Holy Island Group

Short description: Cambro-Ordovician geologic formation in Wales

Holy Island Group
Stratigraphic range: Furongian-Tremadoc
~497–477 Ma

PreЄ

Folded meta-sandstones and mudstones of the South Stack Formation, a subdivision of the Holy Island Group

Type| Geological group
Unit of| Monian Supergroup
Sub-units| South Stack, Holyhead, Rhoscolyn and New Harbour formations
Underlies| Cemaes Group (unconformity)
Overlies| not exposed
Thickness| over 3,000 m (9,800 ft)
Lithology
Primary| Metamorphosed sandstone
Other| Metamorphosed mudstone, quartzite, schist, phyllite etc
Location
Coordinates|  [ ⚑ ] : 53°18′N 4°40′W﻿ / ﻿53.3°N 4.667°W﻿ / 53.3; -4.667
Region| Holy Island and Anglesey
Country|  Wales
Type section
Named for| Holy Island, Anglesey
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The Holy Island Group is a sequence of metasedimentary rocks of Cambro-Ordovician (Furongian to Tremadocian) age found in northern and western Anglesey and the adjacent Holy Island in North Wales.[1] It comprises four formations; a lower South Stack Formation, an overlying Holyhead Formation, a succeeding Rhoscolyn Formation and an uppermost New Harbour Formation. The South Stack Formation outcrops on Holy Island, between Holyhead and South Stack and at Rhoscolyn, and also inland on Anglesey itself between Mynydd Mechell and Carreglefn.[2] The Holyhead and Rhoscolyn formations are restricted to the Holyhead Mountain and Rhoscolyn areas of Holy Island.[3] The New Harbour Formation, which previously enjoyed 'Group' status, is some 2km thick and conformably overlies the Rhoscolyn Formation. It extends across much of northern and western Anglesey and Holy Island.[4]

## References

1. ↑ "Holy Island Group". The BGS Lexicon of Named Rock Units. British Geological Survey. http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=NHI. Retrieved 3 November 2020.
2. ↑ "South Stack Formation". British Geological Survey. https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=NSS. Retrieved 3 November 2020.
3. ↑ "Holyhead Formation". British Geological Survey. https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=HHD. Retrieved 3 November 2020.
4. ↑ "New Harbour Formation". British Geological Survey. https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=NNH. Retrieved 3 November 2020.

## Further reading

Campbell,S., Wood,M. and Windley,B. (2014) Footsteps through Time: the rocks and landscape of Anglesey explained. GeoMôn, Isle of Anglesey County Council

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