Holy Island Group

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]

Holy Island Group
Stratigraphic range: Furongian-Tremadoc ~497–477 Ma PreꞒ Ꞓ O S D C P T J K Pg N
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
Region	Holy Island and Anglesey
Country	Wales
Type section
Named for	Holy Island, Anglesey
Holy Island Group (the United Kingdom) Show map of the United Kingdom Holy Island Group (Wales) Show map of Wales