Geophysical investigations at the ancient royal site of Rathcroghan, County Roscommon, Ireland

Archaeological ProspectionArchaeol. Prospect. 12, 3–18 (2005)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.242

Geophysical Investigationsat theAncientRoyalSiteofRathcroghan,CountyRoscommon,Ireland

KEVINBARTON1* AND JOE FENWICK2

1 AppliedGeophysicsUnit, NationalUniversity of Ireland,Galway, UniversityRoad,Galway,Ireland2 Department ofArchaeology, NationalUniversityof Ireland, Galway, UniversityRoad,Galway, Ireland

ABSTRACT Thepre-ChristianroyalsiteofRathcroghan(ancientCruachain) issituatedabout5 kmtothenorthwestof thevillage of Tulsk in Co.Roscommon,Ireland.Thearchaeologicalremainsofover 50 visiblemonu-ments, of various type and antiquity, are concentrated over an area of some10 km2. Prominently lo-catedat itsapproximate centre is RathcroghanMound, averaging 88mindiameterand 6minheight.The ArchaeoGeophysical Imaging Project, funded by the Heritage Council, has completed a majorprogramme of archaeological and geophysical surveys at Rathcroghan, concentrating primarily onRathcroghanMound.

Detailedmicrotopographic surveyshaveenabled theproductionof computergeneratedmapsandterrainmodels.Thesehaveaidedtheidentificationofanumberoflow-relieffeaturesthathadremainedunnoticedduringreconnaissance fieldinspection.Magnetic susceptibilitysurveyhasdelimitedzonesofactivityon the top of themoundandhasalso identified an ancient field systemsurrounding it.Mag-neticgradiometrysurvey,conductedatdifferingsampleresolutions,complementsandextendsthere-sults of the magnetic susceptibility survey. An enclosure, 370m in diameter, is identified encirclingRathcroghan Mound and a number of its satellitemonuments. Also identified are the subsurface re-mains of foundation trenches for a series of large-scale, circular structures or enclosures, approxi-mately 20m to 32m in diameter, which once occupied the summit of the mound. Another, 26m indiameter, liesimmediately toitsnorth-northeast.ElectricalresistancesurveyonRathcroghanMound,usingdifferent array types, confirm thepresence ofa series of circularandarcuate features, manyofwhich correspond to thosenotedin themagnetic surveys.

ThecompellingresultsoftheRathcroghansurveyhaveenabledmoreextensiveandmeaningfular-chaeological comparisons to bemadebetween this and the othermajor, pre-Christian, royal centresof Tara (Temhair), Co. Meath, Navan Fort (Emain Macha), Co. Armagh and Knockaulin (Du¤ n Ailinne),Co.Kildare. Copyright� 2005 JohnWiley&Sons,Ltd.

Keywords: geophysics;mound; enclosure; royal; Rathcroghan; Ireland

Introduction

The ArchaeoGeophysical Imaging Project,funded by the Heritage Council and based atthe National University of Ireland, Galway, hascompleted a major programme of archaeological

and geophysical survey at a number of importantarchaeological sites at the Rathcroghan andCarnfree groups of monuments in Co. Roscom-mon, Ireland.

Rathcroghan, equated with the Cruachain ofearly texts, the pre-Christian royal centre forthe ancient province of Connacht, consists of acomplex of over 50 visible monuments concen-trated in an area of some 10 km2 (Waddell, 1983,1988). It is situated about 5 km to the northwest of

Copyright # 2005 John Wiley & Sons, Ltd. Received 19 May 2003Accepted 21 October 2004

* Correspondence to: Kevin Barton, LGS, James Street,Claremorris, County Mayo, Ireland.E-mail: [email protected]

the village of Tulsk (Figure 1). The monuments,of various type and antiquity, are scatteredacross the eastern end of a broad, elevated pla-teau recorded from antiquity as Maigh nAı (theplains of the sheep). They represent a longsequence of human activity from the neolithicto the medieval periods. This gently undulatingterrain remains an area of prime agriculturalgrassland—a legacy of its glacial origins—although today it is subdivided into large rec-tangular fields by substantial limestone wallsand wire fences. Rathcroghan Mound, situatedin the townland of Toberrory, is prominentlylocated at the approximate centre of the complex.

