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439 Views 1 Reply Last post: Aug 29, 2014 10:55 PM by MikeHardman RSS
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Aug 29, 2014 6:42 PM

What kind of rock is this?

Hey I live in Colorado.

Someone sent me these photos and this is on the eastern plains. I asked if I can see if in person to figure out other things about it. But maybe it is a dead give away on what it is based on what it looks like.


I haven't seen a small nubbin of rock with this kind of pattern.

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    Aug 29, 2014 10:55 PM (in response to Cassomophone)
    Re: What kind of rock is this?

    Welcome to NaturePlus, Colorado.


    Well, it is almost certainly banded ironstone, in a general descriptive sense.


    But it is curious in that it shows a mix of features of two main types:


    - Sedimentary banded ironstone - flattish bands, perhaps with folding.


    and do  a Google image search for 'banded ironstone'.


    - Concretionary banded ironstone - bands forming closed surfaces, often concentric.

    Sometimes the shapes of the surfaces can be seen to parallel the sides of blocks of rock defined by fractures, eg.

    Such banding, representing variations in mineralogy and hence potentially hardness, may come to stand out as a result of weathering. ...Or not, eg.


    Because the surfaces in yours are at least partly closed, I can rule out the former.


    A third possibility is that the iron-rich layers represent a fossil, the original mineral having been replaced with an iron-rich one. But what sort of fossil? None comes to mind that is a good fit. And I can't see any organic details.


    I don't get much insight from the location, but thanks anyway.


    So I think it is probably a concretionary banded ironstone, but there are some aspects that leave me unconvinced. That's the best I can do from the photos.

    Somebody else here might have an idea. So stay tuned a while.



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