1.1 Case Study: Clogherhead
How was the island of Ireland formed? If we visit the small town of Clogherhead on the east coast of Ireland, we will find some physical clues embedded within the landscape that tell a millions-of-years old story about the making of the Irish landscape.
When the island of Ireland was formed approximately 420 million years ago, it was created from two separate land masses – known as Laurentia and Gondwana (as well as the adjacent micro-continent of Avalonia) – that were separated north and south respectively by an ocean called Iapetus; the two land masses slowly moved together over time, and eventually collided into each other forming a unified land mass (Geological Society of London 2012; Rao, Jones, and Moorkamp 2007).
The consequent geological makeup, on either side of what is referred to as the Iapetus Suture, includes:
- a northern section that originated from Laurentia
- a middle portion, known as Ganderian terrane, that split from Gondwana and surrounds the collision fault line
- a small portion of Ireland’s south-eastern tip, found in Rosslare (of present-day County Wexford), which originated from part of the microcontinent Avalonia
The collision line can be drawn from Dingle, on the west coast of Ireland, all of the way up to Clogherhead in the east (Geological Survey Ireland 2024). The line itself is visible to the naked eye through the folding evident in the rocks at Clogherhead. This collision line has real implications on the makeup of Ireland today; for example, if you look at the placement of mountains on the island, you will notice a high concentration of mountains trending north-east and south-west, with few mountains in the area of the collision zone between Dingle and Clogherhead (Geological Survey Ireland 2024).
In the below Figure 1.3, compare the location of Clogherhead pin-pointed on the eastern side of the island in the satellite image to Figure 1.2 above; the below image can be used to contextualize where the Iapetus Suture occurred and is still visible:
The collision of these two land masses is not only visible through the fault line that runs between Clogherhead and Dingle – it is also evident through the diverse geology of the island itself, despite Ireland’s relatively small area (70,000 km square) (Geological Survey Ireland 2024). This is because the northern land mass, from Laurentia, was once connected to what now forms North America (Geological Survey Ireland 2024). This may be easy for you to remember if you are familiar with the Laurentian Mountains and Laurentian Uplands that span the eastern and central regions of Canada. It is also noteworthy that Scotland was once also a part of North America, as its landmass collided with modern-day England and Wales at the same time and in a similar fashion (Geological Society of London 2012).
If you are familiar with the Appalachian Mountains (that span the Northeast of the United States), this region shares a geological history with the northern region of Ireland. This is evident in the similarities between the mountains of Donegal in Ireland and the Appalachian Mountains in the United States (Rainbolt 2020). In fact, in 2011, the Blue Stack Mountains in Northern Ireland have been classified as a part of the International Appalachian Trail (Rainbolt 2020).
In the video clip below (Figure 1.4), researcher Niall Groome discusses the story of Avalonia, a geological segment of Ireland that is visible in the south-east portion of the island, which you can see in Figure 1.2 (above). While Groome focuses on Avalonia in the context of the geological making of England, Wales and Scotland, this video is useful to our understanding of Iapetus Suture on the island of Ireland, as the maps within this clip provide information on the different terranes that makeup both present-day Great Britain and the island of Ireland. Around 18 minutes into the video, Groome further provides context on the collision of Laurentia and Gondwana (as well as Avalonia) that is useful to the discussion in this section. Pay attention throughout to the maps that show the island of Ireland so that you can make connections between what Groome is saying about Great Britain, and how it correlates to the island of Ireland. The video helps to understand the making of the island of Ireland in tandem with Great Britain through the lens of Iapetus Suture:
Figure 1.4 “The Story of Avalonia: How England & Wales crashed into Scotland.” Cardiff University lecture, given by Niall Groome, available on Youtube. This video is included on the basis of Standard Youtube License.
What, then, does the folded rock at Clogherhead reveal about the island of Ireland? In this section we have learned about the geological makeup of Ireland, and how it developed out of the convergence of very different geological land masses. At the same time, we have learned how the making of the island of Ireland during this period reveals physical connections between modern-day Ireland and North America (i.e., through the connection of the Laurentians and Appalachian Mountains) and also similarities between the makeup of the island of Ireland and Great Britain, where England and Wales share geological traits with Southern parts of the island of Ireland, while the north of Ireland shares geological origins with Scotland.
Multiple Choice Quiz
Test your knowledge on the formation of Ireland!
Figure 1.5 Image of the coastline in Clogherhead. © Pavel, www.adobestock.com. View source. Included under Adobe’s Education Licence – Standard Image terms.