Of equal importance to the archaeologicallegacy of the area is the wealth of early historical,pseudo-historical, mythical and legendary refer-ences. These texts repeatedly emphasize the per-ception of its royal and sacred associations priorto the coming of Christianity and its pivotal roleas a political and symbolic power-base for theprovince of Connacht. Rathcroghan served as aroyal seat, royal cemetery and place of assemblyor oenach. It was from here, so the celebratedlegendary tale, the Tain Bo Cuailnge (The Cattleraid of Cooley) recounts, that Queen Medb con-spired to lead her armies into Ulster to claim theprized brown bull (Kinsella, 1969).

This paper will outline selected results of thegeophysical surveys conducted on RathcroghanMound and its immediate environs. An over-view of the topographical, magnetic susceptibil-ity, fluxgate gradiometry and electrical resistancearea-surveys will be presented along with a briefassessment of the archaeological implications ofthese results. Extensive electrical tomographyand ground penetrating radar (GPR) surveyswere also conducted at Rathcroghan in order toinvestigate the internal structure of the mound;these will be the subject of another paper and arenot discussed here.

This is the first time that such an extensiverange of geophysical techniques has beenapplied to a single archaeological monument inIreland. Initial investigations addressed thequestion of the nature of its internal structureand whether it is of natural or artificial construc-tion. The parameters of the investigation broa-dened as the significance of the results fromsuccessive surveys became apparent. Rathcro-ghan Mound has proved to be a model site onwhich to test and compare the responses of avariety of geophysical instrumentation. In addi-tion, the research provided an ideal platform fora number of student-assisted studies and train-ing courses towards the development of an indi-genous expertise in the field of archaeologicalgeophysics—an aspiration endorsed by theHeritage Council.

Geology, geomorphology and topography

The county of Roscommon comprises part of theCentral Lowland region that developed ongently folded Lower Carboniferous (Brigantian)limestone. The limestone sequence containsmuch reef limestone and is generally light togrey, jointed, fossiliferous, and is oolithic inplaces (Noranda, 1975). At Rathcroghan, oolithic,flaggy limestone is exposed at a depth of about1 m in two small quarries, one lying some 200 mto the northeast of Rathcroghan Mound andanother 200 m to its south.

Although the limestone is karstified in parts ofCo. Roscommon, an extensive glacial drift pre-dominantly obscures the bedrock, typicallybetween 1 m and 20 m in depth (Noranda,1975). The major landform features of the region

Figure 1. Location map: Rathcroghan Mound and (inset) themajor prehistoric royal centresof Ireland.

4 K. Barton and J. Fenwick

Copyright # 2005 John Wiley & Sons, Ltd. Archaeol. Prospect. 12, 3–18 (2005)

are composed primarily of drumlins, kames,eskers and glacial ridges that form tracts ofundulating pasture. A drumlin-dominated land-scape occurs to the west of Tulsk and thesedisappear largely on approaching the moregently undulating terrain of the Rathcroghanplateau. Here, long sinuous ridges, largelyoriented north-northeast–south-southwest andperpendicular to the direction of ice movement,occur in the vicinity. It appears that Rathcroghan

Mound was deliberately sited on the summit onone of these glacial ridges, possibly part of amoraine, to exploit a naturally elevated anddominant position in the landscape (Figure 2).This monument remains as a broad, relativelyfeatureless, flat-topped mound (T1) averaging88 m in basal diameter and 6 m in height abovethe surrounding fields. Two ramps, set into theeastern and western flanks of the Mound today,allow access to its summit.

Figure 2. Topographic surveyofRathcroghanMoundandenvirons: contoursat1mintervals,with fieldwallsand fencesindicated.

