The geology of Anglesey, the largest (714 km2) island in Wales is some of the most complex in the country. Anglesey has relatively low relief, the 'grain' of which runs northeast–southwest, i.e. ridge and valley features extend in that direction reflecting not only the trend of the late Precambrian and Palaeozoic age bedrock geology but also the direction in which glacial ice traversed and scoured the island during the last ice age. It was realised in the 1980s that the island is composed of multiple terranes, recognition of which is key to understanding its Precambrian and lower Palaeozoic evolution. The interpretation of the island's geological complexity has been debated amongst geologists for decades and recent research continues in that vein.
History of geological study
The first geological map of Anglesey was published in 1822 by John Stevens Henslow. It was Edward Greenly who provided the first detailed map of the island in 1920, a year after the publication of a two-part geological memoir following painstaking work, during the course of which he identified and named the mélange. Greenly's work was conducted in the era before the recognition of plate tectonics as the broader context within which geological processes take place. It was from the 1960s that plate tectonic interpretations of Anglesey's geology were put forward by geologists such as Dennis Wood who introduced the concept of an olistostrome to account for the intimately mixed lithologies on the north coast. Through the work of Gibbons and others in the 1980s, it was realised that the island is composed of multiple terranes, separated by faulted boundaries. Subsequently, the concept of 'underplating' at a destructive plate margin has been used to help explain the disposition of the disparate rock sequences. More recently again, work has demonstrated structural links with the Caledonian orogen in northeastern America, enabling Anglesey's wider tectonic context to be better understood.
Precambrian
A blueschist belt extends across the interior of the south - the Central Anglesey and Berw shear zones containing Ediacaran age rocks.
The schists of the Gwna Group date from Ediacaran to Early Cambrian times. They extend north from Menai Bridge to the eastern shores of Red Wharf Bay and from Aberffraw Bay northeast to beyond Cefni Reservoir. Smaller outcrops extend northeast from Llanddwyn Island and in the Pentraeth area. Each of these areas includes igneous rocks too, including pillow lavas and tuffs. The schists are also present to the north and east of Llanfaethlu, at Mynydd Bodafon and patchily along the north coast. Though these schemes have been and continue to be subject to change, the Monian Supergroup has in recent times been considered to include the Fydlyn Felsite Formation (or Fydlyn Group), the Gwna Group, New Harbour Group (within which the Skerries Formation, Church Bay Tuffs & Skerries Grits, Lynas Formation and Bodelwyn Formation are defined), the Holy Island Group and the Rhoscolyn, Holyhead and South Stack formations. It thereby extends into the Cambrian.
The Holyhead and Rhoscolyn formations, a succession of metamorphosed quartzites and sandstones with interbedded mudstones, is between 150 and 200m thick. Its outcrop is restricted to the Holyhead Mountain and Rhoscolyn areas of Holy Island. The Rhoscolyn Formation is metamorphosed sandstones and silty mudstones with some quartzite.
Cambrian
Rocks of the South Stack and New Harbour Groups are of Cambrian age. The South Stack Formation which may be over 1 km thick in places comprises metamorphosed sandstones and silty mudstones and is considered to be of late Cambrian/early Ordovician age. It outcrops on Holy Island between Holyhead and South Stack and at Rhoscolyn and also between Mynydd Mechell and Carreglefn inland on Anglesey itself.
Ordovician
Anglesey's Ordovician rock strata, largely interbedded mudstones and sandstones, lie unconformably on Precambrian and Cambrian basement. The main outcrop extends from Dulas Bay inland via Rhosybol and Llanbabo to the coast west of Llanfairynghornwy with an extension southwest via Bryngwran to the coast at Rhosneigr. The basal beds are conglomeratic in nature. A second smaller outcrop extends southwest from Capel Coch through Llangwyllog. Another stretches north from Beaumaris and a fourth one extends southwest from Red Wharf Bay.
Silurian: Parys Mountain
It is at Parys Mountain, just to the south of Amlwch that the only outcrop of rocks of early Silurian age can be found on Anglesey.