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Understanding Ireland’s landscapes


The seam that runs across Ireland

There is a seam running across Ireland from Limerick to Louth. It marks the join where, 400 million years ago, two continents collided and an ocean disappeared. There is still some debate about the precise location of the seam, because much of the evidence for it has been destroyed or buried since the time of the collision. But geologists have pieced together a picture from fossils and geological evidence, especially around Clogher Head in Co Louth, the only place in Ireland where the seam is exposed and visible.

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The half of Ireland that lies north of the seam was originally joined with Scotland, Greenland and North America in the ancient continent of Laurentia. The south-eastern half of Ireland, along with Wales, England and Brittany, formed the smaller continent of Avalonia. Some 500 million years ago Laurentia lay at the Equator, Avalonia lay further south, and between them was the Iapetus Ocean (Iapetus was the father of Atlantis, after whom the Atlantic Ocean is named).

Fossils from this time found in the northern Laurentian, or ‘American’, half of Ireland are very different from those found in the Avalonian, or ‘European’, half. This reflects the fact that they came from distant parts of the world once separated by 3,000 kilometres of ocean.