Ancient Royal Site of Rathcroghan 5


A number of other monuments occur in theimmediate vicinity of the mound. Two barrows,surviving as low-relief circular depressions, aresituated 65 m to the east of Rathcroghan Mound.A squat standing-stone, known as ‘MilleenMeva’ and a more slender pillar-stone ‘MiosganMeva’ (now recumbent) lie 105 m and 100 m tothe north-northwest and north-northeast of themound respectively. In addition, traces of abarely perceptible, arcuate depression, broadlyconcentric with the mound and discernible for adistance of over 300 m, can also be identifiedsome 130 m to its south. The Toberrory/Glenbal-lythomas townland drystone wall (T2), runningwest-northwest–east-southeast, lies 40 m to thesouth of Rathcroghan Mound. Running beside itare the barely perceptible remains of an ancienttrackway. In more recent years a series of wirefences (T3) have been erected to subdivide theopen grassland surrounding the Mound, some ofwhich have been removed since the completionof the survey.

Geophysical survey—RathcroghanMound and environs

Extensive geophysical and topographical sur-veys were conducted at Rathcroghan between1993 and 1998 (Waddell and Barton, 1995; Fen-wick et al., 1997, 1999). In addition to a detailedtopographical survey, a broader magnetic sus-ceptibility and fluxgate gradiometry survey wasconducted on and in the immediate vicinity ofthe mound, with electrical resistance surveysrestricted to the mound proper. The geophysicaldata were processed using InSite, Geoplot andGeosoft software packages. The images pre-sented here, produced in Geosoft, have under-gone only minimal processing procedures andno filters have been applied.

Magnetic susceptibility

The magnetic susceptibility survey was con-ducted using a Bartington MS2 with MS2Dsearch-loop, which has a depth of investigationin the order of 0.1 m (Clark, 1996). In thisinstance, the sampling strategy utilized two dif-fering survey resolutions, dictated by the level of

detail required. A relatively coarse sample inter-val (2 m station and traverse separation), wasadopted as a reconnaissance sampling strategyin the fields immediately adjacent to the mound.This was done in order to map broad trends inthe susceptibility values, identify zones of poten-tial archaeological interest and to enable a com-parison between these results and those of thegradiometry surveys. The survey of the moundwas conducted at a higher resolution (0.5 m sta-tion and traverse interval) in order to identifyand map potential features of archaeologicalsignificance in greater detail. The combined mag-netic susceptibility image has revealed a numberof anomalous zones and patterns (Figure 3).

The broader survey surrounding the moundreveals two very distinct zonations of magneticsusceptibility to either side of the townland wall(Figure 2, T2). That to the south (S1), in thetownland of Glenballythomas, consists ofbroadly homogeneous, low magnetic susceptibil-ity values whereas that to the north (S2), inToberrory, displays significantly highervalues—with the notable exception of Rathcro-ghan Mound (S3) at the centre of the image. Thezone to the north displays a particularly complexpattern of susceptibility response reflecting fossilagricultural activities that today display noappreciable surface expression. Even at the rela-tively coarse resolution at which the survey wasconducted, it is possible to identify a number ofancient field-plots and associated boundary sub-divisions surrounding the mound. Althoughephemeral, these are expressed as small, regu-larly defined areas of broadly uniform suscept-ibility values (S4) and broad linear anomaliesrespectively (S5). In some instances, it is evenpossible to identify the direction of cultivationridges (S6) within individual plots. Also of noteis the presence of a broad arcuate anomaly (S7)lying 150 m to the west-northwest of Rathcro-ghan Mound.

The comparatively featureless homogeneity ofthe susceptibility values to the south of the town-land wall (S1) is likely to reflect the fact that thisfield was ploughed in more recent decades. Thisactivity is largely responsible for erasing all butthe most deeply buried archaeological features.Fortunately, the area to the north of the townlandwall (S2) has remained relatively undisturbed in



more recent times, which accounts for the pre-servation of near-surface features.

