No passage-tomb under Millmount?

Millmount might not have been built on a Stone Age passage-tomb, a public presentation of archaeological data heard last night in Drogheda.

However, one intriguing possibility based on the results of advanced archaeological techniques is that it might perhaps have been built on a clay mound, like a barrow, a type of burial monument dating to the Bronze Age, and therefore later than the likes of Newgrange, Knowth and Dowth.

Millmount might have been built on an ancient burial mound.

But it's still too early to say for certain what lies beneath the mound of Millmount, if anything. The new data is from the latest phase of the Millmount Archaeological Remote Sensing project, aptly shortened to MARS, because some of the technology being used is out of this world!

The presentation was given by Kevin Barton of Landscape and Geophysical Services (LGS), Conor Brady of the Department of Archaeology, Dundalk Institute of Technology, and Brendan Matthews, the Old Drogheda Society Community Historian.

The data presented last night was generated using Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT). The GPR data was taken along eight transects of the mound, and Kevin Barton revealed that the signal "was totally absorbed" by the mound, indicating that it might consist largely of clay. This would seem to rule out the possibility that it is a cairn, built largely of stone, like Newgrange, Knowth and Dowth. There was a possibility of a slight "overlapping of layers" of clay in the mound, but this was by no means definite.

The Electrical Resistivity Tomography (ERT) data was perhaps the most exciting. This involves putting stainless steel electrodes into different parts of the mound and measuring the resistance based on Ohm's Law (Resistance equals voltage divided by current). A series of measurements were taken at the base of the mound, just above the modern revetment wall, spaced at 5 metres apart. There were four distinct "high resistance" areas along the perimeter of the base of the mound, with low resistance areas underneath them. Eighteenth century drawings show that Millmount had four towers around its base, and these four areas of high resistance might correspond to the location of those four towers.

Four sections of the mound were also measured using ERT, and Kevin Barton revealed results from these measurements which appeared to show a quite distinct area of low resistance at the core of the base of the mound. He said this area of low resistance would correspond in his mind to a large clay mound. However,  he said he could not rule out the possibility of a "cavity" in the interior - something like a passage-tomb or maybe a cist grave.

It would not have been uncommon for Bronze Age barrow to be built over a burial, which sometimes might be contained in a cist grave. This is basically a "box" consisting of large slabs of stone.

Community Historian Brendan Matthews suggested that a Bronze Age barrow-type mound might tie in with the mythology of the site, and that as the reputed burial place of Amergin, the figurehead of the Milesians who landed at the Boyne Estuary, a Bronze Age date might be more apt.

Earlier in the night, Conor Brady of DkIT gave an overview of the types of mounds found in Meath and Louth, including passage-tombs, barrows and a large number of unclassified mounds.

Brendan Matthews revealed that a small section of what might have been an earlier revetment wall, set back some distance from the modern wall, was found during a recent "cleaning up" of the site. He also showed a photo of a stone-lined drain which was found near the top of the mound.

Millmount as it looked in the 1740s, drawn by Thomas Wright.

He revealed that no major modifications had taken place at Millmount from around 1672 until the British fortified the site and built the martello tower on top of the mound in 1807-8. But in the mid 1780s a ditch around the base of the mound was filled in, and local people used it for gardening!

The presentation provoked a lot of questions from the audience, and there's no doubt it has focused minds on the possibility that Millmount is not a passage-tomb, and not, as suggested locally, contemporary with the great mounds of Brú na Bóinne.

However, there have been several people who reported in the 20th century that they had been inside a tunnel under Millmount, and so it is obvious from contemporary accounts that there is at least one subterranean passageway in there. Whether this is part of a passage-tomb structure, or maybe a medieval souterrain, or even perhaps tunnels built linking the 17th century towers is a matter of conjecture right now.

At this point, the Old Drogheda Society will be discussing the possibility of moving on to the next phase of the project, which involves a seismic technique that measures the response of sound waves through the monument. That could provide a more definitive overview of the mysterious mound of Millmount.