Clogherhead and the Shannon Estuary, Ireland
Folding at Clogherheadin Co. Louth: © Geological Survey of Ireland
The Iapetus Suture is a large geological fault that marks the collision zone where the north of Ireland and Scotland collided with southern Ireland, England and Wales. Up to then, the vast Iapetus Ocean had existed between them and over a period of about 200 million years, it gradually closed until collision was complete by about 400 million years ago.
The Iapetus Ocean was located in the southern hemisphere between the ancient continents of Laurentia (of which the north of Ireland and Scotland were a part), Baltica, and Avalonia (of which southern Ireland, England and Wales were a part). In Ireland, the Iapetus Suture runs from the Shannon Estuary in Co. Limerick across to Clogherhead in Co. Louth, before continuing on to the Solway Firth and then the Cheviot Hills in England. As the ocean disappeared, these three continents joined together to form one single landmass. The north-west of the island of Ireland belonged to Laurentia (or proto-North America), while the south-east belonged to Avalonia.
Clogherhead offers some of the best opportunities to see the different rock formations and structural features associated with this major episode of plate tectonic
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Location of Shannon Estuary: © Asarlai
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activity. The structural evolution of the area was complex following and during closure of the Iapetus Ocean but evidence for this is easily seen in the spectacular folding in the cliff exposures and the associated cleavage. The rocks are predominantly Silurian age greywackes and mudstones that formed on the bottom of the Iapetus Ocean. These are intruded by Caledonian lamprophyre dykes which are a relatively uncommon type of intrusion, and have been linked with deep melting in subduction zones such as that associated with the closure of the Iapetus Ocean.
At the Shannon Estuary, the evidence for the Iapetus Suture is nowhere near as striking. The estuary is the outflow of the River Shannon, the longest river in Ireland and the UK, draining nearly one-fifth of the island of Ireland. But a length of around 100km follows the course of the westerly section of the Iapetus Suture indicating that its location has been dictated by the presence of this large geological structure.
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