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feature Hello,

We were walking in Bundoran in Donegal County, Ireland today and we encountered the geological feature shown in the picture (from above from cliffs and from very close up at beach level). We have three questions about it?

  1. How would you call it? It looks to me like the tall rocks on the sides are sea stacks but isn’t the eroded bit in the middle more like a sea stump? Or are they something different altogether?

  2. What type of rock is it (in the close-up pics) Our guess is that it is shale, but it could also be limestone. Which one do you think it is?

  3. Please notice the ubiquitous fossils in the close-ups. Are we correct to think they are crinoids?

I have just realised a good view from top of this feature is available on Google, so I am sharing it here as well: https://www.google.com/maps/@54.4852881,-8.2823865,3a,75y,49.15h,69.55t/data=!3m8!1e1!3m6!1sAF1QipNS26PrjyPwTfKnxdxVE29LHtp7A8TctpDVSjl1!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipNS26PrjyPwTfKnxdxVE29LHtp7A8TctpDVSjl1%3Dw203-h100-k-no-pi-0-ya103.88327-ro-0-fo100!7i8192!8i4096

Than you in advance for answering all our questions.

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  • $\begingroup$ The fossils are crinoids, that's correct $\endgroup$
    – JulPal
    Commented Sep 17, 2018 at 16:22

1 Answer 1

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1) I would call the formation a sea stack. Effectively you have a band of rock which is either more resistant to the forces of erosion and weathering from the sea, or is located on top of a more resistant strata. The more eroded portion is just that, a more eroded portion, and by its absence tend to identify the rest as a sea stack. 2) The matrix, the rock with the fossils, looks like a limey shale, or a limestone with a lot of shale or clay in it. It appears to have a component of lime or CaCO3 in it. This could be confirmed by placing a small amount of dilute HCL or Muriatic Acid on the rock and seeing if it fizzes.
3) There are definitely crinoid stem sections in this rock, as mentioned by JulPal. If you look carefully at this rock you will likely find a number of other fossils.

To get a tighter description you will have to give more information about the rocks

Treatment of this rock with a dilute acid bath might show a number of fossils. See the article here https://museum.wales/articles/2011-06-07/Fossils-in-stone-acid-preparation-of-fossils/

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