[3] 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.
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.
[4][5] 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.
A blueschist belt extends across the interior of the south - the Central Anglesey and Berw shear zones containing Ediacaran age rocks.
[7] 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.
A second smaller outcrop extends southwest from Capel Coch (Welsh: 'red chapel', reflecting the colour of building stone), through Llangwyllog.
Another stretches north from Beaumaris and a fourth one extends southwest from Red Wharf Bay (Traeth Coch: 'red beach', reflecting the colour of the extensive sandflats at low tide).
The coastal outcrop at Dulas Bay is considered to be important both for its fine displays of cyclical fluvial sedimentation but also for being the first place where epsilon cross-bedding was recognised in ancient rocks.
Calcretes (carbonate palaeosols) are well developed in the middle two formations,[11] Rocks of Carboniferous age occur in three parts of the island, the largest part of which, known as the 'Principal area', extends from the east coast between Lligwy Bay and Red Wharf Bay, narrowing southwestwards to the south of Llangefni to reach the southwest coast though is almost wholly concealed beneath younger deposits at Malltraeth Marsh (Cors Ddyga).
These sandstones and conglomerates with some siltstone and mudstone record the initial influx of sediment from the Wales-Brabant massif immediately to the south, onto an eroded surface of lower Palaeozoic and late Precambrian rocks.
[12] As elsewhere in Britain, subdivision of the Carboniferous Limestone has undergone many changes over the years and reference is found in the literature to a confusing array of names.
The outcrops on Anglesey form part of a wider assemblage across North Wales which is interpreted as recording the gradual transgression of the sea southwards and the establishment of a carbonate ramp and shelf during the early Carboniferous.
The Coal Measures and Millstone Grit strata are only found to the south of Llangefni and are largely concealed by modern tidal flat deposits which extend far inland hereabouts.
Interbedded mudstones and sandstones of Westphalian age which crop out along the western parts of the Menai Strait were traditionally known as 'Red Measures' but have more recently been designated the Plas Brereton Formation.
Global sea levels were suppressed during the ice age and it was only just over 5000 years ago that they had once again risen sufficiently to separate the island from the Welsh mainland.
[17] The sands which form extensive beaches and dune systems such as that at Newborough Warren derive from the erosion of glacial deposits and redistribution of the sediment by rising seas and by wind.