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1013 Views 28 Replies Last post: Jan 27, 2014 9:56 PM by Tabfish RSS
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Jan 19, 2014 6:34 PM

Any help to identify please. Fossilised egg??

Myself and my two boys look for fossils at mappleton regularly.

We find crynoids, belemnites and ammonites but this has me puzzled.

It looked like an egg  encased within the rock. I managed to retrieve it in one piece.

Any ideas? The rock was perfectly formed around it.

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    Jan 19, 2014 6:42 PM (in response to Nobby145)
    Re: Any help to identify please. Fossilised egg??

    It looks like a cannonball nodule in flint which could contain a fossil.

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        Jan 19, 2014 9:31 PM (in response to Nobby145)
        Re: Any help to identify please. Fossilised egg??

        Hi Nobby145

        Dans right it is a nodule and could contain a fossil but not a cannon ball nodule, cannon ball nodules (Jurassic) come from bed 33 cannon ball doggers near Whitby but are found on the Holderness because of the last ice age and rocks and fossils being transported inside glaziers and deposited when they melted.

        I have found similar nodules that contained coprolites and looking at the matrix could have come from Speeton (Cretaceous) but it's hard to tell.

        Found Speeton ammonites in similar martix.

         

        Tabfish

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        Jan 19, 2014 9:48 PM (in response to Nobby145)
        Re: Any help to identify please. Fossilised egg??

        Near enough, Dan.

         

         

        Nobby124,

         

        It is a concretion. That word covers a range of different types of object, but they are all the result of local chemical changes (sometimes with physical changes) in the rock.

        Think of rain drops as an analogy. Given supersaturated air, water will try to condense out of it, and it will start to do so using particles of dust/pollen/etc as a nucleus. Once a rain drop has formed, the air around it will be less saturated; the system will be in better equilibrium, so the raindrop will persist.

        Back to the rock: Since the sediment was deposited and turned to rock (or at least since the sequence of events leading up to rock had begun), the physical and chemical environment changed, for instance due to burial by other sediment and/or by heating. That state of disequilibrium caused chemical and physical processes to rearrange the material comprising the rock (minerals/molecules/elements being altered and moved). Such changes often use fossils as a nucleus, growing outwards and enveloping the nucleus in a volume of rock of somewhat different chemistry. That, in outline, is a concretion. A concretion may grade gradually into the surrounding rock, or it may, as in your example, be relatively distinct (and hence physically separable). They can also show concentric layering, which you can see to a small extent in yours: there is a bluish-grey halo around the discrete concretion.

         

        Although the words concretion and nodule are often used somewhat interchangeably,

        "There is an important distinction to draw between concretions and nodules. Concretions are formed from mineral precipitation around some kind of nucleus while a nodule is a replacement body."

        (from the link below)

         

        Canonball concretions are a common name for these things in some places. There are many other common names. Bowling Ball Beach in California gets its name from the conretions on the shore (see the first photo here - http://en.wikipedia.org/wiki/Concretion).

        Your example looks a bit like what are called Moqui marbles in Utah (see same link)

         

        Some types of concretion have mineral veins dividing them into numerous chunks. These are called septarian concretions (commonly also septarian nodules, which is less correct). There is a hint of that in your specimen - note the short pale mineral veins (probably calcite or quartz).

        Septarian concretions are also discussed in that link.

         

        I can't see a fossil at the core of your concretion. It may still be within it, or it could be too small to see unaided, or something else might have acted as the nucleus (eg. an particular mineral grain), or there may simply not have been an obvious nucleus.

         

        You'll find concretions in other discussions on NaturePlus, eg:

        - http://www.nhm.ac.uk/natureplus/message/27257#27257

         

        Mike

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