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

2011-2012 Survey Phase I

The current survey work commenced late in 2011 with funding from the Department of Arts, Heritage and the Gaeltacht Environment Fund.

We had divided the hill onto four zones or sectors as a means for managing the survey. In this first phase of fieldwork the focus of activity was in Area 1 around the motte at the western end of the complex concentrating in particular on the small enclosure feature to the SE of the motte. Work was also undertaken in Area 3 around the College and Graveyard at the core of the complex.

Hill of Slane Archaeological Project survey control points.

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The first task undertaken was the establishment of a series of survey control points. Thirty-seven were established all around the site (see image above).

Area 3 – The College and Graveyard

In this area we carried out three surveys: magnetic susceptibility, magnetic gradiometry and earth resistance. Magnetic susceptibility measurements were taken at 5m intervals along transects spaced 5m apart on the survey grid. The results of this survey are presented below in two formats; firstly as a proportional symbol plot and secondly using a colour ramp with an interpolated grid.

The plot shows a relatively uniform susceptibility enhancement across the survey area with values up to 30 SI units. This level of enhancement can be described as elevated and could be due to weakly magnetic soils derived from the volcanic bedrock known to outcrop in the area. The enhancement may in part be also due to settlement, industrial and domestic activity associated with the monument types on the top on the hill. There are a number of isolated ‘hot spots’ to the NW and SW of the survey area. Those to the NW lie on an area of rock outcrop. The ‘hot spot’ to the N and SE of the College may be due to debris or ferrous debris.

Area 3 magnetic susceptibility, proportional symbol plot.

Area 3 magnetic susceptibility, interpolated grid, colour ramp.

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Magnetic gradiometry survey was carried out on 20m x 20m grids along traverse lines set 1m m apart with a sampling interval along each line of 0.25m using a Bartington Grad601-1 magnetic gradiometer over the same area as the magnetic susceptibility and earth resistance surveys.

The main features seen are a series of small anomalies lying E-W near the northern margin of the map. These could be due to shallow magnetic bedrock and/or areas of burning. There is an area of intense magnetic response associated with the present entrance to the College which is due to the large iron bars forming a barrier to livestock entering the building. There is a similar area of intense activity to the E of the College associated with the floodlights and their security cages. At present the most interesting features are the positive gradient linears extending from the SW and SE corners of the College. These are likely to be due to ditches or cut features. The feature to the SE may relate to cabling to the floodlights, the narrower feature to the SW may be an old boundary.

Area 3 magnetic gradiometry plot.

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Earth resistance survey was carried out carried out in zig-zag mode along lines 0.5m apart with a sampling interval of 0.5m using a 0.5m twin probe array.

There is a double linear feature comprised of both higher and lower resistance elements trending W from the SW corner of the College, parallel to the current 19th century graveyard wall. There is a possibility that the S wall of the College is built on top of the line of this feature as part of the linear is visible immediately to the south of this wall. There may be a return in the linear approximately at the College entrance where the linear veers towards the S. This linear feature could be an enclosure/boundary feature or alternatively a trackway. There is a general area of higher resistance in the central part of the W end of the panel. This area has no particularly clear form and may be related to bedrock close to the ground surface. There is some outcrop just outside the survey grid to the NW. This higher resistance may correspond with an anomaly in the gradiometry data from this area.

Area 3 earth resistance plot.

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Area 1 – The motte, motte enclosure and ‘barrow’

Hill-shaded model of the motte, its enclosure and the ‘barrow’ from the NW.

Survey work here concentrated on the low circular feature to the SE of the motte.

The Ordnance Survey Ireland (OSi) LiDAR and orthophoto data has proved invaluable in preliminary mapping of the features in Area 1 which is heavily overgrown in places. The model showed that the large enclosure surrounding the motte overprints the ‘barrow’ feature and so is later in date.

A partial survey of the ‘barrow’ feature was carried out because parts were inaccessible due to dense vegetation. The terrain model indicates that the feature is unlikely to be an enclosure, having a more platform-like profile with central area above surrounding ground level, although there are traces of a ‘lip’ or low bank along the S side of the feature.

‘Barrow’ feature terrain model.

‘Barrow’ feature orthophoto. Note dense vegetation.

Surveys carried out in this area were magnetic susceptibility, magnetic gradiometry and earth resistance.

The susceptibility results within the 60m x 60m survey area are presented below as a colour-coded grid. The background response indicates a lower background enhancement relative to that found in Area 3.  There is a zone of enhancement within the curtilage of the ‘barrow’. This extends to the N from an area of high susceptibility. A further zone of enhancement appears to coincide with the eastern ‘edge’ of the ‘barrow’. A small isolated zone of enhancement is found at the western edge of the ‘barrow’ where it is intersected by the outer enclosure surrounding the motte. This area is close to a hollow in the ground surface and the enhanced susceptibility value here may be indicating a change in the material used to construct the small enclosure/barrow.

Magnetic susceptibility (MS) data over area of ‘barrow’ draped over LiDAR terrain model. (LiDAR data with permission of OSi)

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The gradiometry results from the extended 60m x 60m survey area (below) show a curvilinear anomaly coinciding with the northern edge of the small ‘barrow’ indicating a ditch defining northern edge although there is no ditch evident on the ground or in the LiDAR data. This possible ditch is cut at the W by a weaker, curvilinear feature running in an approximate N – S direction. This weaker feature appears to lie outside the outer enclosing element of the motte. There is a SW – NE zone or possible line of pits/cut features running from the central northern area of the ‘barrow’ across the ditch into the ground outside the barrow to the NE. There is a curvilinear feature of positive gradient in the NE corner of the survey area indicating an arcuate ditch outside the ‘barrow’. The trend of the gentle curvature of the ditch indicates that it may be associated with a presently unknown feature lying to the NE of the survey area.

Magnetic gradiometry data over the ‘barrow’ draped over LiDAR terrain model. (LiDAR data with permission of OSi)

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Earth resistance was carried out in a 60m x 40m area. The general area of the ‘barrow’ is composed of higher resistance with the exception of two areas of lower resistance. The lower resistance area to the W corresponds with the hollowed out area of the ground surface. A larger area of lower resistance in the central northern sector of the ‘barrow’ appears to have a circular form with a small area of higher resistance in the centre. This has the appearance of a second feature perhaps superimposed or cut into the ‘barrow’. There is a possible correlation with anomalies in the MS and MG data which also show anomalous responses in this area.

Earth resistance plot over ‘barrow’ feature draped over LiDAR terrain model. (LiDAR data with permission of OSi)

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