Exploring the archaeology, landscape and history of Slane, Co. Meath

2010 Results

The fieldwork undertaken in the 2010 season focused on the area of the motte to the west of the College and Graveyard on the Hill of Slane.

The fieldwork was undertaken by Kevin Barton of Landscape and Geophysical Services with assistance in the field from Igor Murin and Mark Nolan. The aims of the fieldwork were to use topographical and geophysical survey to build a picture of the morphology and internal structuring of the motte. The geophysical techniques used included Electrical Resistivity Tomography (ERT), earth resistance and topographic survey. ERT is a technique not often used in archaeological geophysical surveying in Ireland to date, although it is ideally suited to looking at problems like the structural composition of mound features. Instead of producing a bird’s-eye plan or map view of features within a defined survey area, this technique produces a vertical ‘slice’ downwards into the ground along a defined line between two points. The resulting image is a section, i.e., we are looking at the results side-on. ERT has been used very successfully to investigate the structure of the Rathcroghan Mound in Co. Roscommon (Waddell et al. 2009). It was decided to use earth resistance on the top of the mound to explore the near-surface features there, which are likely to include the foundations of a timber or stone structure.

Figure 1: Locations of ERT lines and earth resistance panel.

A reconnaissance GPS survey was carried out to provide an initial field map and control points for the survey. The heavy vegetation in the area created difficulties for the reception of a clear GPS signal in places. In spite of this, Kevin was able to set out a 20m x 20m survey area on the summit of the motte for the earth resistance survey (Figure 1).

Figure 2: Earth resistance data, motte summit, Hill of Slane

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The earth resistance survey mapped some significant variations in the material making up the summit of the mound (Figure 2). The lighter area on the SE side of the mound is an area of higher resistance and may indicate a stone layer close to the motte surface. Within this area there is a roughly linear feature trending NW-SE of lower resistance. There is also a zone of lower resistance material ringing the summit of the motte on the S, W and N sides.

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The electrical resistivity tomography (ERT) was carried out as four separate lines crossing the mound and intersecting in the centre (Figure 1). The individual electrodes were inserted into the ground at 3m intervals which, although less detailed than a closer electrode spacing, gave a modelled depth of 9m which was sufficient to explore the full height of the mound. The output from each of the sections is given in Figures 3 to 6. Each pseudosection is draped on the corresponding topography which is exaggerated x2 for presentation.

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Figure 3: ERT 1 pseudosection.

In Figure 3 which shows the east-west section, there are a number of features apparent. ER1 is an area of low resistivity, likely due to the fill material here being clay. This corresponds to an area of similar low resistivity to the west which is where the low resistivity mound material was probably dug from. ER2 is an area of high resistivity which lies to the eastern side of the flat-topped summit. It probably represents either very compact soil or stony material and appears to have a depth of c.3m. This corresponds with the area of high earth resistance in Figure 1 above.

Figure 4: ERT 2 pseudosection.

In Figure 2 showing the northeast-southwest section, there are four features of interest. ER1 is an area of low resistivity, ER2 is a narrow zone of high resistivity below the mound which may be due to bedrock, ER3 indicates a low resistivity core to the mound and ER4 is a small area of higher resistivity corresponding to the area of high earth resistance identified in Figure 1 above.

Figure 5: ERT 3 pseudosection.

In Figure 3 we see the north-south section through the mound and there are four areas of interest. ER1 is an area of lower resistivity associated with the ditch and the southern face of the motte. ER2 is part of the higher resistivity zone on the summit which correlates with the earth resistance data (Figure 1). ER3 is an area of lower resistivity within the mound. ER2 may be cut into this feature. The material in ER3 may have originated in ER4 which is a substantial area of low resistance to the north of the mound, probably representing an extensive clay deposit.

Figure 6: ERT 4 pseudosection.

Figure 6 shows the northwest-southeast section through the mound. ER1 is a higher resistivity zone on the summit which matches the sections in Figures 3, 4 and 5 above and also the earth resistance data in Figure 1. ER2 is the extension of the area of low resistance to the north of the mound already noted in Figure 5 above.

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The results above indicate that there is a broad correspondence between the nature of the material making up the mound and the nature of the material from the ground surrounding the monument. At this level of detail, it appears that there much of the material making up the mound was simply dug out of the very substantial surrounding ditch. However, there are hints that the mound was not constructed all in one operation – there are some indications that there may be separate phases in the construction of the monument, some of which might be earlier than the 12th century AD when the motte was created.

There is also some very interesting evidence from the summit of the motte. The area of high resistance suggests a stony area. Given that the surface of the motte summit is still largely intact, this high resistance area seems to be a deliberate feature of the construction rather than rubble from a collapsed structure. This may have served as a foundation layer for a medieval wooden tower. Alternatively, it might be something earlier. We hope that more detailed work later on might help to clarify these questions.

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