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Ireland has a rich and diverse geological history spanning from 1.7 billion years ago to today. The geology of Ireland charts the opening and closing of the Iapetus Ocean, the opening of the North Atlantic Ocean, and the joining and movement of Ireland from the Southern Hemisphere to Ireland’s current location in the Northern Hemisphere. Below is a brief account of the formation of Ireland through geological time from the Precambrian (bottom) to the Quaternary (top).

0 – 3 million years ago

Ireland’s landscape was formed during the Quaternary. There have been a number of glacial events, the last one in Ireland ending around 12000 years ago.  The ice-shaped mountains, glaciated valleys and glacial deposits of eskers and drumlins all formed due to these glacial events. These glacial events also left the foundations for our lakes, waterways and bogs.

3 – 23 million years ago – Neogene
23 – 65 million years ago – Palaeogene

The Palaeogene saw the opening of the North Atlantic Ocean and the associated generation of volcanic and magmatic rocks. Palaeogene age rocks are mainly found in the north eastern part of the island and form the prominent features of the Cooley Mountains, Co Louth, the Slieve Gullion ring dyke which straddles the Ireland-Northern Ireland border between Co Armagh and Co Louth, the Mourne Mountains, Co Down and the Antrim plateau.

65 – 145 million years ago

Rocks from the Cretaceous are only found in Northern Ireland. They may have formed on other parts of the island but have since been eroded. They survive along the Antrim coast because they are overlain by other rocks that protected by them. Most of Ireland was covered by a sea during this period and therefore these rocks, typically white limestone or chalk, tend to be rich in fine marine fossils.

145 – 201 million years ago

The continent of Pangaea started to break up during the Jurassic Period and most of Ireland was mainly dry land, apart from the northeast and southeast which were shallow seas. Jurassic rocks are mainly found in exposed sections along the Antrim coast and are rich in marine fossils, such as ichthyosaurs which are large marine reptiles.

201 – 252 million years ago

Ireland was part of a desert during the Triassic. Fine-grained red sandstone and siltstone were deposited during this time.

252 – 299 million years ago

Ireland was just north of the equator during the Permian and was a low-lying, arid area. Fine desert red sandstone and coarser conglomerates were deposited. Cycles of shallow seas and high evaporation rates occurred towards the end of the Permian. The supercontinent of Pangaea began to form at the beginning of this period.

299 – 359 million years ago

Carboniferous bedrock accounts for over half the bedrock of Ireland. In the early Carboniferous, Ireland was periodically covered in shallow tropical seas and fossil-rich limestone formed. Later sea-level changes formed swamps which led to the accumulation of swampy plant matter and the development of coal found in Co Roscommon and Co Kilkenny.

359 – 419 million years ago

There are two rock types associated with the Devonian in Ireland – the Caledonian granites and the Old Red Sandstone. The Leinster, Galway, Donegal and Newry granites and other smaller bodies were also intruded during this time period due to the large amount of melt associated with the subduction of the Iapetus Ocean and the formation of the Caledonian mountains. Ireland was in the southern mid-latitudes by this time. Sedimentary basins formed and with the high sediment supply from the Caledonian Mountains to the north, large deposits of sandstone (Old Red Sandstone) formed. This is seen mainly in Co Cork and Co Kerry.

419 – 444 million years ago

This period is marked by the deposition of huge volumes of sediment in various terrestrial and marine environments and by volcanic activity. The Iapetus Ocean closed completely during the Silurian and this is reflected in the sedimentation. Silurian sediments are found in Co Longford, Co Down, Co Galway, Co Mayo and Co Kerry.

444 – 485 million years ago

The Iapetus was closing during the Ordovician, bringing north west and south east Ireland together, to create the island of Ireland we know today. Subduction of the plates during the closing of the Iapetus Ocean generated volcanoes and earthquakes. At the start, the Iapetus oceanic crust was subducted beneath continental crust, but eventually the oceanic crust was gone, and continents collided. To the north, the Caledonian Mountains formed, and it is thought they were as high as the modern Himalayas when they formed. The presence of the Caledonian Mountains led to an increase in sediment being carried by rivers into the closing seaway at this time. The sedimentary rocks deposited in the Precambrian were folded and underwent metamorphism during the Ordovician too.

485 – 541 million years ago

During the Cambrian a new ocean, the Iapetus Ocean, formed and separated two continents: Laurentia and Gondwana. Each half of Ireland was on each continent; north west Ireland was on Laurentia and south east Ireland was situated on Gondwana. Rocks from the Cambrian were deposited in the sea and from these we have our earliest known evidence of life in the form of trace fossils. Trace fossils are the preserved tracks or feeding tunnels of organisms. Cambrian rocks are found on Howth Head, Co Dublin, Bray Head, Co Wicklow, and in Co Wexford.

541 – 1000 million years ago – Proterozoic
541 – 4500 million years ago – Precambrian

The oldest rocks in Ireland are 1.7 billion years old and are found on the island of Inishtrahull, Co Donegal.  In Ireland, all rocks from the Precambrian have become metamorphic rocks during a later Period. We find rocks from the Precambrian in west Co Galway, Co Mayo, Co Sligo, Co Donegal, Co Tyrone, northeast Co Antrim, and in small sections of Co Wexford and Co Waterford.

1 : 1,000,000 Bedrock Geological Map of Ireland; a companion to Geological Survey Ireland’s ‘Understanding Earth Processes, Rocks and the Geological History of Ireland’ book by Andrew Sleeman, Brian McConnell and Sarah Gatley.