Geopuzzle #12

This is not so much a puzzle as a mystery, in that I’m not entirely sure myself what this is:


I do have a guess, of course, but I’ll be interested to see what all you clever folk think. Here’s a wider shot of the outcrop this feature is found in; hopefully you can see that it’s not alone.


Have at it in the comments.

Categories: geology, geopuzzling

Comments (17)

  1. Tegumai Bopsulai, FCD says:

    Obviously, it’s a coin. I can’t make out the nationality or denomination though. It’s got smooth edges, so it appears not to be a U.S. quarter.

  2. Tegumai Bopsulai, FCD says:

    I found some helpful pictures on Teh Interwebs. I’m guessing it’s a South African 5 Rand piece.

  3. Bob O'H says:

    It’s a geologist’s coffee stain. Obviously a pair of them, taking a break from knocking chunks of Africa off the continent with their glorified toffee hammers.

  4. Matt Platte says:

    Right beside the coin, there used to be a swimming pool ladder. It was taken away by a metal scrapper for recycling. The coin is the profit from the transaction, representing simultaneously the meagre wages for society’s recyclers, the appalling disparity between the recycled value of the stolen ladder versus replacement cost and the frustration of the pool owner who, having lost the ladder, decided to drain the pool and move to Las Vegas.

  5. Mel says:

    Iron Oxide concretions. But I like the coin joke 😉

  6. Lynn says:

    Geniuses, all.

  7. Andrew says:

    I just posted a photo of similar-looking rocks, finer grained with thinner beds. They were intermittently exposed sediments with mudcracks and iron concretions formed in them. Yours look like a coarse-grained version.

  8. MrvnMouse says:

    That seems to be a digital picture of a coin on a rock ledge.

  9. Perhaps it used to be an armoured mudball?

  10. BrianR says:

    I agree w/ Mel and Andrew … some sort of iron-rich concretion/diagenetic feature. The ring-like pattern with the lighter-colored middle part reminds me of cherty concretions that form in some fine-grained carbonates.

  11. Lynn says:

    BrianR, we have the Boone formation here in north Arkansas with similar-looking cherty concretions, but I have to agree with those who say it is a coin.

  12. Silver Fox says:

    I like the iron-concretion idea, though it looks like some of the unknowns are in layers or lenses. The layers and lenses appear to be disturbed or swirled as in soft-sed deformation, perhaps?
    If it’s PreCambrian, could you have an incipient banded iron-formation of some kind?

  13. Old Bogus says:

    The coin humor was funny, anyway. I found similar things in rocks at Mesa Verde. Some of them looked like pipes embedded in the rocks from a ancient civilization. But I assumed they were some kind of fossil of a stemmed sea creature. I did notice they seemed more resistant to weathering than the rock.

  14. Divalent says:

    What you are showing are (to me) less remarkable than what I observed in a limestone formation in NW Georgia. (but then, maybe it’s because I’m not a geologist; for all I know you pros will look at my images and say “piece ‘o cake”) They appear to be similar formations, but yours look less distinct.
    Here’s a sample:
    more pics are a mini-web page:
    the image linked above is #6 on the web page, and photo #4 shows the whole formation (the portion that is in photo #6 is about a metter to the upper left of the persons head)

  15. christie says:

    Definitely concretions! I saw a paper not too long ago… looking for it now… on concretions forming parallel to pore water flow paths along and across bedding. Resultant concretions were long and lumpy (like bean pods along bedding) and egg- shaped within beds. Will look for reference & post here if I find it.

  16. Frank Shearar says:

    In case anyone’s interested, the coin is in fact a 2 Rand piece, identifiable by the kudu head. Its diameter is just about a millimetre more than a 1 Pound coin and about half as thick, if you’re from the UK. (I can’t find my ruler.)

  17. djlactin says:

    it’s a coin that had too much to drink, with the expected messy consequences.