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Rock markings

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  • 30-08-2013 10:15am
    #1
    Tea Tree
    Closed Accounts Posts: 116 ✭✭


    Hi :)
    I don't know if this is the right forum for this so feel free to move it if appropriate.
    I came across this rock a while back in the West of Ireland, limestone area. I can't for the life of me figure out how those markings might have formed. But then maybe I just don't know enough to know! Any suggestions?

    The rock/ stone is about 1 ft wide.
    5CFCBDC73B754A009B9E3521836A6F5B-0000331913-0003354392-00640L-2C3EFBFB28A047ECA74D281550D245CF.jpg


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    #2
    slowburner
    Moderators, Science, Health & Environment Moderators Posts: 5,218 Mod ✭✭✭✭slowburner


    Tea Tree wrote: »
    Hi :)
    I don't know if this is the right forum for this so feel free to move it if appropriate.
    I came across this rock a while back in the West of Ireland, limestone area. I can't for the life of me figure out how those markings might have formed. But then maybe I just don't know enough to know! Any suggestions?

    The rock/ stone is about 1 ft wide.
    Limestone is a sedimentary rock composed of deposited and compressed layers. I'm presuming this is limestone?
    If it is, what you have here are two exposed layers with the 'grain' running in opposite directions.
    The stone has been well rolled, tossed about and eroded by water. Various impacts have caused break off of linear fragments - probably along cracks or lines of weakness.
    That's my guess for what it's worth but I am certain that the markings are natural.


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    #3
    slowburner
    Moderators, Science, Health & Environment Moderators Posts: 5,218 Mod ✭✭✭✭slowburner


    ...and welcome :)


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    #4
    Tea Tree
    Closed Accounts Posts: 116 ✭✭Tea Tree


    Thanks Slowburner for the answer and the welcome :),
    slowburner wrote: »
    Limestone is a sedimentary rock composed of deposited and compressed layers. I'm presuming this is limestone?
    If it is, what you have here are two exposed layers with the 'grain' running in opposite directions.

    I'm trying to visualise it and that makes sense. I'm assuming it's limestone since everything around it is limestone (with lots of fossils) . I should have payed more attention in class all those years ago.

    The stone has been well rolled, tossed about and eroded by water. Various impacts have caused break off of linear fragments - probably along cracks or lines of weakness.
    That's my guess for what it's worth but I am certain that the markings are natural.

    I picked the right forum then:) . It's in a lovely little stony inlet which at the right tide sounds fantastic with the waves crashing over all those little stones so yes I imagine it's been tossed around a lot through the years. I must look more closely at the bigger rocks there next time.


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    #5
    NotCarrotRidge
    Registered Users Posts: 160 ✭✭NotCarrotRidge


    I would probably disagree (respectfully) with SB. It doesn't look like limestone to me, more like sandstone. The other rocks all look like limestone, and are a different colour (pale grey). The weathering is more typical of sandstone than limestone.

    The feature running bottom left to top right looks like a joint, and there are a couple of other linears parallel to that. The other direction (top left to bottom right) looks like it could be bedding or another joint set. The prominent, well-defined triangular feature above and to the right of the centre looks like the intersection of two joints.

    I'm not entirely convinced by any of the explanations and would need to take a look at it in real life to be sure.


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    #6
    slowburner
    Moderators, Science, Health & Environment Moderators Posts: 5,218 Mod ✭✭✭✭slowburner


    NotCarrotRidge wrote: »
    I would probably disagree (respectfully) with SB. It doesn't look like limestone to me, more like sandstone. The other rocks all look like limestone, and are a different colour (pale grey). The weathering is more typical of sandstone than limestone.

    The feature running bottom left to top right looks like a joint, and there are a couple of other linears parallel to that. The other direction (top left to bottom right) looks like it could be bedding or another joint set. The prominent, well-defined triangular feature above and to the right of the centre looks like the intersection of two joints.

    I'm not entirely convinced by any of the explanations and would need to take a look at it in real life to be sure.
    I will defer to your wisdom on all the above (most respectfully) :)
    I am however, 100% confident that the marks are natural.


