GEOPHYSICAL SURVEY
Geophysical surveys have been carried out on the entire route. The survey of the Dunshaughlin-Navan section (Section 2) and of an area at Nugentstown on the Navan-Kells section (Section 4) was carried out in 2000 and 2001 as part of the Environmental Impact Assessment by GSB Prospection. In 2002, the remainder of the scheme was surveyed by Bartlett-Clark Consultancy.
SECTION 2 (DUNSHAUGHLIN - NAVAN)
A magnetometer survey (also known as gradiometer survey) was carried out in two stages on the 15.5km Dunshaughlin-Navan section (Section 2). Click here to download the geophysical survey report for the Dunshaughlin-Navan section.
Magnetometer scanning (unrecorded) was used during a gridded walkover of this section of the route (105 hectares) with traverses spaced at intervals of approximately 10m. During this operation, fluctuations in magnetic signal were observed on the instruments' display panel. Any significant variations were investigated more closely to determine their likely origin. The location of these anomalies considered to have archaeological potential was recorded on a map for subsequent detailed survey.
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| Geophysical survey in progress using Fluxgate Gradiometer with handheld trigger .(Courtesy of Shane Rooney) |
The level of magnetic background response along the entire route was found to be low. This helped to identify relatively low fluctuations and areas where only a slight increase in magnetic response was encountered.
Variations in magnetic response that were thought to be of archaeological potential were identified at 30 areas along the route of the proposed road. These varied in character; some contained a number of anomalies and linear responses that could clearly be traced. Others comprised a single response in an otherwise quiet background. In a few cases distinct regions of ferrous interference were observed that
are likely to be of modern origin. However, all potential anomalies were subjected to detailed survey even where the archaeological potential was uncertain.
The second phase of survey involved detailed recorded magnetometer survey of all 30 areas covering 26 hectares (approximately 25% of the available area). Readings were taken at 0.5m x 1m intervals.
The results were spectacular in their clarity. They indicated six definite archaeological sites (one of which was spread over two survey areas). The road was subsequently moved to avoid three of these sites. Twenty-three additional areas were described as possible archaeological sites or areas of archaeological potential.
The sites identified were three large enclosure complexes at Roestown (Fig.2a/b), Baronstown (Fig.3a/b) and Dowdstown (Fig. 4a/b). The morphology of the sites indicated that both Roestown and Dowdstown were likely to be early medieval settlements while Baronstown could be early medieval or prehistoric. The sites subsequently avoided by the route were another enclosure complex at Garretstown (Fig. 5a/b) and two small circular enclosures at Berrillstown (Fig. 6a/b) and Skreen (Fig. 7a/b). The square shape of the enclosure at Garretstown indicated that it was likely to be a medieval moated site, while both of the small circular enclosures were typical of ring ditch burial sites.
ROESTOWN
The geophysical interpretation (shown in 2a) is based on the greyscale image from the geophysical survey (shown in 2b). Features of archaeological potential (and other anomalies such as ploughing trends) are visible in the greyscale image as dark lines against the lighter grey background.
Each of these features or anomalies is distinguished in the geophysical interpretation (2a) as Archaeology (continuous purple line), Possible Archaeology (continuous light blue line), Ploughing Trend (dotted light brown line), Trend (continuous dark brown line) or Ferrous (yellow). These features are shown here in relation to the CPO landtake of the proposed M3 road, which is indicated on the drawing by a dark blue dashed line.
Figure 2a
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| Results of Geophysical Survey at Roestown (Geophysical Interpretation) |
To the east of the present N3 road, the scan produced a substantial U-shaped enclosure with internal ditched subdivisions and traces of a radial field system to the southeast. There was also a long linear feature extending to the northeast (possibly a trackway) and another linear feature that may have been the result of modern drainage. There were pits within and to the south of the enclosure.
At the western side of the N3 road, the scan identified a substantial D-shaped enclosure (approximately 10m by 55m) with a subcircular annexe with radial subdivisions; there were traces of a radial field system to the south. Some of the radial ditches may be later field-system features superimposed on the enclosure after it had gone out of use. The D-shaped annexe also had internal sub-divisions, one of which was rectangular, the other curved. A deep modern ditch along the southern boundary of the site showed extensive ferrous disturbance, probably due to both the hardcore around the gates and the metal fences and gates.
Figure 2b
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| Results of Geophysical Survey at Roestown (Greyscale Image) |
BARONSTOWN
The geophysical interpretation (shown in 3a) is based on the greyscale image from the geophysical survey (shown in 3b). Features of archaeological potential (and other anomalies such as ploughing trends) are visible in the greyscale image as dark lines against the lighter grey background.
Each of these features or anomalies is distinguished in the geophysical interpretation (3a) as Archaeology (continuous purple line), Possible Archaeology (continuous light blue line), Ploughing Trend (dotted light brown line), Trend (continuous dark brown line) or Ferrous (yellow). These features are shown here in relation to the CPO landtake of the proposed M3 road, which is indicated on the drawing by a dark blue dashed line.
