St. Mary’s Abbey in Trim, County Meath, Ireland is a ruinous Augustinian Abbey dedicated to the Blessed Virgin Mary. The ruins of the abbey, consisting of a seven-storey tower, are located on the northern bank of the River Boyne, overlooking Trim Castle. The abbey served as a prominent pilgrimage site throughout much of the medieval period, until its dissolution in the mid-16th century.
Today very little remains of the abbey above ground, with the exception of the bell tower and ‘Talbot’s Castle’, a Later Medieval manor house that may incorporate a small portion of the original building. However, significant portions of the foundations/abbey outline may survive underneath the ground. In summer 2017 the Irish Archaeology Field School (IAFS), together with the University of Bournemouth and Ithaca college, supported a program of non-invasive survey at this crucial site, commencing with geophysical survey, followed by 3D Lidar ground scanning.
The Yellow Steeple:
The abbey today is commonly referred to as the Yellow Steeple, due to the yellow hue the surviving bell tower assumes during sunset. At c. 40m in height the tower continues to dominate the Trim skyline and is thought to have been the tallest building in Ireland in the Later Medieval Period.
The tower is thought to have been built shortly after the 1368. However, the Augustinian Abbey itself was founded in the early 12th century on the site of the original parish church (whose foundation is often credited to Saint Patrick), possibly after the church was burnt/rebuilt. The Augustinian order was introduced to the Abbey in the mid-12th century. The abbey was burned again in 1368 and shortly after this date a status of the Virgin Mary was erected at the site. This statue became famous for the miracles it worked throughout the medieval period, resulting in the abbey becoming an important place of pilgrimage. The statue was publicly burned in 1538, during the religious reforms as part of the dissolution of the monasteries under Henry VIII. Shortly after this the last abbot left the Abbey in 1542. The Yellow Steeple is alleged to have been ‘destroyed’ by Oliver Cromwell, when it may have been used as an Irish garrison.
As stated very little of the original abbey survives above ground today, where a large green space now occupies the area between the two surviving monuments of the Yellow Steeple and Talbot’s tower. A large green space is also extant directly south of the tower, when the church would have been located. These spaces are maintained by the Office of Public Works (OPW) and the ground is well suited to investigation through geophysical methods. The surviving archaeology of the abbey is likely to be diverse in nature, consisting of smaller ‘cut’ features (such as grave cuts, garden features etc,), as well as large lumps of masonry/wall. Thus a full suite of geophysical means are employed, including:
Earth resistivity – Detects changes in conductivity 50cm-2m below ground surface depending on probe spacing and array. More likely to identify structures and large negative features than small targets, such as individual burials, small metallic objects etc.
Electromagnetic Survey (EM) – Detects changes in magnetic susceptibility and conductivity up to 75cm below the surface depending on orientation and the requirement of a “clean” site. The survey may be able to delineate a cemetery, but not individual graves. However it will detect remaining structures and large areas of burning.
Ground Probing Radar (GPR) – Detects interfaces between different material types. Depending on soil type and antenna frequency, penetration can range from 50cm to 10m. The technique provides approximate depth of any detected anomalies and can detect individual graves (when using a small transect interval) as well as remaining structures, drainage systems, etc.
Gradiometry – The technique uses dual sensors to detect changes in the magnetic field and requires a “clean” site. Gradiometry helps delineate areas with changes in magnetic susceptibility, so changes in material (walls), changes in compaction (backfilled pits, wells, drainage systems), metallic and ferrous objects, and areas where burning occurred repeatedly.
