Today I want to highlight not so much a site, but a geological feature that characterises the MACGeopark region – glacial erratics – those huge boulders and pedestal rocks that we find, especially close to Cavan Burren Forest. But I also have to say it is not unusual for you to see one that, having been unearthed when building a new home, becomes a front garden feature roundabouts. Which makes sense since our earliest ancestors saw them as aesthetic objects made of rock. They use them as their palette for some of the earliest examples of human art on this island. In terms of geoheritage topics you cannot beat the beauty, mystery and mystique surrounding glacial erratics. Surely, some poet can sing a hymn of praise to these earthly wonders for our Geopark Poetry Map!
This is an example of a type of glacial erratic, whereby the huge boulder would have beenMartina O’Neill, MACGeopark Development Officer, Partnership & Engagement
left behind as the ice melted and retreated at the end of the last glaciation. The fact that the
boulder is a different type of rock from the underlying bedrock gives rise to the name ‘erratic’. This type of erratic is known as a pedestal rock and these features are relatively rare landforms. However, there is a significantly high concentration of pedestal rocks within Cavan Burren Park where they are considered to be of international significance. In order fora pedestal rock to form, the erratic must be deposited directly on top of the limeston
bedrock. Other glacial erratics within the forest have no pedestal suggesting that they were
transported within a mass of boulder clay and therefore came to be deposited on top of the
boulder clay and not directly on to bedrock. It is thought that the deposition of the huge
sandstone boulder directly on top of limestone acted as a barrier to erosion, as limestone
erodes readily in weak acidic water such as rainwater. If this is the case, then the amount of
erosion that has taken place since the end of the last glaciation is easily estimated as the
height of the limestone pedestal is the height that all of the limestone would have been whenthe erratic was deposited.
Within Cavan Burren Park there is also a rare example of what is called a ‘modified glacial erratic.’ Signposted as ‘The Boulder Tomb’ it is thought that cremated remains were deposited in the niche created by the modifications. There is also rock art at this location. A small spring can be found towards the bottom of the incline. I have to agree with local ceramic artist Jim Fee that this part of Cavan Burren Park has a special and very peaceful presence.
Enter the site and walk towards the huge boulder in themiddle. It is best to view this feature from below so walk downhill before stopping. This is another example of a huge glacial erratic. It displays evidence of alteration by man with rock art on the top surface. This is another example of a pedestal rock with the erratic beingsandstone and the underlying pedestal being limestone. In this instance the limestone has been carved and has been identified as a prototype tomb. If you look carefully at the sandstone you will see that the layers are contorted in places. This is probably due to some form of disturbance before it became lithified, when the wet layers of sand were disturbed causing the water to be released.Martina O’Neill, MACGeopark Development Officer, Partnership & Engagement
All the Marble Arch Caves UNESCO Global Geopark sites are open to the public and now we are free to travel round the country. Northern Ireland has a bank holiday this weekend and the Republic will have one the first week of June. You may want to visit Cavan Burren Park and hug some of these glacial eratics to inspire some poems that will put them on our Geopark Poetry Map. Email GeoparkPoetryMap@gmail.com for submission guidelines. The closing date is 15th June 2021.