TTTHEHEHE ‘B ROCH ’ I’ I NNN G EOPHYSICAL S URVEY O NNN O RKNEY

Mary K. Saunders and Dr Susan Ovenden College Geophysics Unit, Orkney College, East Road, , Orkney, KW15 1LX

Introduction chaeological responses, a high level of ‘background’ magnetism causes In the last 10 years, geophysical survey has been used readily across the magnetic enhancement of areas of activity to be much greater than Orkney and has offered the opportunity to investigate the use of the tech- that observed over similar sorts of features in areas with other types of nique in the identification of like structures and their surrounding non igneous geology. extramural settlements. The characteristic response of a gradiometer to a dipole (e.g. a ditch) buried at 0.5m for various latitudes can be seen below. It can be seen The Broch in Orkney that in Orkney (59 o N) the characteristic anomaly for a simple cut feature The most coherent study of the brochs of Orkney is perhaps Hedges’ should consist of a strong positive anomaly with a synthesis which appeared as the third part of his Bu, Gurness and the negative component to the north. Brochs of Orkney volume. With this, he makes an attempt to highlight both the similarities and differences which are found within the wider ge- Because of the highly magnetic background and level nus of the ‘Orkney broch’. It is clear from his work that there is no single of enhancement over archaeological features, struc- blueprint for these structures, instead it would appear that there is great tural remains constructed from materials such as variation in the size and form of the sites in question. Further still, sandstone, often manifest themselves as negative lin- Hedges suggests that factors such as the position of the doors and ac- ear anomalies, without a positive component. It also cess to stairs and galleries differs so much that it is even difficult to iden- seems settlement activity creates areas of ‘noise’ as tify sub groups within the structures classed as brochs (Hedges 1987). highly magnetic material such as midden and occupa- tion layers accumulate.

One of the key strengths of Hedges’ work was to bring together a list of Magnetic responses over a dipole 52 known and 80 potential brochs — a staggering number for a group of buried at 0.5m for various latitudes islands measuring c. 60km 2 . In the 20 years since this list was drawn up, (Clark 1990) further broch sites have been identified and others discounted. Excava- tion of all these sites, potential or otherwise, would be unthinkable, but it The ‘broch’ as a geophysical anomaly is with the use of archaeological geophysical techniques such as mag- The key archaeological components of a broch will dictate the nature of netic survey, that we can begin to rapidly characterise and partly interpret the geophysical responses detected over it. Simplistically, we know that these sites in a realistic and affordable fashion. settlement activity should have occurred in and around it and particularly within any extramural structures. The broch itself is likely to have been Magnetic enhancement of archaeological features constructed from a non magnetic stone and the whole settlement sur- The geology of Orkney is characterised by old red sandstone, overlain in rounded by ditches and possibly revetment walls. places by boulder clays. Within this, numerous bands of out cropping ig- neous rock are encountered (BGS 1999). This igneous material is ex- tremely magnetically enhanced and gives rise to the presence of various ferrous oxides and titano-magnetites in the subsoil and topsoil derived from it. In turn, these soils have a greater magnetic susceptibility than may be expected over other types of geology.

Why archaeological features give rise to an enhanced level of magnetic Distribution of broch like structures across Orkney, together with the location of the sites dealt with in this poster. After Hedges 1987 susceptibility in such a specific fashion has been the subject of ongoing research since the phenomenon was first observed in the 1950s. Burn- Geophysically, we would expect to see a large or series of large curvilin- ing and other anthropogenic activities increase the magnetic susceptibil- ear positive anomalies. Some more pronounced negative anomalies ity of the soil in the immediate vicinity and the way that archaeological de- may also be present in this area. Within (and possibly slightly impinging posits are laid down, the levels of magnetic minerals in the surrounding Left: Characteristic responses from an area of settlement activity, Links of Noltland, Westray. Right: Mine Howe, Orkney. on) these anomalies, an area of jumbled positive and negative re- soil matrix and the presence of organic material also contribute to en- Many of the characteristic broch type responses are visible on this site but crucially there appears to be little indicators of internal structures or settlement (image courtesy of GSB Prospection Ltd) sponses would be noted. Within in this noise, there may be a small sub hancement, but the precise mechanism by which this occurs is poorly un- circular ‘quiet’ area, possible enclosed by negative anomalies. derstood (Hesse 1978). What is clear, is that rather than obscuring ar-