Fluxgate gradiometry

A hierarchical strategy was also adopted for thefluxgate gradiometer survey of RathcroghanMound and its immediate environs with differ-ing survey resolutions applied according to thelevel of detail required (Figure 4). This surveywas conducted using a Geoscan FM36 and sur-

vey data throughout was collected in parallelmode. The majority of the fields surroundingthe mound were surveyed at a 0.5 m stationinterval and 1.0 m traverse separation. The arealying to the east and northeast of the mound waslater resurveyed at a higher resolution (0.5 mstation and traverse interval) in order to survey,in greater detail, the low-relief barrows and anumber of new archaeological features discov-ered during the course of the survey. The samesurvey resolution (0.5 m station and traverse

Figure 3. Magnetic susceptibility surveyof RathcroghanMoundandenvirons.Units ofmeasurement are SI�10�5.



interval) was applied to Rathcroghan Mound,which revealed an extraordinary wealth of over-lapping anomalies of archaeological significance.On the basis of these encouraging results it wasdecided to conduct an even more detailed exam-ination over a 35 m� 35 m area on the flat sum-mit of the mound, utilizing a station and traverseinterval of 0.25 m.

With the exception of Rathcroghan Mounditself, perhaps the most striking feature evidentin the large-scale gradiometry image is the pre-sence of a large circular enclosure (G1), 370 m indiameter, defined as a broad negative–positive–negative anomaly. The southern sector ofthis circle corresponds with a broad (ca. 5 m)low-relief arcuate depression noted in aerial

Figure 4. Magnetic gradiometry surveyof RathcroghanMoundandenvirons.



photographs. A possible continuation of thisenclosure was also noted in the magnetic sus-ceptibility image to the west-northwest of themound (Figure 3, S7) and this was confirmedsubsequently by the gradiometry survey. Inter-estingly, Rathcroghan Mound (G2) is placedeccentrically within this enclosure and the smallcomplex of surrounding archaeological monu-ments, mentioned above, now assume a greatersignificance as these also lie within its interior(Fenwick et al., 1999). Although no obviousentrance to the 370 m enclosure has been identi-fied positively, it originally may have been to theeast as there is a predominance of archaeologicalfeatures within its eastern sector, many of whichhave an easterly alignment.

The difference in the magnetic properties ofthe soils to the south and north of the townlandboundary, noted in the magnetic susceptibilitydata, are also readily apparent in the gradiome-try image. That to the south (G3), remains rela-tively featureless, with the exception of the well-defined signature of the 370 m enclosure. Theresponse to the north (G4), however, is entirelydifferent and displays a wealth of previouslyunrecorded features of archaeological signifi-cance. The extensive evidence of the buriedremains of fossil ridge-and-furrow cultivation,as alternating parallel bands of negative andpositive magnetic gradient (G5), confirms thatnoted in the magnetic susceptibility image (Fig-ure 3, S6). The differing orientations of thispattern of cultivation are clearly visible andagain reflect the former existence of a series oftillage-plots (G6). These surround, but do notappear to have encroached on, the mound. Thispattern is readily visible despite being partiallyobscured by the strong magnetic response of themodern wire fences (G7; Figure 2, T3). Cultiva-tion of the fields surrounding RathcroghanMound has not occurred within living memory.It is possible, therefore, that this fossil tillagepattern and associated plots are of some anti-quity and may potentially date to as early as themedieval period. This cultivation patternappears to be partially superimposed over anumber of more impressive archaeological fea-tures, many of which were previously unrec-orded and only came to light during the courseof fieldwork. The two barrows (G8) lying to the

east of the mound are defined as double-rings ofpositive magnetic gradient. The larger averages26 m in diameter and its diminutive counterpartabout 14 m.

Of particular interest is the presence of a pre-viously unknown circular enclosure (G9), 26 m indiameter, situated on a small elevated platformjust 10 m to the northeast of the base of Rathcro-ghan Mound. Although partially obscured by theoverprint of ridge-and-furrow, this enclosureappears to be composed of two concentric ringsof positive magnetic gradient. The outer circleforms a penannular ring with a possible entrancegap, defined by bulbous terminals, facing dueeast. The inner circle, approximately 17 m indiameter, is composed of an almost contiguousring of equispaced discrete anomalies of positivemagnetic gradient. Additional anomalous zonesalso occur within the interior. These geophysicalfeatures may represent the foundation of a large-scale wooden structure or building. The outerring may correspond to a slot-trench for a woo-den wall or palisade, and the inner ring mayrepresent a series of closely set pits, possiblypost-pits for large upright timbers. Interestingly,two positive magnetic features (G10), attached tothe northeast and southeast quadrants of the26 m enclosure, run eastwards for a distance ofover 100 m and beyond the limits of the magneticgradiometry survey. Both are composed of aclosely set parallel pair of positive magneticlineaments, ca. 3 m apart, and appear to definean axial, funnel-like, approach to the eastwardfacing entrance of the enclosure. A similar east-ward-facing funnel-like approach to Rathcro-ghan Mound is formed by two positivelymagnetic linear anomalies attached to its north-east and southeast quadrants (G11).