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    #7
    Tea Tree
    Closed Accounts Posts: 116 ✭✭Tea Tree


    :)Interesting.Thanks for your thoughts on it NotCarrotridge. On this point..
    NotCarrotRidge wrote: »
    I would probably disagree (respectfully) with SB. It doesn't look like limestone to me, more like sandstone. The other rocks all look like limestone, and are a different colour (pale grey). The weathering is more typical of sandstone than limestone..

    I was wondering at first if it was sandstone because it looked a bit different to the other rocks around it. However I looked back at the other photos I had to put a bit of context to it and I think this one shows the variation well (I didnt really pay much heed at the time:o) although "my" stone was mixed in with the paler stones. The blurred bit in the middle is my edited out children :pac:


    95896D2C9BC444C3AC6AB79AD1CA0815-0000331913-0003358165-00500L-EFCDC0D729B74D73B4702AD6425DA86D.jpg



    For further context here is the back of the rock.
    EAFF6BF9B536496AAE788263E5178AAE-0000331913-0003358167-00500L-DE951583BC2E4AA89FE0FCF07FAF1769.jpg

    just out of curiosity I looked back at pics from the same place a year ago and was surprised at how much the bigger rock at the water line has moved! It's on a bed of small stones so I guess that makes sense too. I wouldnt have noticed but for looking back at the older photos but just shows how much things move! I'm guessing "my" stone may be hard to find again!
    CF4BC89403BC4E3C95064F60EC8D3048-0000331913-0003358164-00500L-95752F538D814E88BB95F5E0D64B590A.jpg


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    #8
    Coles
    Registered Users Posts: 1,020 ✭✭✭Coles


    It all looks like limestone to me. North Clare? Near Kinvara?


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    #9
    NotCarrotRidge
    Registered Users Posts: 160 ✭✭NotCarrotRidge


    The bedrock is all limestone for sure. You can see a very strong joint set, pretty much vertical and pointing out to sea. That has influenced the shape and steep sides of the inlet. However, if your rock is sandstone, it's from a different place and those possible joints are nothing to do with the ones in situ.


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    #10
    El Siglo
    Registered Users Posts: 3,803 ✭✭✭El Siglo


    It doesn't look like limestone or sandstone to me (but I'd need to see it physically), it looks like shale. The surrounding rounded material in the plate looks like greywacke. Shale is pretty fine grained (essentially a mudstone) grey in colour, not terribly rounded (angular/subangular). The surface markings probably erosional, but you'd have to look at the mineralogy there to see secondary mineralisation (eg in weathered sandstone you should see sepiolite and smectite on exposed surfaces). Anyway, just throwing my two cents in, probably wrong!


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    #11
    slowburner
    Moderators, Science, Health & Environment Moderators Posts: 5,218 Mod ✭✭✭✭slowburner


    It's not basalt by any chance, is it?


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    #12
    Tea Tree
    Closed Accounts Posts: 116 ✭✭Tea Tree


    The place is Inis Mór, on the Eastern side of the island. I've just been looking at
    this map which is interesting.

    it was the angularity of the marks and indents that intrigued me but now you lot have me staring inquisitively at every rock I see to see if I can figure out what it is :pac: I think in this case, unless I find it again and bundle it up and take it home we will never know for sure !


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    #13
    El Siglo
    Registered Users Posts: 3,803 ✭✭✭El Siglo


    slowburner wrote: »
    It's not basalt by any chance, is it?

    No, it's not basalt. Far less basalt below the Longford South Down suture zone (generally, as they've nearly all been weathered). It doesn't look to have enough magnesium so I doubt it mafic and I don't think it's got te iron (no red colour) to be ferric and I doubt highly I it's andesitic either for that matter. You need to see vesicles or potentially amygdales in the rock which this has neither. Again, the grain size is pretty small and usually basalt is intermediate (of course depending on cooling). I doubt you would see very large heavy minerals in this sample, it looks very fine grained. Knowing some of Clare I would hazard an assumption that it's shale or a fairly hard limestone (some old, probably carboniferous). However making such a suggestion without seeing the country rock is like trying to figure the number of jelly beans in a bag without actually seeing the bag!


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