Use of geophysics leads to the discovery of enclosure at Tara
Hidden beneath the green grass of the Hill of Tara and otherwise invisible to the naked eye, archaeologists have identified a circular enclosure which may date to the late neolithic period (around 2500BC). This suggests that Tara was an important ritual site at a much earlier date than previously realised.
The new enclosure is about three metres wide and 250 metres in diameter, and up to 400 post holes may surround its full circumference. It encircles the Rath of the Synods as well as the Mound of the hostages and the church on Tara. Prof George Eogan, chairman of the Discovery Programme, said this enclosure was very significant.
"The Discovery Programme has been carrying out research on Tara since 1992, and already this work has increased the number of known sites from 30 to 60. This new discovery is by far the largest and most spectacular monument," he said. "Frankly, I'm as excited as anyone else. It highlights the importance of Tara and also highlights the importance of what is not visible."
The new enclosure was discovered through the collaboration of the Discovery Programme and the Centre for Archaeological Survey at the Department of Archaeology, NUI Galway. Mr Joe Fenwick, of NUI Galway, who carried out the survey using a fluxgate gradiometer, explained that geophysical techniques were not new to archaeology, and had been used in Ireland since the 1960s, but the advent of computers has allowed data to be more easily processed. This has caused an exponential increase in the use of geophysical techniques here.
What geophysics brings to archaeology is a non-invasive, non-destructive way of obtaining a picture of what lies beneath the soil, Mr Fenwick said.
A gradiometer measures the vertical component of the Earth's magnetic field. A very sensitive instrument, it can detect minor variations due to buried features which cause magnetic anomalies. Typically, these features distort the Earth's magnetic field by two to 15 nanoTeslas (nT). To put this in perspective, the Earth's magnetic field is in the order of 48,000 nT.
"Human activity in the past has a bearing on the chemical and physical properties of the soil. Some activities, in particular burning and organic decomposition, may enhance the magnetic properties of the soil. This soil may subsequently come to fill ditches and postholes," Mr Fenwick said. "When you go over these features with a gradiometer, they give a magnetic kick above or below the Earth's magnetic field."
The area is mapped in a series of panels. The arcing ditch, illustrated in the diagram (left), was found in this way. Mr Fenwick said he was reasonably sure it was part of a circle as he scanned either side of the arc and found evidence of the feature continuing.
The newly discovered enclosure is cut by Rath na Rig, which dates to the Iron Age, so it apparently predates this ditch, but further work is required to obtain precise dating. Prof Eogan said the nature of the site with the ditch and postholes suggested certain similarities with late Neolithic structures such as Woodhenge in Wiltshire, England.
Mr Fenwick, who previously worked with the Discovery Programme on Tara, said that when they started work on the hill in 1992 they had to call on Geoquest, a British company, to carry out the initial geophysical survey. Since then, Ireland has developed its own expertise in the area.
Magnetic gradiometry is only one of a number of geophysical techniques applied to archaeology. These include electrical resistivity, magnetic susceptibility, electro-magnetic surveying and ground penetrating radar. The use of several complementary geophysical techniques can help interpret the buried archaeology.
The archaeology department of NUI Galway works very closely with the college's applied geophysics unit and has recently opened a centre for archaeological surveying. "We hope to lead the field in the next few years," said Mr Fenwick, who is manager of the centre as well as a college lecturer.