The rocks of the Cliffs of Moher.

As the tourist season is in full swing, one of the most popular visitor locations will be the Cliffs of Moher. All visitors will experience a variety of feelings and emotions as they gaze out over the world-renowned cliffs. It is worth reminding ourselves about the geology they are looking at.

All the rocks in the Cliffs of Moher started out as mud, sand, and silt around 320 million years ago. These sediments were eroded by rivers from a huge mountain range that extended along the equator, south of where we were located at that time. Like the modern Ganges or Brahmaputra rivers, they were fed by monsoonal rains and carried a lot of sediment to the sea where it was deposited as a delta.

When rivers carry sediment to the sea, the coarser sand is deposited first, right at the shore, but the finer mud can be transported huge distances offshore before it finally settles to the seafloor. As the sediment keeps being deposited, the sheer volume of it can form new land as it builds outwards into the sea. So, if you were standing on the seafloor (for a very long time) far out to sea, you would initially see a lot of mud accumulating around you. However, over time you would be buried by increasingly coarser sediment as the shoreline got closer to you. Eventually it would fill the sea, land would appear, and plants would then colonize that land.

Then imagine that sea-level rose and you were inundated again. As long as the rivers keep flowing the process would repeat itself and the sequence from mud to sand would be repeated in successive layers as the sediment filled the sea again. If the sea-level then dropped, the coastline would move offshore and there would be erosion of the coastal area until a new equilibrium was reached.

This interaction between fluctuating sea-level, monsoonal rains and large rivers is what happened 6,000 kilometres away and 320 million years ago to form the layers of sand and mud that became the sandstone and shale of the Cliffs of Moher. The changes in sea-level were caused by an ancient ice age that was happening at that time.

The layers of sandstone are slightly harder than the rest of the cliff face, which is made of softer shale and siltstone. The softer layers get eroded by wind and waves, leaving the sandstone ledges slightly overhanging. These ledges are ideal nesting sites for the incredible bird population that returns here every year, an amazing sight for the visitors who also return every year.

The rocks of Cliffs of Moher are a record of the changing patterns of sedimentation in an ancient delta that formed 320 million years ago, over 100 million years before the first birds had evolved and 320 million years before the first tourist.

Dr. Eamon Doyle, geologist for the Burren and Cliffs of Moher UNESCO Global Geopark.