In addition to these features, a broad sweepingarc, 80 m in total length, can be identified lying35 m to the north of Rathcroghan Mound (G12)and a pair of parallel, positive magnetic linea-ments (G13), orientated northwest–southeastand separated by a space of approximately 8 m,lie to the southeast of the mound roughly alignedto its eastern ramp. On closer examination, thereare numerous other features of potential archae-ological significance evident in the gradiometryimage, but only the principal features aredetailed here.



Rathcroghan Mound—detailed topographicalsurvey

Rathcroghan Mound is an imposing monument,subcircular in plan, with maximum basal dimen-sions of 86 m north–south and 92 m east–west(Figure 5). The monument, exploiting the naturalelevation of a glacial ridge, varies in height abovethe surrounding fields. Approached from thenortheast it is little more than 4 m in heightcompared with 7 m if approached from the

southeast. Access to the summit is facilitated bythe provision of two gently sloping ramps (T4)set into the eastern and western flanks of themound. Its summit rises to a maximum height of149.9 m above Ordnance Datum and is approxi-mately 64 m in diameter. From a distance itappears to be relatively flat but, in reality, thissurface is slightly domed, rising in a gentlegradient from its edge to a relatively level areasituated slightly north of centre of the mound(T5). A small circular mound (T6), 4 m in

Figure 5. Topographic surveyof RathcroghanMound: contoursat 0.5 mintervals.



diameter and 0.3 m high, is located on this flatarea, a little to the northwest of centre.

Rathcroghan Mound—magnetic susceptibilitysurvey

The magnetic susceptibility survey over Rathcro-ghan Mound presents an interesting picture ofdifferential zonation (Figure 6). In general, themound proper (S3) displays a relatively weaksusceptibility response compared with the sur-

rounding fields (S2). Curiously, a well-definedcircular area of enhanced susceptibility values,approximately 30 m in diameter, occupies thecentral area on top of the mound (S8). On closerinspection it is possible to identify a number offaint concentric and overlapping circular anoma-lies comprising this zone, with a particularlywell-defined circle of relatively high susceptibil-ity values, 25 m in diameter, situated a littleeast of centre of the mound (S9). These featuresare particularly well-defined in the detailed

Figure 6. Magnetic susceptibility surveyof RathcroghanMound.Units ofmeasurement are SI�10�5.



gradiometry survey discussed below. The zoneof enhanced susceptibility on the summit of themound is evidently a result of anthropogenicactivities. It possibly may be related to a spreadof ash or burnt material confined in its extent by acircular delimiting barrier, perhaps a fence, wallor palisade. In this instance, the depth of inves-tigation is in the order of 0.1 m and, evenaccounting for biogenic activity, it is likely thatthe source of these anomalies is correspondinglyshallow.

Rathcroghan Mound—magnetic gradiometrysurvey

The magnetic gradiometry survey was under-taken at a station and traverse interval of 0.5 m.The resulting image reveals a remarkable com-plexity of archaeological activity (Figure 7). Theparallel bands of positive and negative magneticgradient (G5), representing ridge-and-furrowcultivation, clearly stop short of the base of themound. The mound itself appears to be

Figure 7. Magnetic gradiometry surveyof RathcroghanMound.



circumscribed by a broad band of positive mag-netic gradient (G14) forming an irregular subcir-cular girdle marginally upslope from the presentbase. This magnetic anomaly may represent thesubsurface remains of some form of revetment(possibly a wall footing or slot-trench for aretaining palisade) built to contain the moundmaterial, which over the intervening centurieshas eroded to its present angle of repose. It islikely therefore that the mound may once havepresented a more steeply defined edge or per-haps a sheer perimeter facade. If this magneticanomaly marks the true perimeter of the moundthen the maximum dimensions of the originalmonument are approximately 81 m north-southby 85 m east–west. Interestingly, a definite gapoccurs in the eastern sector of this subcircularanomaly (G15) and this is likely to represent anentrance gap allowing access to the eastern rampand the summit of the mound (T4).

