320 million years in the making

You’ll be fascinated to learn that the rocks that make up the Cliffs of Moher were formed over 300 million years ago during the Upper Carboniferous period. You’ll experience this as visible bands of Namurian sandstone, siltstone and shale. This is an extraordinary contrast to the recent History of the Cliffs.

How Were the Cliffs Formed?

Looking at the Cliffs of Moher one will notice individual horizontal layers of rock or strata. These layers are clearly defined bands of rock of different thickness. Each stratified layer signifies a separate cycle in the geological development of the Cliffs. The origins of these strata that form the Cliffs date back to over 300 million years ago, to the Upper Carboniferous period (327-299 million years ago). In that period much of Western Europe was covered in shallow warms seas. The area where the Cliffs of Moher now rise from the sea was a warm sub-tropical estuary, the mouth of a large river that flowed down to a shallow warm sea. 

Over millions of years this large river washed down deposits of silt and sand. Heavy rainfall created great floods which eventually blocked up the channels between the sandbanks that formed a river delta. As this occurred the river expanded to seek new channels to the sea, until this delta stretched out to the length of the present Cliffs of Moher. The lower deposits of sand and silt began compacted, turning into sandstone, shale and flagstone. These rocks which are the building blocks of the Cliffs of Moher were formed during the Namurian stage. River erosion, fluctuating sea levels and tectonic plate movement, meant that the rock layers were formed over five great cycles each of which added another stratified deposit on top of the last.  

As a result of the addition of these new layers of sediment, the estuary eventually became a low range of hills. These hills stretched out to sea far beyond the present coastline and was even thought to be part of a landmass that incorporated the Aran Islands. As time went by and the sea levels rose, the elements began slowly eroding the hills away. During the Ice Ages great sheets of ice covered the land, and when these began to melt, glaciers formed and planed the hillsides into long steep slopes or escarpments.  

The continuous wave action of the seas and the winds erode the exposed rock away, eventually forming a great wall of cliffs. 

The battle between the elements and these ancient Cliffs is ongoing and can be observed in the sea stacks and sea arches around the Cliffs. These massive columns of rock have been eaten away from the cliffs by wave and wind action. Slowly, eroding this rock away until it disintegrates into nothing. 

Cliffs of Moher aerial view


Looking out from the top of the cliffs you’ll 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 can’t 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 which are visible from sea level.

One of the Cliffs sea caves featured in the movie ‘Harry Potter and the Half Blood Prince’.

One of Ireland’s most famous sights, the Cliffs of Moher are entirely vertical and the cliff edge is abrupt. On a clear day the views are tremendous, with the Aran Islands etched on the waters of Galway Bay. From the cliff edge you can just hear the booming far below as the waves crash and gnaw at the soft shale and sandstone.

With a due-west exposure, sunset is the best time to visit.

Lonely Planet