Erratic Rocks

A notable feature of the exposed rock of the Burren are the large boulders that sit on top of the limestone. Some of the best examples are seen along the coast road between Ballyrean and Black Head or Rock Forest near Mullaghmore. These are called ‘glacial erratics’ and they were transported to their present position by large ice sheets that moved across Ireland during the last period of glaciation which was at its maximum about 22,000 years ago.

Boulders such as this are known from across northern Europe and North America and during the 17^th Century they were originally thought to be evidence of the Great Flood. It wasn’t until the 19^th Century that the idea that major glaciations in the past had shaped the Earth’s surface became accepted thanks to publications by eminent geologists such as Charles Lyell’s Principles of Geology in 1830.

The name comes from the Greek word ‘errare’ meaning to wander. The term is applied to rocks that have been carried some distance from their original outcrop. We have some excellent examples of large granite boulders that have come from across Galway Bay. There are no outrops of Galway Granite in the Burren so the boulders must have been transported here. The vast majority of our erratic boulders are limestone and local to the Burren, so the transport distances aren’t huge.

One of the useful features of erratics is that they can tell us the direction that the ice flowed, so we know that the Galway granite boulders were carried south to the Burren by ice sheets flowing from Connemara. In southern County Clare at the Bridges of Ross we have erratics that were carried there from County Kerry whereas in eastern Clare we find erratics carried from the Slieve Aughty mountains. This tells us that local ice sheet flow spread out from regional upland areas and that while the overall flow of the main ice sheets in Ireland was from the northeast, the local pattern can be more complex.

While currently these boulders stand proud and exposed it is possible that they were once part of  glacial Boulder Clay deposits (a mixture of boulders and ground up rock clay) which have lost all the finer material due to erosion. The amount of Boulder Clay that has been eroded, the erosional processes involved or when that erosion happened has yet to be established. Some boulders are split in half or thirds, this is most likely due to the action of freeze-thaw processes in the periglacial period after the rock was deposited when severe seasonal freezing and thawing would have been common.

More recently, these erratics can be used to establish when the last ice sheets melted. Dr. Gordon Bromley, a glaciologist from NUI Galway is currently studying our Buren erratics. By calculating when these boulders were last exposed to cosmic radiation, it is possible to estimate when they were exposed after the last ice sheets melted.

For an alternative and thoroughly entertaining story about how the Burren got covered in boulders I can recommend Eddie Lenihan’s book ‘Irish Tales of Mystery and Magic’ published by Mercier Press.

Learn more about the history of the Burren formation here.

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