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1262 Views 16 Replies Last post: Nov 7, 2013 9:30 PM by Tabfish RSS
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Nov 5, 2013 12:08 PM

What is this?!

Hi all,

 

I found this 5 years ago while building a dry stone wall. It's about a foot long and has been in the living room ever since.

 

If any body knows what it might be, I would be very interested to know.

 

Thanking in advance

 

Brendan

photo.JPG

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    Nov 5, 2013 6:21 PM (in response to Bren)
    Re: What is this?!

    It May or may not be a fossil.  It could be a sendementery rock which was covered with rocks so each part formed at different speeds.  It also could be a fossil sea floor.  It does look like it has the right patterns.  Whatever it is I don't think it is ' incredibly' rare but fossil sea floors are quite rare.

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        Nov 5, 2013 10:00 PM (in response to Bren)
        Re: What is this?!

        Bren,

         

        In some way it resembles sole marks, which come in quite a range of forms. But there are some aspects that cause me to be cautious. Sole marks are often assicated with turbidite sediments; they occur on the sole (bottom) of sedimentary beds and are often seen as casts (where the overlying bed fills-in the hollows in the underlying bed). Flute casts and groove casts are perhaps the best known types.

         

        Please post a photo of the oppisite side of the specimen.

         

        It might help if we could see a close-up or two of the sides. That might just enable me to see if there is a grading in the particle sizes. Turbidite beds often have a fining-upwards grain size distribution.

         

        Please tell us more about the dry stone wall. It was presumably made of fairly/very local rock. But where abouts is the wall? Please be as specific as you can. That will enable me to check the geology, which may reduce the number of possible IDs.

         

        Mike

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        Nov 5, 2013 10:09 PM (in response to Bren)
        Re: What is this?!

        Hello Bren

        Your specimen looks to be a peice of a lava flow, did you find it in the UK? please don't say were exactly or you won't have any walls left!

        only joking.

         

        Tabfish

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      Nov 6, 2013 5:55 PM (in response to Bren)
      Re: What is this?!

      The fossils in Devon are aquatic fossils, so I don't think it can be lava.  I still think it a jurassic sea floor

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        Nov 6, 2013 6:15 PM (in response to Dan)
        Re: What is this?!

        Bren,

         

        Thanks for the further info and photos.

        From that I can't discount either lava or sole markings.

         

        I suggest you take it to the Tavistock Museum, to see what they say about it

        (I'm hoping they will have somebody mining-related they can call on)

        https://www.cornish-mining.org.uk/areas-places-activities/tavistock-museum

        First thing is to find out the rock type. They may be able to do so using a hand lens. However, the rock is clearly fairly fine-grained, so a hand-lens might not be good enough.

         

        Microscopic examination would give the definitive answer about rock type, but that looks to be tricky

        (unless you have a contact in a university geology dept.).

         

        But we might be able to deduce the rock type from the locality.

        The rocks around Tavistock are Carboniferous in age, comprising lavas, tuffs, slate, siltstone, sandstone and chert (and a microgabbro intrusion). If you know the location more accurately, that might help narrow it down (grid ref, street name, quarry name, etc).

         

        Once we have determined the rock type, we can think about the particular features of your specimen.

         

        Mike

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      Nov 6, 2013 8:03 PM (in response to Bren)
      Re: What is this?!

      I don't think it is rare, but it is strange.  The quarry will probably be full of them.

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      Nov 6, 2013 8:36 PM (in response to Bren)
      Re: What is this?!

      Ah - well then...

      Mill Hill is a slate quarry

      - http://www.millhillquarriesltd.co.uk/

       

      Your piece is not slate per se, but it may be reject material from adjacent beds, which would have been subject to similar metamorphism; or it could have been overburden (stripped to allow access to the slate).

      Having said that, the structure shown by your specimen does not strike me as metamorphic.

      Believe it or not, despite a metamorphic setting, both possibilities (volcanic rock and sedimentary rock with sole marks) remain valid. Volcanic rocks can occur intercalated in sedimentary or metamorphic strata (dykes and sills). Sands within muds can sometimes withstand low grade metamorphism.

      So the true identity of the rock type will still rely on in-lab examination.

      ...Unless...

      You keep your fingers crossed and take it to Mill Hill Quarry, where they may recognize it, tell you what it is - and perhaps show you a whole pile of reject rock (spoil) similar to yours!

      Tavistock museum might have an opinion, too.

       

      The question of rarity...

      It is difficult to talk about rarity for this type of specimen.

      There is probably lots of it in particular localities, so elsewhere it may be rare. But also, it is not as collectable as fossils or minerals, so in a collectable sense it would not be valuable, which one might associate with rarity. See also end of next paragraph.

       

      If I really had to guess, I would now side with you, Tabfish, as regards it being volcanic. However, I would lean towards a sill (shallow intrusion) rather than lava (marine or subaerial surface eruption). In that situation, the unusual texture shown in this specimen could represent the flowing of the molten rock during intrusion and its cooling at the margins. That is somewhat analagous to how basic lava forms a crust which becomes ropey and twisted as it flows. In the sill (or dyke) situation, I think it is rare to find such marginal structures.

       

      Of course, what to do next is up to you.

      If you do take it further, please let us know!

       

       

      Mike

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        Nov 6, 2013 9:38 PM (in response to MikeHardman)
        Re: What is this?!

        Thank's Mike

        I just realised I have a piece from a quarry that you would not believe came from a cement quarry.

        I will take some images in daylight.

         

        Tabfish

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