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    Ireland’s Weird Geology

    By Dawn Rainbolt, PR Manager
    More by Dawn

    Strange Geology

    For a small island, Ireland sure does have some odd, interesting, varied and fascinating geology.

    From basalt columns to dramatic cliffs to exposed limestone landscapes, explore some of Ireland’s strangest geological wonders with us. 

    A bit of geological Irish history

    Ireland is made up of bits of two today non-existent continents – Laurentia and Gondwana, who collided some 420 million years ago. Other parts of Laurentia went on to create parts of North America, hence why we see geological similarities between the mountains of Donegal and northwest Ireland and the Appalachian Mountains. From 450 million years ago, Ireland was under the sea, resulting in limestone, mudstone, and of course, plenty of fossils.

    The bedrock – deeply embedded hard rock – of Ireland is made up of a wide variety of rock types, each with different ages, though the oldest rock is found in Donegal, formed some 1750-1780 million years ago, though Kerry’s Valentia Island is also quite old (and home to dinosaur footprints!). Sections of quartzite are also quite old, and found on the mountains of a few regions.

    Ireland has a good bit of sedimentary rocks as well, most notably the exposed limestone landscape of the Burren region, as well as the rocky, mountainous terrain of the southwest. Learn more here.

    Read on to discover a few of Ireland’s most iconic geological features.

    The Giant's Causeway

    Probably the most important site of Ireland’s unique geology is the Giant’s Causeway. Located on the Antrim coast of Northern Ireland, the Giant’s Causeway is made up of 40,000 interlocking basalt columns, each with up to several irregular sides. 

    This place looks as if it belongs in a fantasy world. Myth claims that this strange, rocky outcrop is the result of warring giants, but geology says the Giant’s Causeway is actually the result of volcanic activity 50-60 million years ago during the Paleogene period. Overlapping flows of lava erupted and cooled upon reaching the ocean, resulting in basalt columns sculpted into polygonal shapes of varying heights.

    Since 1961, the Giant’s Causeway has been designated as a UNESCO World Heritage Site. 

    Learn More: The Giant’s Causeway

    The Burren, Co Clare

    Wild and fantastical, it isn’t hard to see why this place helped to inspire JRR Tolkien; the Burren looks like a scene pulled straight from the pages of the Lord of the Rings, and one half expects to see dwarves mining the landscape. 

    The Burren is an exposed limestone landscape of the Visean Period, formed from the undersea sediments from a sea that once covered Ireland some 350+ million years ago. Compressed and flattened, this area is a fossil-rich landscape, dotted with corals, urchins, and ammonites as well as others. 

    In some areas, the Burren takes on a steppes look – caused by extra layers of clay slotted between the layers of limestone. Together, this forms the rugged hills characteristic of the Burren, the most famous of these being Mullaghmor. Learn more about the Burren’s geology.

    Slieve League are Ireland’s highest cliffs. Jutting out of a remote peninsula in Ireland’s northwest county of Donegal, Slieve League is about 600 metres high at its highest point. 

    Developing over the past 100,000 ages, the cliffs are largely comprised of Dalradian Age quartzite. Not as sheer as the Cliffs of Moher, Slieve League cliffs slope at an angle, a result of erosion and “slumping.” 

    Interestingly, the geology making up the Slieve League cliffs is actually connected to the famous Appalachian Mountains that run the length of the eastern part of the USA; the mountains that are renowned for the 2,200-mile-long (3,500 k) Appalachian Trail (AT) that traverses their ancient peaks.

    Over 450 millions of years ago, the continents had yet to take their present shape, and parts of Ulster and Northwestern Ireland made up the same tectonic plate as what would eventually become the Appalachian Mountains, formed when it collided with the rest of North America. 

    Due to this geological connection, Slieve League and the rest of the Blue Stack Mountains through Northern Ireland are all part of the IAT or the International Appalachian Trail. The IAT was formed in 2011 as an optional extension of the original AT, and runs about 280 miles (450 km) from Slieve League, through the Bluestack Mountains, the Sperrin Mountains and along sections of the Causeway coast. (More info here). 

    Though you get the iconic view of Slieve League at the Bunglas Viewpoint, the best way to appreciate their amazing geology and stunning views is by climbing the back of the cliffs along the Pilgrim’s Path, to stand on the cliff’s edge. More about the cliffs’ geology.

    Learn More: Slieve League

    Steep, sheer, stratified – these cliffs are the iconic image of Ireland.

