Holy Island Group Explained

Holy Island Group
Type:Geological group
Age:Furongian-Tremadoc
~
Period:Furongian
Prilithology:Metamorphosed sandstone
Otherlithology:Metamorphosed mudstone, quartzite, schist, phyllite etc
Namedfor:Holy Island, Anglesey
Region:Holy Island and Anglesey
Coordinates:53.3°N -44°W
Unitof:Monian Supergroup
Subunits:South Stack, Holyhead, Rhoscolyn and New Harbour formations
Underlies:Cemaes Group (unconformity)
Overlies:not exposed
Thickness:over 3000m (10,000feet)

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]

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

Notes and References

  1. Web site: Holy Island Group . British Geological Survey . The BGS Lexicon of Named Rock Units . 3 November 2020.
  2. Web site: South Stack Formation . BGS Lexicon of Named Rock Units . British Geological Survey . 3 November 2020.
  3. Web site: Holyhead Formation . BGS Lexicon of Named Rock Units . British Geological Survey . 3 November 2020.
  4. Web site: New Harbour Formation . BGS Lexicon of Named Rock Units . British Geological Survey . 3 November 2020.