More Than Meets the Eye: Geophysics in the Midlands

More Than Meets the Eye: Surveying in the Midlands (Ashely Green and Denis Shine)

If you have been keeping track of our blog post in recent times you will be aware that we have been supporting the PhD research of Ashely Green (University of Bournemouth) in recent years (see Shine et al. 2016). In our last blog we promised to follow up on geophysical surveys Ashely conducted in the midlands of Ireland – we are keeping that promise!

Since December 2016, Ashely has surveyed at three locations in and close to Birr Town, Co. Offaly: Maigh Leana, Saint Brendan’s Church and Graveyard and Roscomroe Church and Graveyard.

GPR survey, a technique which is still underused in Ireland (Bonsall et al. 2014; Green 2017), was the preferred technique for both surveys. Ground-penetrating radar (GPR) detects slight variations in the subsurface conditions (e.g. geological variations, anthropogenic activity, services) by emitting electromagnetic pulses through the ground from a transmitting antenna. If the emitted signal interacts with changes in the subsurface material, it is reflected back to the ground surface to the receiving antenna and converted to wavelets presented on a monitor held by the surveyor. As such GPR detects changes in the subsurface matrix, whether they are significant archaeological features or variations in geology. The amount of time passed from the emission of the electromagnetic signal to receiving is used to determine the approximate depth of any detected objects. Higher frequency antennas are suitable for detecting near surface objects, while lower frequency antennas have a greater potential penetration depth (up to 20m under certain conditions) but with lower resolution.

St Brendan’s Church was once the medieval parish church for Birr (Biorra) founded in the 6th century. The monastery grew in prosperity due to its central location, making it an ideal location at which to call synods and meetings, and is probably best known as the possible site for enactment of the Law of the Innocents (Cáin Adomnáin) in 697 (the historical significance of the foundation will be the subject of a later blog). The standing church, based on its architectural detail, probably dates to the 14th century, around the time the O’ Carrolls regained control over the ‘Birr region’ from the Anglo-Normans. Later additions, such as the bell tower, most likely date to 17th century (Callaghan and O’Brien 2016). No geophysical surveys had been conducted within the church and graveyard; however, archaeological testing within the town (Tierney 2008) recovered burials indicating the original church precinct extends beyond the graveyard boundary that exists today. There are a number of post-medieval and later headstones and grave slabs surviving within the graveyard, but no indications of the medieval precinct boundary. As such, Ashely conducted a GPR survey of the known graves as well as the areas of the graveyard that should contain graves, but showed no signs of grave markers on the surface, in order to identify any unmarked medieval and post-medieval graves. I was also able to survey two areas outside of the current graveyard boundary to ascertain if there is any indication of the original precinct boundary or medieval burials. One of these, a laneway connected to church lane, contained a ‘feature’ of undetermined type/function (several such features were identified inside the current graveyard). A number of anomalies, interpreted as unmarked graves, were also recorded within the confines of the current graveyard.

Roscomroe Church was originally an Early Christian (Early Medieval) church attributed to St Molua, whom the holy well in the adjacent field is dedicated to (O’Brien and Sweetman 1997). Today, the church remains as the wall footings of a late medieval rectangular church and the west gable (with bellcote) within an irregularly shaped graveyard, the boundary of which is marked by a stone wall.

A high resolution GPR survey of the area was conducted immediately to the north and west of the gable to determine if there are any unmarked medieval graves in this area. While I didn’t interpret any distinct medieval graves manually, this site will play a key role in testing the effectiveness of the automatic feature detection software Ashely is producing as part of her PhD. Of note a sub-rectangular feature, which may be of archaeological significance, was observed during the survey as was a curving feature, which has been tentatively interpreted as a possible enclosing ditch.

While the features encountered at both sites through archaeogeophysics cannot conclusively be classified without follow-up ground-truthing, being able to roughly characterise the material beneath the ground surface allows archaeogeophysicists and archaeologists alike to improve the archaeological record and/or form appropriate excavation strategies – should there be a need to appetite to excavate the site. Further to this, the surveys discussed above form a portion of PhD research Ashely is undertaking at Bournemouth University, which aims to minimise the risk of disturbing archaeological burials where possible or accounting for them prior to excavation.

This work was made possible by the support of the local partners, landowners, the Birr 20/20 group and Offaly County Council. The preparatory and survey works were undertaken between December 2016 and June 2017.

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