    Ireland’s most famous cliffs are surely the Cliffs of Moher.  These horizontal stratifications each represent different eras in the geological history of the Cliffs of Moher. The rocks and their stratified layers themselves originate from the Upper Carboniferous period, about 300 million years ago.

    We might think of Ireland as a windy, rainy little place where the temperature rarely rises above 25 C (77 F), but 300 million years ago, Ireland (and neighbouring countries) was actually under a tropical sea – to be precise, County Clare was a sub-tropical estuary. For millions of years, the region was a dumping ground for sediment and sandbanks, which compacted into a layer of sandstone, shale and flagstone.

    Time wore on, and events such as fluctuations in sea levels, tectonic activity, erosion and other events caused the stratification of these hills over 5 cycles. Rising sea levels eroded away the layers and the hills, and glaciers formed and melted, creating steep slopes, continuously eroded by wind and wave.

    As with much of Ireland, you’ll be able to see some fossils at the Cliffs of Moher, as well as sea arches and sea caves and layers of sedimentary rocks. Learn more about the geology.

    Lear More: The Cliffs of Moher

    Sea Stacks of Rathlin and Donegal

    Sea stacks are elongated pinnacles of rock rising up from the ocean, usually fully or partially separated from land. The type of rock can vary, but se stacks are usually formed on coasts comprised of rocks of medium hardness, such as limestone (a rock we have quite a lot of in Ireland). Wind and wave beat against the coast, attacked the weakest segments. These result in fissures or caves, and eventually, these land bridges break away from the mainland, creating a free standing sea stack, usually paired with mainland cliffs.

    Rathlin Island

    Rathlin Island’s stacks and cliffs, comprised dually of Cretaceous chalk and Tertiary basalt, are partially vegetated. These steep slopes make up a good home to seabirds, in particular puffins, to whom the inaccessible outer-reaches make perfect nesting spots. In fact, there is actually a puffin sanctuary on the island.

    Donegal Coast

    The coasts of Donegal is a particularly good place to spot these, as are the coasts of Rathlin Island, along the shores of Northern Ireland.Donegal is home to many dramatic and hard-to-reach sea stacks, such as Cnoc na mara, Tormore and Dunoff Head.


    Glacial Landscapes: Erratics and Corrie Lakes

    During the Ice Age roughly 10,000 years ago, glaciers once covered the island. Movement and eventual melting during this era caused the creation of ‘corries’ – small, rounded lakes tucked into pockets of mountains, caused by glacial melt trapped in the peaks. Large boulders were carved out of the mountains, leaving behind cavities or deep, rounded craters that were filled with slowly melting ice.

    Some landscapes are strewed with these glacial boulders, called erratics, which got pulled and shaped (a bit like a smooth stone found in a river) by the ice during the slow movement of glaciers.

    Coumshingaun Lake, Wexford

    A good example of a corrie lake is Coumshingaun Lake in the Comeragh Mountains of southeast County Wexford. There is a great 7.5 km walk (which takes about 4 hours) following the ridge of the mountains that form a horseshoe around the lake. The cliffs stand 365 m (1,200 feet high) above Coumshingaun Lake, though the highest point above sea level is 793 metres. So expect great views! At the the beginning or end of the path, there is an optional side track leading to the lake’s shore.

    The Eglone Stone, Sligo

    A glacial erratic is a stone deposited in a landscape via glacial movement. There are many such stones throughout Ireland, though a good example of the geological wonder is on the Moyrura Plateau in County Sligo. Outside the wee village of Highwood on the Historical Trail, find the huge monolith, the Eglone Stone. Once thought to be carved by Neolithic peoples, we now know that it, like many other the huge boulders strewn throughout this region, is a glacial erratic. (Though there are plenty of other Neolithic monuments in this region too!)

    Wilderness at Home

    During the pandemic, check out what Team Wilderness is up to while we #stayathome in this video and blog series. 

    Watch Now

    Or, bring Ireland to your sofa through our comprehensive list of useful resources such as virtual exhibits, classes and workouts. 

    Read Now

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    Meet the Author: Dawn Rainbolt

    American by birth but European in spirit, Dawn has called the US, Costa Rica, Spain, England, Poland, France and now Ireland home over the years. While she has travelled to more than 30 countries, she has fallen in love with the rich Irish culture and sweeping landscapes of Ireland. Armed with a Masters Degree in Tourism Marketing and a love of writing and photography, she is Wilderness Ireland's Marketing Executive since 2017.

    View profile More by Dawn


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