Anglesey and North Wales

Rhinog Mountains

Underlying the mountainous landscape of North Wales is over 600 million years of Earth history.

Hidden in the geological foundations of the region, which includes some of the oldest rocks to be found in the principality, is a detailed record of some of the most fundamental events which occurred during the formation of southern Britain.

These events include: the birth, life and death of an ancient ocean — Iapetus; the evolution of 450 million-year-old (Ordovician) volcanoes which spewed ash and lava over the area which was to become Snowdonia; to the blossoming of life on the floor of a shallow, tranquil, tropical Carboniferous sea; as well as the later dramatic modification of the landscape during the last ice age, which reached its 'icy peak' around 22 000 years ago, when glaciers carved the spectacular valleys and cwms of the Snowdonia National Park.

Explore ice age Anglesey using our interactive map — Anglesey i-Map

iMap logo

Menai Strait

Geological evolution of North Wales

The BGS research programme in this region is continuing to work on unravelling the geological evolution of North Wales. Our team of Earth scientists are currently working on Anglesey (Ynys Môn) where they are focusing on tackling some of the key issues relating to the amalgamation of this geologically complex island.

Although relatively small, Anglesey represents one of the key areas in the UK for understanding the large-scale tectonic processes that eventually led to the formation of southern Britain, and as such, is widely considered to be a 'classic' area of British geology. Its classic status also extends to the glacial landforms, with the island being used as a natural laboratory from which to investigate the process that occurred beneath a fast flowing corridor of ice which swept down through the Irish Sea during the last ice age.

Anglesey has a rich legacy of scientific investigation and was originally surveyed in detail by Edward Greenly, whose geological map (published in 1920) will be revised during the course of this project.

Selected detailed geological mapping and landform analysis, combined with new HiRES airborne geophysical data, will help to deliver an exciting, new and up-to-date model of the geology of the island. This will provide a more accurate environmental baseline for Anglesey which will not only help to underpin ongoing infrastructure development, but will also yield additional societal benefits by delivering a much richer understanding of the geological heritage of this part of North Wales.

Evidence of ancient plate tectonics

Lleyn Peninsula

The bedrock geology of Anglesey comprises a complex collage of igneous, sedimentary and metamorphic rocks that were formed between 300–650 million years ago.

This 'geological jigsaw puzzle' preserves evidence of several cycles of ocean basin formation and destruction, with the famous Anglesey Blueschists having formed during the closure of one of these ancient oceans as oceanic crust was forced deep (20–30 km depth) beneath the Earth's surface during the process known as plate subduction.

Taken together, this geological record provides evidence of most of the major plate tectonic episodes responsible for assembling southern Britain, which in itself only records a small part of the history of formation and break-up of a global supercontinent known as Pangea.

Indeed the youngest rocks (less than 65 million years old) on Anglesey, an array of narrow, sheet-like, igneous dykes, record the final breakup of Pangea and the early stages of the opening of the Atlantic Ocean.

During the on-going project, bedrock geological research has focused on the Ordovician succession of sandstones and mudstones that underlies the central part of Anglesey. These rocks preserve the highly foreshortened and fragmented remains of a sedimentary basin that existed along a continental margin (peri-Gondwana) located in southern Britain.

A combination of detailed field surveying, interpretation of remotely sensed data and hi-resolution biostratigraphy has been used to unravel the dynamic stratigraphy and evolution of this succession and is providing new insights into the tectonic and palaeoenvironmental processes that operated between around 470 and 430 million years ago.

The 'footprint' left by a fast-moving corridor of ice

Rhoscolyn erratics

During the last ice age, known as the Late Devensian period, which began some 30 thousand years ago, Britain and Ireland were plunged into 'deep freeze' with a large part of the land and surrounding seas being covered in a thick layer of ice and snow known as the British and Irish ice sheet.

Anglesey occupied a unique position beneath this ice sheet, occurring close to the eastern margin of a fast flowing corridor of ice, or ice stream, which helped regulate the size, shape and thickness of the ice sheet.

This ice stream, known as the Irish Sea ice stream, transported ice from its source in south-west Scotland, through the Irish Sea and across Anglesey, to as far south as the Isles of Scilly.

The low lying, gently rolling hills of Anglesey preserve the unique 'footprint' left on the landscape by the ice stream. The landforms, such as egg-shaped drumlins, and glacial sediments left as the ice retreated provide a record of the processes occurring beneath the Irish Sea ice stream.

Work by the BGS has shown that the variation in the morphology and distribution of the landforms demonstrates that the speed of the overriding ice was highly variable within this part of the Irish Sea ice stream.

Marked changes in landform morphology have been shown to have been locally controlled by large-scale faults and/or major lithological boundaries within the bedrock, with less durable lithologies controlling the location and lateral extent of relatively faster flowing portions of the Irish Sea ice stream. This provides important evidence for the potential control exerted by bedrock geology on the location and size of ice streams draining contemporary ice sheets.

Cemlyn Bay drumlin

Ice streams and drumlins, landforms in western Anglesey (mp3)


Contact Alan Holden for more information on bedrock research in North Wales.

Contact Dr Emrys Phillips for more information on research into glaciation on Anglesey.