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1223 Views 4 Replies Last post: Apr 18, 2014 9:40 AM by Heart+Dart RSS
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Apr 17, 2014 10:37 AM

Osmington Mills

Found at Osmington Mills, is it a fossil?


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    Apr 17, 2014 11:16 AM (in response to Heart+Dart)
    Re: Osmington Mills

    Please could you post photos of the other sides.



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        Apr 18, 2014 8:46 AM (in response to Heart+Dart)
        Re: Osmington Mills

        It is a bit enigmatic...

        Let me take you through my thought process...


        I am sure it is not biogenic (animal life not involved in its formation).


        There are aspects which could be interpreted as soft-sediment deformation (the 'creases' in the bottoms of the 'valleys' and a hint of 'load structure' shapes). But that is not quite right.


        But the best ID I can come up with at the moment is calcite-cemented gravel.

        That can occur in three main ways:

        1. A gravel sediment gets cemented by calcium carbonate in a similar way to many rocks.

        2. In cave systems in limestone rocks, or other rocks with a significant lime content, dissolved calcium salts crystallize to form stalactites and stalagmites. The stalagmites build up from the floor of the cave, which means that, to start with, any rubble/gravel/sand on the cave floor gets cemented together with the same material. Between stalagmites, the lime-rich water flows away, also cementing any rubble/gravel on its way. One should bear in mind that the rubble/gravel on the cave floor may include fallen stalactites and toppled stalagmites, which can have distinctively banded interiors. A similar process can occur on a much smaller scale - in gaping fractures or faults.

        3. At or near the surface, gravel can become cemented by calcium carbonate dissolved from overlying rock by recent rains. This is called calcrete or caliche. It typically occurs in arid or semi-arid environments.


        Considering those with respect to your specimen:

        1. Your specimen shows what is probably a free surface (originally open to the air), and the lumps (pieces of gravel) are not close-enough to each other to have been self-supporting. So we are not seeing part of a solid rock, so (1) does not apply.

        2. I am not aware of significant modern cave systems in the area of Osmington Mills. Open fractures/faults are possible. Calcium carbonate is known to have been mobile in the area (it is a very common cement in many sedimentary rocks, but see also the text to image 3.131 here). However, the presence of 'creases' in the bottom of the 'valleys' is not right, or at least it is atypical. Usually, such 'valleys' tend to get smoothed-over (a bit like shown here)

        3. There are some similarities with calcretes, as shown here (from this page) and here (top-right photo), for instance. But the palaeoenvironment at Osmington Mills was not right for calcrete formation: it was marine.


        On balance, I favour (2).

        The 'drip pool' photo here (look at the left of the photo) shows how cave gravels can be cemented while the individual gravel grains remain relatively distinct (not too 'flowed-together'). Still that does not quite explain the 'creases' in the 'valleys'. However, looking at the photo of the reverse of your specimen, I see signs of pressure solution creases (where minerals have been selectively dissolved under pressure, leaving less-soluble darker minerals forming narrow bands). I suspect the 'creases' in the 'valleys' are an expression of the same process.


        Your 'reverse' photo shows a curious hard patchy pale veneer. I suspect that is a red herring: I think it might be modern (adhesive, or resin paint perhaps); maybe the stone had been used on a building at some time. If that is the case, the rock may have come from somewhere else - somewhere where cave deposits are more likely, perhaps.


        In summary, I don't have a reliable ID; all I have is hypotheses.

        Those could be refined given microscopic analysis (and perhaps chemical analysis of that pale veneer).

        Good ref on the area

        - (my thanks to Ian West)



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