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436 Views 2 Replies Last post: Aug 31, 2014 12:33 PM by Sapipsmu RSS
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Aug 30, 2014 11:38 PM

What is this? Reighton Sands Find.

Hi there, I visited Reighton Sands just a few days ago, and found a strange rock type. I'm not sure if it's a fossil or if it's simply nothing.  I've posted 2 images showing both sides next to a 20p.

 

Thanks in advance,

Sapipsmu.

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    Aug 31, 2014 8:12 AM (in response to Sapipsmu)
    Re: What is this? Reighton Sands Find.

    Sapipsmu,

     

    There are two main explanations for these things:

     

    - Mud cracks - where damp sediment shinks and cracks as it dries out. That happens just after the sediment has been deposited. To get preserved as rock, another layer must get deposited gently on top. Then, probably after deposition of many more layers perhaps totaling hundreds of metres thick, and the passage of time, it becomes compacted and/or cemented to form rock.

    The cracks in mud crack structures tend to be filled with sediment from the overlying bed (and unless that is of a different colour, the cracks would probably go unnoticed).

    See http://en.wikipedia.org/wiki/Mudcrack

     

    - Concretion - where sediment may be distended as localized chemical changes cause a bit of expansion to internal material, stretching the outer parts, which crack as a consequece. This tends to happen in the time between a sediment being deposited and it becoming rock (such cementation and/or compaction may take from years to millions of years).

    The cracks in such concretions tend to be filled with mineral; ie. they are veins.

    (There are many and various types of concretion, with a corresponding range of explanations.)

    See http://en.wikipedia.org/wiki/Concretion

     

    In this case, I think you have a pice of mudstone or siltstone (slightly coarser grained) showing mud cracks (we call them mud cracks even if shown in slightly silty material).

     

    Bear in mind that the top and/or bottom of sedimentary layers can be irregular in various ways.

    One side of your specimen appears to be bulged-up a bit. I suspect that is just such irregularity; but there might be a concretionary process as well.

    If you can identify the material filling the cracks (mineral or sediment, as above), you may be able to firm-up the ID. I can't tell from the photos; need a magnifying glass or microscope.

     

    I have not checked with you that this is solid rock.

    It does not matter much: the ID is the same whether it is rock or recent sediment.

     

     

    Mike

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