The central summit of the mound presents apattern of overlapping large-scale, annularanomalies. The particularly distinct signaturesof two circular features of archaeological signifi-cance dominate this area. The circumference ofthe larger, ca. 31 m in maximum diameter,appears to be defined by a regular and discon-tinuous series of anomalies (G16). The smallerring, 20 m in diameter and situated eccentricallywithin the larger, displays as a particularlystrong positive magnetic gradient against a nega-tive ‘penumbra’ (G17). The larger circular featurecorrelates exactly with the larger circular zonenoted in the magnetic susceptibility image(Figure 6, S8). The strong isolated anomaly onthe north-eastern quadrant of the mound (G18)marks the position of the metal National Monu-ment notice and is, therefore, of no archaeologi-cal consequence.

Detailed gradiometry survey

On the basis of these geophysical results a moredetailed gradiometry survey was undertaken onthe summit at a station and traverse interval of0.25 m (Figure 8c). The resulting image is parti-cularly sharp and has helped to resolve somefeatures noted in the preliminary gradiometry.The largest circular feature (G16) is in fact com-posed of two concentric rings, 32 m and 28 m in

diameter, with circumferences (although largelyobscured to the east) that appear to be defined bya series of equispaced (1.5 m centre-to-centre)discrete, roughly circular, positive magneticanomalies (G19). It is likely that these two ringsare contemporary and part of the same structuralphase as, in addition to being concentric, thediscrete anomalies composing them occur inmutually opposed pairs. It is probable that eachof these discrete anomalies may represent a sub-stantial post-pit (averaging of the order of 0.5 min diameter).

In contrast, the smaller circle (G17), 24 m inmaximum diameter, is composed of a singlecontinuous negative-positive-negative band.On closer examination, however, it is clearlypenannular in plan with a distinct gap in itswestern sector (G20) marked by bulbous term-inals. If this penannular anomaly constitutes thefoundation trench of a wooden enclosure orbuilding it is likely that the interruption in itscircuit represents an entrance gap flanked by apair of upright portal timbers set in large post-pits. There are other potential annular featuresthat can be identified in the image, althoughthese are largely obscured by the strong mag-netic signature of the penannular enclosure(G17), which all but overwhelms lesser mag-netic anomalies in its environs. A hint of onepotential circle (G21) can be discerned in thespace between the large concentric circularanomaly (G16) and the penannular anomaly(G17) and another within its interior (G22).The particularly distinct dipolar anomaly (G23)lying a little to the northeast of centre of thepenannular anomaly marks the position of anOrdnance Survey trigonometric station, lyingflush with the ground surface.

It is interesting to note that the larger post-pitcircle (G16) and the penannular ring (G17) seemto correlate broadly with the larger and smallercircular zones noted in the magnetic susceptibil-ity image (Figure 8b, S8 and S9).

Rathcroghan Mound—electrical resistancesurvey

Square-array surveyThe relative merits of electrode arrays used in

archaeological surveys are discussed in Clark



Figure 8. Composite image of the topographical: (a) magneticsusceptibility; (b) magnetic gradiometry; (c) electrical resis-tance square-array; (d) electrical resistance twin-probe array;(e) surveys of a 35m by 35m area on the summit of Rathcro-ghanMound.



