Huge storms and tsunamis may have eroded Inis Mor and turned two inland forts into clifftop fortresses. Lorna Siggins, Marine Correspondent, reports.
Archaeologists studying the famous Aran island forts of Dún Aonghusa and Dun Duchathair may have been misled over the years by their "cliff top" locations.
In fact, both forts were originally built on inland sites on Inis Mór, according to Dr Michael Williams, of NUI, Galway's geology department.
Marine erosion by the Atlantic has virtually eaten away part of the island in intervening centuries, says Williams. He believes it is reasonable to suggest that both forts date from 2,500 BP (before present), and were probably constructed a good distance from the Atlantic precipice.
Although there are several conflicting theories on Dún Aonghusa's purpose, the prevailing view is that it was built on a cliff top to make full use of its "defensive advantage". Archaeologists also describe Dun Duchathair, the Black Fort, as a promontory fort. The extensive construction of chevaux de frise (an obstacle to cavalry) outside the ramparts of both is quoted as further proof of this defensive context.
The fact that both are on westerly facing cliffs exposed to some of the most extreme sea and weather conditions in western Europe, and without any permanent water supply, has led to some debate about the fort's intended purpose. Now, Williams has calculated the extent of marine erosion since the original construction, suggesting they were built 1,000 metres inland from the south-west shoreline.
It was during his general research into marine erosion, and the existence of "tsunamis" in the north Atlantic, that Williams came to his conclusion. Tsunamis are caused by tremors or earthquakes on the sea floor, and are normally associated with the Pacific region. Although Ireland is on a relatively stable part of the Earth's crust, it is exposed to seismic events - such as the tremors that affected Dublin and Wexford in the 1980s.
The one tsunami on record to have hit the Irish coastline occurred after the Lisbon earthquake of 1755. More than 50,000 people died in the Portuguese capital, and three tsunamis crashed over the harbour area after the initial ground shocks. The waves are reported to have devastated the Iberian coast as far south as Morocco, and they then propagated in a radial pattern across the North Atlantic.
Computer simulation suggests that they would have struck the west Irish coast some two to three hours after the first ground shocks in Portugal, and the castle of Coranroo, on the southern shore of Galway Bay near Kinvara, is said to have collapsed.
Williams has trawled through historical records to find evidence of previous occurrences, dating back as far as AD 830, when the annals recorded violent seas in Co Cork. He has become a "storm" collector, tracing events, such as the loss of 15 people from Inis Mór's limestone ledges in a freak wave back in August, 1852. Two years later there were reports of a giant wave that struck the Wexford coast near Kilmore Quay.
Just over 50 years ago, at the end of January 1953, the great storm that halted the construction of the slipway at Gort Na gCapall on the Aran Islands also sank the Princess Victoria in the Irish Sea, with the loss of 132 people. "The storm became a legend in Holland where, as it swept from Scotland into the North Sea, the gigantic waves breached the dikes of the Dutch coast, with the resulting deaths of over 1,000 people and the loss of hundreds of square kilometres of agricultural land," he says.
Willliams has carried out research on the product of some of these storms - substantial accumulations of megaclast on the vertical clifftops of the Aran Islands. Similar megaclast deposits can be found on the Scottish islands of Shetland and Orkney.
In a paper written with Adrian Hall, which is due to be published shortly, he notes that some of these clasts weigh over 117 tonnes, with the heaviest calculated at 250 tonnes. Imbrication measurements indicate a "well defined and consistent mean direction from the south-west" for waves tossing them up on the cliffs - a direction consistent with prevalent storm winds for the area.
So how big were these rollers? Williams and his team have looked back over a non-continuous record of wave data recorded by passing ships off the Irish west coast from 1870, which shows that large wave heights of over 14 metres occur regularly in this part of the Atlantic. Some waves recorded were as big as 20 metres, and between 1879 and 1934 there were 20 "large wave events" of over 14 metres recorded in the summer months.
He maintains that the erosion caused by such waves is greater on the Aran Islands due to a combination of their geology and their exposed position. He calculates that erosion rate at 0.4 metres per annum for the islands' southeast coastal cliffs - a rate which would be accelerated by major storm events.
Assuming this mean rate of erosion, the southwest coastline of Inis Mór has receded by approximately 1,000 metres since the two forts were built, he says. This would also explain the shape of Dun Duchathair, which was probably similar to Dún Aonghusa but on a smaller scale.
Williams also notes that the site at Dún Aonghusa has produced evidence of occupation around 3,000 BP. "It seems unlikely that Bronze Age settlement would occur on a site as exposed as the present location," he concludes. "However, a site in a wooded environment in an upland area and perhaps one kilometre away from the worst effects of the Atlantic storms would seem reasonable."
Williams will publish his findings on the Aran island forts in GeoArchaeology, while his paper with Adrian Hall on extreme waves and the cliff-top megaclast deposits of Ireland and Scotland, is due to appear in a forthcoming issue of Marine Geology.