The site had never been geophysically surveyed before so a large scale, landscape survey, using a magnetometer was preferred initially. The survey was completed by Ashely Green of Bournemouth University. Surveying the entire area west of the town wall helped to narrow down areas would make suitable targets for high resolution survey with other techniques. A few areas with high concentrations of modern ferrous material in the magnetic survey data were noted and consequently chosen for an earth resistance survey. For the survey a multi-probe array with a 1.5m beam and six probes was preferred, which acquires data from up to six different depths (in multiples of 0.25m) up to 1.5m below the surface; a single set of probes only measure the resistance at one depth. Approaching the survey in this way allows the surveyor to not only investigate deeper beneath the ground surface but to potentially determine the morphology (size, shape, depth, thickness, etc.) of any potential archaeological features.
Even though the area covered by both survey techniques at the Yellow Steeple thus far is marginal, there appears to be a number of potential archaeological features (possibly relating to the occupation of the abbey) visible on the geophysical survey results and aerial imagery.
In summer 2017 a team from Ithaca College (New York), lead by Prof. Michael ‘Bodhi’ Rodgers, also returned to the Trim – having surveyed the site of Trim Castle the previous year. They continued 3D scanning the landscape surrounding Trim Castle, including the Yellow Steeple monument. At historic sites such as the Yellow Steeple, 3D laser scanning can document every small detail of the archaeological landscape. Laser scanning not only records every detail down to the centimetre and millimetre level it allows us to look at the site from different perspectives by removing trees and/or modern features and taking slices or profile cuts in areas of interest without having to physically alter the landscape. Put differently it is possible to digitally reconstruct and deconstruct both the modern and relict landscapes, allowing a more layered examination of space through time. This application can prove particularly useful at sites like Yellow Steeple, where much of the original site no longer remains.
The Ithaca College 3D Archaeological Visualization Laboratory used a Leica C-10 laser scanner. This scanner, as with all 3D laser scanners, works by sending out pulses of laser light, which travel out from the scanner, reflect from the object being scanned, and return to the scanner. The scanner records the time it takes each laser pulse to travel out and back from the object they reflect from. This two-way travel time is converted to a distance and the scanner’s precise horizontal and vertical angular measuring tools facilitate the conversion of each pulse to an X, Y, and Z location in space for each spot the laser reflects from. Modern laser scanners pulse the laser 50,000 to 1,000,000 times per second. While pulsing this fast they spin the laser in the vertical and horizontal directions to cover 360 degrees horizontal and 270 degrees vertical. The scanners cannot ‘see’ 360 degrees in the vertical because they cannot image beneath them where they sit upon a tripod. Most scanners also have a digital camera onboard, which is used to take a full dome of photographs. These photographs are used to map an RGB colour to each point recorded using the laser. Photographs can also be taken with a standalone camera and registered to the laser scan post-acquisition.
The data from each scan location are loaded into the Leica program Cyclone. Because each scan location has coordinates within the local grid system the data from different locations automatically come together to form a 3-dimensional data set called a point cloud. The photographs are adjusted to ensure uniform exposure and colour vibrancy, and then extraneous data are removed from the point cloud. The point cloud can now be manipulated to look at features of interest.
But how might the data be manipulated, and to what end? The 3D scanning at the Yellow Steeple has generated a detailed model comprised of billions of data points, which can be used in a host of innovative ways relating to heritage management, archaeological research, outreach and education, remote touring, virtual reality modelling etc. As with more traditional archaeological work, in practice the data will be most effectively used in a holistic fashion drawing from a range of disciplines and expertise. All data has been given to the custodians of the Yellow Steeple, the OPW.
The full results of the survey have been communicated to the National Monuments Section (NMS) and OPW, both in terms of regular updates and the submission of a final report. The entirety of the 3D scan data was also made available to the OPW. The results of the geophysical survey have been prepared into poster form and presented at a conference. The project results have been communicated to both the local and academic community through a series of presentations. Finally, the results of the 3D scan have been published in the 2018 summer issue of Archaeology Ireland.
For further information see also Rodgers, M., Bouricius, R., Shine, D. Mandal, S. and Stull, S. 2018. Laser-Scanning Trim Castle. Archaeology Ireland, 32 (3), 34-39.