Translations
Cyfieithu i'r Gymraeg
Aistrigh go Gaeilge
Nestled beneath Holyhead Promenade lies a geological treasure and window on the Cambrian seafloor, formed at a time before the age of the Dinosaurs. So ancient is the exposed bedrock that at half a billion years old, it predates the emergence of true dinosaurs by a factor of two.
This exposed bedrock is a wonderful example of the New Harbour Group, a particularly striking coloured rock that tells of the prehistoric origins of what is now Holyhead – at a time when it was in quite a surprising location!
You will notice a green-hued rocky outcrop embedded within the modern concrete sea defences. The rock displays distinct layers, which you might normally associate with a simple sedimentary rock. This is indeed how the rock started out, a silty precursor of deep water marine turbidites consisting of finely laminated mudstones and siltstones with quartzose silt-mud couplets. This protolith (parent rock) was deposited at the bottom of a deep sea as the underlying bedrock of Anglesey was transported from near the south pole to its present location through aeons of continental drift.
During this time, this protolith underwent metamorphic transition (the “Greenschist facies metamorphism”), at temperatures of some 300-450°C and pressure of some 1-4 kilobars, a process which destroys almost all evidence of early fossils. These conditions recrystalised some of the minerals and folded the layers onto themselves, converting the weak mud and siltstones into the hard rock we see today. The resulting foliated rock has a characteristic tendency to split along its layers, breaking away into thin sheets. These layers are, however, folded and contorted, as if a crumpled sheet. There is clear evidence of great thrusting and folding resulting in the mesmerising patterns seen.
This is one of many examples of greenschist found along Newry Beach - a metamorphosed rock composed of minerals including quartz, muscovite mica and chlorite (the mineral responsible for its distinctive green colour).
Scattered around below on the beach are assorted other pebbles of different colours and textures – their existence is much younger than the bedrock, having been deposited by the Irish Sea Ice Shelf some 25-27,000 years ago. Not all, however; some of the darker more jagged pebbles are somewhat rounded pieces of bedrock schist, broken away. This formation forms part of the Monian Supergroup that makes up the geology of Anglesey.
Cap Corse, Corsica, France showing the amazing wild formations of Greenschist.
Logging Tasks:
Please send your answers within two days. If not received, your Found It log may be deleted. You may log a Found It before receiving a reply. Either use the "Message this owner" button at the top of the page or use the Message Center.
As this is part of the GeoMôn UNESCO Global Geopark in Anglesey, please DO NOT damage the formation in any way, or take any pieces with you.
- At what temperature and pressure did the Greenschist facies metamorphism take place? Referring to the diagram, what would have been different at a cooler temperature but the same pressure?
- How many fossils can you see with the naked eye? Bearing in mind the original mudstone was formed on a seabed, why do you think this is?
- Describe the texture, colour patterns, structure and erosion you observe on site. Is the erosion even? Why do you think it is eroded like this?
- Photo Task: Take a photo of you, your GPS, name tag or other personal item looking towards the sea.
Image Credit:
Top Image is our own
Diagram 1 Credit of https://en.wikipedia.org/wiki/Turbidite
Diagram 2 Credit of https://www.alexstrekeisen.it/english/meta/greenschist.php
Bottom Image Credit of https://en.wikipedia.org/wiki/Greenschist
Main Citations:
https://accessanglesey.leeds.ac.uk/home/the-geology/
https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=NNH
https://www.geomon.org.uk/exploring-geology-in-holyhead/
https://www.geomon.org.uk/green-schist/