Figure 3a
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| Results of Geophysical Survey at Baronstown (Geophysical Interpretation) |
A substantial circular enclosure (45m in diameter) within an oval system of earthworks was recorded at the top of a slope in the southwestern corner of a near circular field (approximate diameter of 170m), suggesting the field boundary itself is of archaeological significance. Extending out from the enclosure was a series of ditch anomalies, some enclosing small fields on the northern side. The linear responses on the western side of the survey area coincided with steep slopes down to a stream and may, therefore, be partly topographic in origin.
A semi-circular ditch within the enclosure appeared to divide off a possible dwelling. While no anomalies indicative of occupation were apparent within this feature, pit anomalies both inside and outside the circular enclosure indicated occupation. A series of linear trends aligned approximately north-south is likely to be due to more recent cultivation. Other weak linear features may relate to field systems associated with the enclosure.
Figure 3b
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| Results of Geophysical Survey at Baronstown (Greyscale Image) |
DOWDSTOWN
The geophysical interpretation (shown in 4a) is based on the greyscale image from the geophysical survey (shown in 4b). Features of archaeological potential (and other anomalies such as ploughing trends) are visible in the greyscale image as dark lines against the lighter grey background.
Each of these features or anomalies is distinguished in the geophysical interpretation (4a) as Archaeology (continuous purple line), Possible Archaeology (continuous light blue line), Ploughing Trend (dotted light brown line), Trend (continuous dark brown line) or Ferrous (yellow). These features are shown here in relation to the CPO landtake of the proposed M3 road, which is indicated on the drawing by a dark blue dashed line.
Figure 4a
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| Results of Geophysical Survey at Dowdstown (Geophysical Interpretation) |
The scan of the northern section of this area revealed a large rectangular enclosure or field defined by narrow ditches. A large complex settlement site consisting of a circular enclosure (30m internal diameter) with a large annexe subdivided into smaller sections was recorded on the southern section; traces of a substantial field system were found to the east and north of the central enclosure. There was another possible circular enclosure defined by much narrower ditches at the corner of one of the fields. There were also traces of modern pipes and field boundaries in this area, as well as some magnetic interference caused by electricity poles.
Figure 4b
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| Results of Geophysical Survey at Dowdstown (Greyscale Image) |
GARRETSTOWN
The geophysical interpretation (shown in 5a) is based on the greyscale image from the geophysical survey (shown in 5b). Features of archaeological potential (and other anomalies such as ploughing trends) are visible in the greyscale image as dark lines against the lighter grey background.
Each of these features or anomalies is distinguished in the geophysical interpretation (5a) as Archaeology (continuous purple line), Possible Archaeology (continuous light blue line), Ploughing Trend (dotted light brown line), Trend (continuous dark brown line) or Ferrous (yellow). These features are shown here in relation to the CPO landtake of the proposed M3 road, which is indicated on the drawing by a dark blue dashed line.
Figure 5a
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| Results of Geophysical Survey at Garretstown (Geophysical Interpretation) |
The scan revealed a substantial, virtually square enclosure (approximately 60m by 60m), with a smaller subcircular enclosure with several internal divisions. There was an extensive attached field system to the north, part of which was formed by an enclosure that appeared to show traces of a hearth; there was also a sub-triangular field with a subcircular internal enclosure to the northeast. The site appears to be a settlement site, and the shape of the enclosure and size of the ditch suggest that it may be an Anglo-Norman moated site. The readings also suggested that the complex extends further south from the scanned area. The road alignment has been shifted to only a small area at the northeast corner of the site.
Figure 5b
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| Results of Geophysical Survey at Garretstown (Greyscale Image) |
BERRILLSTOWN
The geophysical interpretation (shown in 6a) is based on the greyscale image from the geophysical survey (shown in 6b). Features of archaeological potential (and other anomalies such as ploughing trends) are visible in the greyscale image as dark lines against the lighter grey background.
Each of these features or anomalies is distinguished in the geophysical interpretation (6a) as Archaeology (continuous purple line), Possible Archaeology (continuous light blue line), Ploughing Trend (dotted light brown line), Trend (continuous dark brown line) or Ferrous (yellow). These features are shown here in relation to the CPO landtake of the proposed M3 road, which is indicated on the drawing by a dark blue dashed line.
Figure 6a
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| Results of Geophysical Survey at Berrillstown (Geophysical Interpretation) |
This site was located at the top of a hillock and was represented by linear features and a penannular enclosure (30m diameter) associated with a group of pits. The features could represent either a partially enclosed settlement with storage pits or a burial site, possibly a type of barrow with associated burial pits. A cluster of pits and short ditches was recorded on the centreline of the proposed road; the road alignment has been shifted to the east to avoid the circular enclosure. The numerous pit anomalies surrounding the enclosure could represent individual burials, although it is rare to detect such features using geophysical techniques. To the east were several weak anomalies, possibly buried ditch features representing part of a settlement site, although they may be due to natural soil variations or agricultural disturbance. Recent plough disturbance was indicated by a series of lines running east-west. The results suggest that archaeological deposits have been damaged or destroyed by ploughing. Linear anomalies running east-west to the south may relate to former field boundaries.
