Advanced laser technology is giving archaeologists a new way to pick out signs of past habitation, writes Dick Ahlstrom
Advanced laser technology is giving archaeologists a completely new view of Ireland's passage graves and ancient monuments. The aerial technique shows up hidden features that can't easily be spotted while on the ground, identifying promising new areas for archaeological excavation. Researchers from University College Cork have teamed up with colleagues from Cambridge University to study in detail the landscape around the famous Loughcrew passage graves in west Co Meath using the technology.
The aerial surveying system is known as LiDAR (Light Detecting and Ranging) and Cambridge's Dr Colin Shell has been key in applying its use in archaeological studies, explains Dr Elizabeth Shee Twohig, head of UCC's Department of Archaeology. "Colin has really been pioneering this," she says. "He has done work at Stonehenge and at Loughcrew."
An aerial LiDAR survey uses an eye-safe laser pulsing at 33,000 times a second in a zig-zag pattern to give highly accurate distance readings between the aircraft and the ground. This data is combined with aircraft position and orientation taken every second using GPS satellite information and an onboard inertial measurement unit.
The result is a three-dimensional image showing surface features with a height accuracy of 20cm and overlaid on the Irish National Grid. "The resolution is extraordinary," says Shee Twohig. "You have almost an X-ray which shows you where you want to go."
Shee Twohig has been studying the passage graves at Loughcrew for many years. Located on the Slieve na Calliagh hills, the main features include passage graves on three hill tops lying on an east-west axis and generally pointing towards the larger and much better known Newgrange passage graves, she explains.
The public might not know about them but they are no secret to archaeologists, she adds. "While they are not as big or spectacular, we know them to be really interesting too."
These structures are Neolithic, built during the Stone Age at around 3,500 to 3,000 BC. "They are part of the same monument building phenomenon as seen at Newgrange," she says.
The largest passage grave, known as Cairn T, is aligned to pick up sunlight at dawn on the mornings of the Vernal and Autumnal Equinoxes in March and September. And while it is unproven, the Loughcrew cairns intentionally point towards Newgrange, another noted passage grave on Slieve Gullion in Co Armagh seems to be aligned towards Loughcrew, she adds, hinting at some intended connection between the three locations on the part of the Neolithic tomb-builders.
"In more recent years I have been aware there was a lot of monuments in the area, not on the hilltops but on lower ground, particularly to the north of the (Loughcrew graves)," says Shee Twohig. These lie in the townlands of Ballinavally and Summerbank and include a stone circle, standing stones and other features dating somewhere between 2,500 and 1,500 BC, she says.
Trying to assess topographical features associated with these smaller monuments is a challenge, but then the opportunity for a LiDAR survey came up by a "happy coincidence", she says.
Shell's colleague, Dr Corinne Roughley, was looking for a promising site and Shee Twohig suggested Loughcrew. Roughley applied for funding from the UK but also to the Irish Heritage Council and in the end she received €20,000 in support from our Heritage Council. While that amount wouldn't allow much digging, it covered the cost of an extensive LiDAR survey, and provided much new information about the younger monuments at Loughcrew, says Shee Twohig.
Now the UCC/Cambridge team plans to return to Loughcrew for further analysis using more advanced geophysics technology that will help them "see" what lies beneath the surface.
If the results are promising, she will look for funding to support a full excavation of the Loughcrew monuments and the potential to reveal more ancient secrets about the people who built them.