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524 Views 7 Replies Last post: May 18, 2014 8:10 AM by MikeHardman RSS
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May 17, 2014 9:32 PM

Ribs?

Hi

 

We were in westbury on Severn today and found this. Would love to know what it is. Seems to have a lot of pyrite in it also. Many thanks in advance if you can help!

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    May 17, 2014 9:16 PM (in response to Sailorjerry)
    Re: Ribs?

    Was it loose (in river bed/scree/soild/etc.) or in an outcrop?

    If an outcrop, can you describe it - the layer it came from and adjacent strata.

     

    Please post photos showing the end-on views and the other side.

     

    Mike

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      May 17, 2014 10:45 PM (in response to Sailorjerry)
      Re: Ribs?

      Hi

       

      45 years back I haunted that area of the Severn I regret I have no idea what you have, can you post a simple picture showing the subject in its totality and why you think its a rib I suspect is a caste of a mud bank/ shore line with water lines ie ripples

       

      Steve

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          May 18, 2014 8:10 AM (in response to Sailorjerry)
          Re: Ribs?

          Steve,

          Many thanks for your local knowledge.

          That's a good account of the Rhaetic.

           

          Sailorjerry,

          I am sure these are not fossil in themselves.

          They are eroded ripple marks.

          The end-section shows the various laminae, of different particle sizes, and the ripple structure, including one part where the ripple top(s) have been planed off and flat laminae deposited ontop. Your specimen shows alternating bands of silt and mud because the silty ripples protrude and the troughs retain the mud that overlaid them.

          These would have been little ripples much as you would see on a beach today. As with such modern ripples, ancient ones can show a variety of forms: parallel, sometimes with bifurcations, often with more complex modifications. Your ones are mainly parallel though I note a slight departure from that in one of your photos (much more likely with rirples than ribs!).

          As well as that basic sedimentary structure, on a smaller scale one sees slight desiccation cracking (fine T-shaped marks and dashes), and bioturbation (tiny burrows showing in cross-section as dots and in longer section as elongate sreaks).

           

          Mike

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