Figure 6b
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| Results of Geophysical Survey at Berrillstown (Greyscale Image) |
SKREEN
The geophysical interpretation (shown in 7a) is based on the greyscale image from the geophysical survey (shown in 7b). Features of archaeological potential (and other anomalies such as ploughing trends) are visible in the greyscale image as dark lines against the lighter grey background.
Each of these features or anomalies is distinguished in the geophysical interpretation (7a) as Archaeology (continuous purple line), Possible Archaeology (continuous light blue line), Ploughing Trend (dotted light brown line), Trend (continuous dark brown line) or Ferrous (yellow). These features are shown here in relation to the CPO landtake of the proposed M3 road, which is indicated on the drawing by a dark blue dashed line.
Figure 7a
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| Results of Geophysical Survey at Skreen (Geophysical Interpretation) |
This site is overlooked by Skreen and is in the immediate vicinity of an enclosure site (ME032:031) and a ringfort (MEO32:032). The scan produced a series of linear features that appeared to represent the remains of a field system and a penannular ditch enclosure, probably a barrow. One linear feature extended southwestwards from the barrow and may be an integral part of the feature. Several pit features near the northern edge of the survey area may be natural in origin, as they appeared to coincide with a hollow area on the ground. Another anomaly, showing as an area of burnt material, appeared to coincide with an area of modern ferrous debris and was probably modern. The road alignment has been shifted to the west to avoid the barrow feature.
Figure 7b
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| Results of Geophysical Survey at Skreen (Greyscale Image) |
SECTION 1 & SECTIONS 3-5
A magnetometer survey was also carried out over a 9-hectare area in Nugentstown on the Navan-Kells section (Section 4), where a possible burnt mound was identified. The remainder of the route (Sections 1, 3-5) was investigated by magnetometer and magnetic susceptibility survey. Magnetometer readings were recorded along parallel strips 9m wide spaced 11m apart running along the length of the landtake. This amounted to some 45% of the total site area. The magnetic susceptibility survey was generally carried out along the magnetometer strips, with readings taken at 12.5m intervals.
The magnetic susceptibility survey was used to complement the magnetometer survey. The results obtainable from magnetometer and magnetic susceptibility surveys are related, but they will not necessarily detect the same features or disturbances. The magnetometer survey responds to cut features such as ditches and pits when they are silted with topsoil, which usually has a higher magnetic susceptibility than the natural subsoil. It also detects fired materials such as kilns and hearths. Magnetic susceptibility survey detects the disturbed (either by agriculture or worm action) traces of such features in the topsoil.
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| Geophysical survey in progress using a Bartington Magnetometer. (Courtesy of GSB Prospection) |
Although this survey sampling strategy was arguably more scientifically objective, by relying less on the skill of the surveyor and more on achieving a higher and more evenly spaced coverage, the results of this survey were less clear and definitive than that carried out on the Dunshaughlin-Navan section.
In this survey no sites were clearly identified as being definitely archaeological. Sixty-four areas were identified as potentially being archaeological, these were classified as 16 probably archaeological, nine possibly or probably archaeological and 39 possibly archaeological or modern.
GLOSSARY OF TERMS USED IN GEOPHYSICAL INTERPRETATION [AFTER GSB PROSPECTION]
Archaeology
This term is used when the form, nature and pattern of the response are clearly archaeological but where no supporting evidence exists. These anomalies, whilst considered anthropogenic, could be of any age.
Ditch / Pit
This category is used only when other evidence is available that supports a clear archaeological interpretation e.g. cropmarks or excavation.
Ferrous (Response)
This type of response is associated with ferrous material and may result from small items in the topsoil, larger buried objects such as pipes or above ground features such as fencelines or pylons. Ferrous responses are usually regarded as modern. Individual burnt stones, fired bricks or igneous rocks can produce responses similar to ferrous material.
Natural
These responses form clear patterns in geographical zones where natural variations are known to produce significant magnetic distortions e.g. palaeochannels or magnetic gravels.
Possible Archaeology
The interpretation of such anomalies is often tentative, with the anomalies exhibiting either weak signal strength or forming incomplete archaeological patterns. They may be the result of variable soil depth, plough damage or even aliasing as a result of data collection orientation.
Ploughing Trend
These are isolated or grouped linear responses. They are normally narrow and are presumed modern when aligned to current field boundaries or following present ploughing.
Trend
This is usually an ill-defined, weak or isolated linear anomaly of unknown cause or date.
