Friday, January 27, 2012

The Other California: A Friday Mystery for Armchair Geologists

Here's a bit of a mystery for a friday night...what do you make of this outcrop? I missed it's significance when I was there looking at it. What do you think it might be? We are in southern California, somewhere in the San Gabriel Mountains.
Answers in the next post...
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7 comments:

  1. SciGuy315January 27, 2012 at 11:35 PM

    It sure looks like debris flow deposits. Maybe from intense orographic precipitation?? How tall is the outcrop? 10-15' x 30'?

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  2. Ron SchottJanuary 27, 2012 at 11:48 PM

    This is just a stab in the dark, but when you mentioned the San Gabriel Mts as its general locality, the thought crossed my mind that this might be the anorthosite, however a quick web search suggests that this is not the case.

    There appear to be drill holes on the surface of the outcrop, and their spacing and weathering condition suggests an older mining or quarrying operation at this site.

    That's about all I've got.

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  3. Brian RomansJanuary 28, 2012 at 6:22 AM

    I agree w/ SciGuy315 ... kinda looks like a chunky debris flow deposit. But, perhaps an 'old' one that is partially lithified. But the current shape of the outcrop is strange, seems like it's been molded into that shape by humans, which Ron alluded to.

    good one for a puzzle for sure

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  4. Mike JurasiusJanuary 28, 2012 at 6:35 AM

    Rhyolite tuff? It's too pixelated to verify petrography and igneous origin.

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  5. Mike JurasiusJanuary 28, 2012 at 6:45 AM

    I was able to zoom in a bit more on my phone. What I can assess from lying supine on my sofa about 225 miles north of this out crop, is that it's masssive and granular with white "sand" at the base. I'm sticking with igneous, though intrusive rather than extrusive...let's say granodiorite.

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  6. AnonymousJanuary 28, 2012 at 10:16 AM

    The most significant thing to my eye are the vertical tubes. Overall, the lithology looks to be a debris-flow deposit of some sort. I'm going to guess that the vertical tubes are moulds of a thicket of small trees that was inundated by the flow, then subsequently weathered out. If this is so, the debris flow would have to be very low-energy, otherwise such small trees would have been bent over or flattened. Perhaps the deposit was emplaced gradually, a few inches at a time, over weeks or months, during a very rainy period.

    So, how wrong am I?

    --Howard

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  7. Andrew Alden, Oakland Geology blogJanuary 28, 2012 at 5:33 PM

    I'm betting it's artificial, probably an old flood-control dam or stock pond built by a rancher

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