geological feature questions

[tadzio]

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?

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?
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?
Please notice the ubiquitous fossils in the close-ups. Are we correct to think they are corals?
Than you in advance for answering all our questions.

 

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[DDancer]

Pictures are not real clear but what I do see describes a limestone dike. A sedimentary rock formation that in this case was made from ocean deposits, hence the fossils and the layering, long ago. Depending on the hardness of the rock it tends to erode at different rates and that shelf of rock is likely harder than the rest. That harness resulted in the formation your inquiring on. My opinion.

 

[tadzio]

Thank you. And what makes you think it is limestone rather than shale?

 

[stdenis_jd]

some might call it a karst type formation due to the limestone eroding at different levels, the areas that haven't yet eroded have probably silicified which makes it much much harder and less prone to erosion than the limestone. As far as why limestone? Because it has the appearance of it and is what fossils are usually preserved in (limestone, dolostone, siltstone, various sediments). Shale tends to be harder and darker as well - testing a small sample with an acid (muriatic or even white vinegar) will produce effervescence if it is a calcium (or calcium/magnesium) carbonate.

 

[tokameel]

Welcome to T-Net, tadzio!

I hope that you have a chance to enjoy all of the cool folders here.

 

[Eu_citzen]

No way to tell from the pics. You'd need to be there or check their geological maps from the Irish geological survey.
Like the others, I find it likely to be limestone.