be a student working on a thesis or a dissertation in that area and engaging the late Jesse Machado.
That it remained "unconfirmed" until 1990 is testimony to the difficult terrain as well as a "dating" of the general onset of availability of high resolution satellite imagery. All we had in the ancient times of the 1970s was the first two Landsat satellites (well, that's all the NSA would let us see), and that imagery was far, far beyond the price range for a poor student or an unfunded researcher. For good or ill (mostly for good, I'd estimate), Google Earth imagery has enabled the discovery, or confirmation, of so many structures, features, and occurrences that it boggles the mind.
Thanks SO much for including the link to the story behind the outcrops.
Foy