(1980, 1996). The square array is regarded asparticularly effective in the detection and resolu-tion of small-scale features. A Campus Geopulseresistivity meter and prototype square-array,with 0.5 m electrode spacing, was used for thesurvey of Rathcroghan Mound. Measurementswere taken at 0.5 m intervals along parallel trans-ects 0.5 m apart in zig-zag mode. The results ofthe square-array electrical resistance survey

appear to confirm and, in places, complementthose of the susceptibility and gradiometry sur-veys and have identified a number of additionalfeatures of potential archaeological interest(Figure 9). The mound proper (R1) is of relativelyhigh electrical resistance against a backdrop ofgenerally lower values found in the surroundingfields (R2)—with the exception of a curiousarcuate anomaly, of high electrical resistance,

Figure 9. Electrical resistance survey of Rathcroghan Mound: square-array. Units of measurement are Ohms, no geometricalfactor hasbeenapplied.



situated some 10 m from the southeastern base ofRathcroghan Mound (R3). The steeply slopingside of the mound is, for the most part, defined asa broad irregular band of high electrical resis-tance (R4). This may reflect increased drainageand the presence of a greater proportion of stone,which is exposed in places around its perimeterand suggests the presence of an informally con-structed inclined rubble revetment. Of interesttoo, is the fact that the eastern and western ramps(T4) display contrasting signatures. That to theeast is of low electrical resistance (R5) comparedwith the significantly higher values obtained onits western counterpart (R6). This may be indi-cative of the fact that only one may be an originalfeature, possibly the eastern ramp, whereas thewestern approach is arguably the result of laterquarrying activities.

The perplexing complexity of overlappingarcuate and circular anomalies on the summitof the mound presents something of an inter-pretational challenge. The majority of these areill-defined, ephemeral anomalies of relativelylow electrical resistance. Despite this, it is pos-sible to identify a number of distinct features.Two annular features of low resistance, 30 mand 20 m in diameter, are situated centrally ontop of the mound (R7 and R8). Hints of otherpotential circular anomalies, although lessclearly defined, lie in the northwest (R9) andsoutheast (R10) quadrants and appear to over-lap with the larger central circle (R7). There isthe possibility that one (or more) of these mayform a figure-of-eight configuration with thesmaller central circle. In addition to the circularfeatures, numerous low-resistance arcuate ano-malies also can be identified. The most notableof these is a pair of parallel arcs in the south-eastern quadrant (R11). These are set about 4 mapart and run from the edge of the moundtowards the centre in a sweeping curve. Thesmall earthen mound situated on the summit(T6) registers as an amorphous low resistanceanomaly (R12) within the western sector of thesmaller central circle (R8).

It is likely that these overlapping low-resis-tance circular, arcuate and linear anomalies (R7–11) situated on the summit of the mound repre-sent the subsurface remains of drains, palisadeand foundation trenches. It is possible that a

number of these represent, in accordance withthe tentative interpretation of the gradiometrysurvey, a variety of slot-trenches or post-pits dugto support the wooden structural timbers of aseries of substantial enclosures or buildings.

Twin-probe array surveyAs part of a comparative study of the

responses of different electrode arrays, a surveyusing a Geoscan RM15 using the same sampleinterval and electrode a-spacing as that of thesquare-array was conducted over the summit ofthe mound. Although the results of the twin-probe array are broadly similar to the square-array there are a number of notable differences inthe anomaly pattern (Figure 8e).

The larger circle noted in the square-arrayimage (Figure 8d, R7) is also clearly defined asa broad annular, low resistance, feature ofapproximately 30 m in diameter in the twin-probe image (r1). The smaller circle within itsinterior, however, appears not to be defined as aring (Figure 8d, R8) but as a roughly circularmass of low electrical resistance values (r2). Thatarea within the western sector of this smallercircle (r3) corresponds to the small earthenmound (Figure 8a, T6: Figure 8d, R12) situateda little to the northwest of the centre of Rathcro-ghan Mound. In this instance, however, it is notparticularly well defined. The amorphous area ofrelatively high electrical resistance values lyingto the west of the larger circle (r4) correspond toan area lying immediately to the east of the top ofthe western ‘ramp’ (Figure 8a, T4). A number ofthe linear and arcuate anomalies noted in thesquare-array survey are not as well defined in thetwin-probe survey. The pair of parallel arcs(Figure 8d, R11) noted to the southeast of thecentral circles, for instance, is not expressed withthe same degree of clarity in the twin-probesurvey (r5).

The differences in detail between the twoimages may be due largely to the differing sensi-tivity of the array type (Clark, 1980) and theirrespective depth of investigation. In addition,the possibility of differing soil moisture balanceconditions at the time each survey was con-ducted may also have a significant influence onthe results and the nature of features detected(Clark, 1996).



