320 million years in the making
You will be fascinated to learn that the rocks that make up the Cliffs of Moher were formed over 300 million years ago during the Carboniferous Period. At that time, we were located close to the equator almost 6,000 km from where we are now!
Looking at the Cliffs of Moher you will notice individual horizontal layers of rock or strata. These layers are clearly defined bands of rock of different thickness and composition.
The lighter coloured layers that form narrow ledges are sandstone. The darker layers which make up most of the Cliffs are made of softer siltstone and shale. The sandstone ledges are slightly more resistant to erosion; however, they cannot support their own weight for long and eventually crash into the sea below. Therefore, it is so important to stand well back from the edge of the Cliffs and do not venture out onto the ledges.
The layers are mud, silt, and sand that was carried into an ancient sea by large rivers that flowed from distant mountains that have been eroded.
These sediments built up in the shallow sea and formed a delta-like the modern Mississippi delta, eventually filling in the sea and forming land. We know that some of the shale layers contain marine fossils such as ammonoids and crinoids. We know the sea was eventually filled in to become land because we find fossil roots preserved within fossil soils in these rocks further along the coast.
The typical squiggly patterns on many of the walls and pavement along the cliffs are the remains of the burrows of organisms that lived in the shallow mudflats at the edge of the ancient sea. The creature that formed them has yet to be discovered.
These sediments were buried deeper by more layers and eventually turned to rock. It took 300 million years for those rocks to get to where they are now. They travelled slowly (about 2cm per year) on a giant tectonic plate on the Earth’s crust. During that long bumpy journey, the rocks were fractured by colliding with another tectonic plate. This collision led to the formation of deep vertical fractures; it is these fractures that give the cliffs their vertical appearance now because when the rocks are eroded they fall along these vertical layers of weakness.
So, when you look at the magnificent Cliffs of Moher you are watching a 320-million-year-old story of rivers and mud and mountains and continents colliding and the tracks of mysterious creatures that lived and died and are preserved in those layers of rock.
The prominent sea stack below O’Brien’s Tower is the result of thousands of years of coastal erosion which has removed all the surrounding rock, leaving it standing isolated from the cliff to which it was once attached. Coastal erosion is likely to accelerate due to global warming.
The Cliffs are under constant attack from the enormous waves that batter them every day and this leads to erosion which results in fairly regular rockfalls, so please stick to the marked path and let the Cliffs be what they are; an active, dynamic landscape that provides a home to thousands of birds and a world-class visitor attraction that tells the amazing story of our geological journey and a vital part of the Burren and Cliffs of Moher UNESCO Global Geopark.
Looking out from the top of the cliffs you will see many coastal landforms: Sea Stacks, Sea Stumps and caves that have been formed as a result of erosion. These features will be regularly populated by up to 20 different species of birds.
You cannot miss the great sea stack – Branaunmore, standing 67 metres high. Once part of the 214m high Cliffs, it was separated by erosion.
The Cliffs of Moher lets you see a rare example of a sedimentary basin – rock formation that is normally only visible under the sea.
There are different types of trace fossils in the Cliffs of Moher flagstones which are the feeding trails and burrows of unidentified marine creatures.
These patterns of the rocks have led to a demand for Liscannor flagstone and in the 19th century, a strong quarry industry grew in the area. You can see this stone in the paving of the Cliffs of Moher Visitor Centre and on the pathways and viewing platform.
Look out for the sea arch at Hag’s Head – just below the Napoleonic signal tower – as well as many smaller sea arches that are visible from sea level.
One of the Cliffs sea caves featured in the movie ‘Harry Potter and the Half-Blood Prince’.