A ‘Ringfort – Rath’ is defined by the NMS Scope Notes as: A roughly circular or oval area surrounded by an earthen bank with an external fosse. Some examples have two (bivallate) or three (trivallate) banks and fosses, but these are less common and have been equated with higher status sites belonging to upper grades of society. They functioned as residences and/or farmsteads and broadly date from 500 to 1000 AD.
The NMS also use the term ‘Ringfort – unclassified’ where the surviving remains of a Ringfort are insufficient to determine whether the monument was originally a rath or cashel. Cashels are comprised of stone – which exhibit different geophysical responses – as opposed to the earthern bank and fosse of a rath. In these instances, geophysical surveys are often able to help classify such monuments by offering an interpretation depending on the presence or absence of a classic stone/ditch response.
Geophysical response of a Ringfort
In magnetometer data, the fill of the external fosse (commonly known as the ‘ringfort ditch’) should exhibit a positive magnetic contrast compared to the background soils. The magnetic contrast of the ditch fill can range from no contrast, to a very slight contrast through to a very strong contrast, depending on the host soils and drainage conditions.
The ditch fill of Ringforts on limestone or tills, or on poorly draining soils such as gleys and peat can be difficult or impossible to image: the magnetic contrast may be very weak or not present at all. Negative magnetic contrasts for the ditch fill can also be present, depending on the background soils (e.g. igneous bedrock) or the fill itself, which may contain natural back-filled stony deposits.
A priori information concerning the presence of a monument (or a redundant record or crop mark anomaly) is often vital. Geological anomalies – particularly in areas of limestones – can mimic the morphology and magnetism of Ringfort ditches. The earthern bank is often not imaged at all due to the host soils being the same redistributed material as the bank, offering no magnetic contrast (unless other deposits/layers/artefacts are incorporated). High resolution assessments (data acquired at 0.5 m x 0.25 m) are highly recommended when surveying Ringforts located on low/weakly contrasting soils.
When surveying Ringforts (or any ditched feature), it is advisable to assess the soil drainage in advance of survey (see ‘Groundwater Recharge: Soil Drainage’, mapping on the Geological Survey of Ireland’s Groundwater Data Viewer), as this will influence the choice of an appropriate geophysical technique. Dry, well drained soils on a favourable geology will produce a clear and coherent magnetic anomaly. Poorly draining soils allow rainfall and other water runoff to stagnate and silt up in the base of circular Ringfort ditches, which often have limited opportunity (if any) for natural drainage (Bonsall 2014: 454). Waterlogged conditions are known to impede the magnetic susceptibility enhancement of anthropogenic soils (Weston 2004), which in turn leads to very low magnetic contrasts (or ‘no-contrast’ at all), prohibiting visualisation in magnetometer data.
In some circumstances magnetometer surveys are inappropriate and earth resistance or electromagnetic apparent electrical conductivity surveys are better suited to areas of poor drainage – where low magnetic contrasts (or an absence of contrasts) can be expected for earthcut features.
In earth resistance data, the external fosse is typically measured as a low resistance response compared to the background soils (allowing for normal seasonal variations as necessary) and the bank can often be measured as a higher resistance contrast.
Ringforts in Magnetometer Data
The following examples of ‘Ringfort – Raths’ are located on various limestone bedrocks beneath peat or tills. The magnetic content of the ditch fill in these examples are either weakly contrasting or non-contrasting. In some cases, the contrast is significant enough to identify the ringfort ditch and further, isolated contrasts, are indicative of pits or even post-pit structures within the ringfort. In the weak/non- contrast examples, some or none of the ringfort ditch is visible.
- Post-hole structures and hearths are typical internal features, however Ringforts can contain other areas of burning, cultivation and souterrains, meaning that a wide variety of anomalies may be encountered within the monument.
- Some Ringforts contain trees or hedges, which help protect them from agricultural damage.
- Small (sub-20 m) survey grids may be needed across a Ringfort interior (the bank/ditch or vegetation, may prevent a continuous survey within and beyond the Ringfort).
- Access/egress for some survey carts may be prevented by thick vegetation or steep slopes of the bank and fosse.
Bonsall, J. 2014. A reappraisal of archaeological geophysical surveys on Irish road corridors 2001-2010. Thesis submitted for the degree of Doctor of Philosophy, Archaeological and Environmental Science, School of Life Sciences, University of Bradford.
Weston, D.G. 2004. The Influence of Waterlogging and Variations in Pedology and Ignition upon Resultant Susceptibilities: a Series of Laboratory Reconstructions. Archaeological Prospection 11.2: 107-120.