Discussion

The results of the archaeological and geophysicalsurvey have proved to be particularly rewardingand have revealed a wealth of previouslyunknown and unexpected archaeological fea-tures on, and surrounding, Rathcroghan Mound.Indeed, the true significance of the small groupof archaeological monuments in its immediatevicinity was not fully realized prior to the com-mencement of the project. The extensive suite ofcomplementary survey techniques, when inter-preted as a whole, has enabled a more completepicture of the successive development of the siteto be proposed. These are expressed as a myriadof geophysical anomalies representing a palimp-sest of overlapping archaeological features andlater agricultural activities.

It is now evident that Rathcroghan Mound andits surrounding monuments are of especial sig-nificance and perhaps originally perceived asinterrelated elements in a greater ritual precinctdefined by the spatial limits of the 370 m enclo-sure. In addition to the barrows, standing-stonesand trackway, and in spite of the overprint oflater agricultural activity, a complex pattern oflinear, arcuate and circular features have beenidentified and these possibly represent the sub-surface remains of palisade trenches, ditches,drains, and the foundation slot-trenches of anumber of large circular structures or buildings.The overlapping and concentric circles on thesummit of Rathcroghan Mound, for instance,may represent a series of construction phases—perhaps the building and successive replacementof a large circular ceremonial structure or templeon the same spot over a protracted period of time.

Even without the benefit of excavation—stra-tigraphical sequences, associated material cul-ture and scientific dating techniques—it isstill possible to draw compelling comparisonsbetween the survey results at Rathcroghan(Cruachain) and the archaeological remains atthe other, more extensively explored, pre-Christian royal centres of Tara (Temhair), Co.Meath, Navan Fort (Emain Macha), Co. Armaghand Knockaulin (Dun Ailinne), Co. Kildare(Figure 1). The ground plans of excavated exam-ples of large-scale ceremonial timber structuresat Navan Fort, Knockaulin and possibly Tara

bear remarkable similarities to the geophysicalpicture emerging at Rathcroghan (O Rıordain,1965; Wailes, 1990; Cooney and Grogan, 1994;Waterman, 1997). Furthermore, geophysical sur-veys at Navan Fort and Tara have identified awealth of previously unrecorded subsurfacearchaeological remains, among them featuresinterpreted as enclosures, palisade trenches andthe foundation slot-trenches of large-scale timberstructures, broadly comparable to the geophysi-cal signatures of equivalent features at Rathcro-ghan (Ambos et al., 1996; Newman, 1997; Ambosand Larson, 2002; Fenwick and Newman, 2002).

The successful, systematic deployment of arange of geophysical techniques has demon-strated that this approach is preferable, indeednecessary, when surveying multiperiod, com-plex archaeological sites. The value of this ap-proach is amply demonstrated in Figure 8, wherethe various survey techniques conducted overthe summit of Rathcroghan Mound have beenillustrated as a composite image. The archaeolo-gical implications of this multifaceted surveywill necessitate a complete re-evaluation of thenature and significance of ancient Cruachain inits local, regional and wider national context.Furthermore, the successful application of abroad range of geophysical techniques at Rath-croghan has demonstrated its great potential fornon-invasive investigative purposes in otherarchaeological contexts throughout Ireland.

Acknowledgements

The ArchaeoGeophysical Imaging Project, direc-ted by Professor John Waddell and Kevin Barton,would like to express its gratitude to the HeritageCouncil for its generous grants towards thisresearch programme. We are also indebted tothe Tulsk Action Group, the staff of the CruachanAı Heritage Centre, and the local community andlandowners for their continued help and sup-port. We are particularly grateful to Michael andGeraldine Kelly, on whose land RathcroghanMound lies, for their co-operation, support andgenuine interest in the work. Thanks, of course,are also due to all those who assisted with thetopographical and geophysical mapping at Rath-croghan, most especially, Yvonne Brennan, FinnDelaney, Angela Gallagher, Gerard Healy, JohnMadden and Vikram Unnithan.



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Geophysical investigations at the ancient royal site of Rathcroghan, County Roscommon, Ireland

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