SKAILL THE CAIRNS This site was identified in August 2008 during a large The Cairns can be found on the East side scale survey by OCGU on the Heart of Neolithic Ork- of the Bay of Ireland, south of Brig o’ ney World Heritage Area near . Waithe. It was surveyed by OCGU as The geophysical anomalies identified on this site part of the SCAPE coastal erosion pro- indicate the presence of a very large scale feature with a ject and was also listed as a possible large amount of settlement activity occurring in and broch by Hedges. around it. The large concentric anomalies may repre- Although the survey area was lim- sent ditches but the strong negative components indi- ited, the anomalies encountered were ex- cate that revetment walls may also be located in this 333 tremely strong and definitely suggestive

area. It is possible that the ‘noisy’ anomalies, partly ob- of settlement activity of some sort. scuring these responses, are the result of later activity— nTnTnT There is some suggestion of an en-

perhaps the continuing development and reuse of the 333 closing anomaly on the seawards side of site. the site, together with a series of re- ---1-111 A small quiet area is visible within the inner circu- nTnTnT sponses which may relate to further lar anomaly and could represent the location of the cen- ditches. The very strongest anomalies

tral structure. 000 50m were seen within the scheduled area but ---1-111 it is clear that the site extends to the east of this. Further survey should allow bet- HODGALEE ter characterisation of the site, particu- Hodgalee, on the island of Westray, was 333 larly in identifying the nature of enclo- 000 50m listed by Hodges as a potential broch site. sure on the east side. Much of the site is eroding into the sea and nTnTnT

large structural remains area visible in sec- tion. A large mound remains extant but HESTIGEO ---1-111 this appears to be formed by tips of rela- Located in South Walls, east of Hoy, this site has been subject tively sterile material overlying some occu- to a great deal of coastal erosion, leading to the loss of approxi- pation layers. The site was surveyed in mately one third of the site. This was listed by Hodges as a possible broch. 2008 by OCGU as part of the SCAPE pro- 000 50m ject. Here one large circular anomaly dominates the results and 333 suggests the presence of a large enclosure ditch. In form it is The anomalies observed on this site are very much weaker than those seen on the other broch sites, suggesting that in this area, at least, intensive settlement did not not dissimilar to the responses found at St Peter’s Bay or indeed nTnTnT occur. Several negative linear anomalies are indicative of the presence of walls but to the Mine Howe ditch. 000 50m Smaller anomalies on the same alignment may indicate fur- there is no clear enclosure ditch. ---1-111 ther ditches, revetment or a combination of both. The ‘noisy’ positive and negative anomalies suggest very definite settle- ment activity but it is interesting to note that these anomalies do BIG HOWE not extend much beyond the ditch. Big Howe is located on a small mound in the fields to Within what was once the centre of the site, a ‘quiet’ area is visible. It is very possible that this indicates the position the south east of the within the WHA of the central structure. It is also interesting to note the igneous dyke running through the site. inner buffer zone. It was surveyed by GSB Prospection Ltd. ST PETER’S BAY This site is dominated by a ‘light bulb’ shaped anomaly, suggestive of the presence of a ditch, perhaps St Peter’s Bay is located on the west side of with associated walls. The shape of this anomaly is Deerness in east Orkney. The site was sur- quite different to that observed on the other surveyed veyed by students on the MA Archaeological sites, perhaps indicating a difference in date, form or Practice degree as part of their geophysics module. construction methods. Within this outer anomaly, an in- 333 ternal sub-circular response can be observed. This may The most notable aspect of the results is the strength of the anomalies found across the relate to some kind of defensive inner wall. nTnTnT site. It is clear that there has been a great Once again, the characteristic settlement deposits are evident but their spatial location is curious. The deal of settlement activity in this area. ---1-111 333 The site is enclosed by a ditch like fea- area around the enclosure entrance seems very quiet but the extension of the anomalies across the opposite ditch ture, very similar in size and form to that nTnTnT found at Hestigeo. The large negative com- may suggest a continuation of settlement long after the ponent to this anomaly may well suggest the initial structure fell out of use. Once again an igneous dyke can be seen extending ---1-111 presence of some large masonary remains, al- through the site. 000 50m though it could be an artefact of the shear 000 50m strength of the observed responses.

Conclusion Clearly there are limitations to what can be identified that there are many different types of broch structure Bibliography British Geological Society, 1999, Orkney Islands, solid and drift geology It is clear that geophysical survey provides a useful purely by geophysical survey alone, but it does identifiable throughout Orkney. The systematic use seem particularly useful in identifying large enclos- of geophysical survey on more sites can only serve Clark, A. J., 1990, Seeing Beneath the Soil. Batsford. London means by which to begin to characterise the nature Hedges, J. W. 1987, Bu, Gurness and the Brochs of Orkney, Part III: The and form of broch type sites, without out the use of ing features and areas of intense settlement. to increases our knowledge about the types of Brochs of Orkney. BAR 165. BAR. Oxford anomaly we can expect to encounter and to what ar- Hesse, A., 1978, Manuel de prospection geophysique appliqué a la recon- excavation. naissance archeologique. Universite de Bourgogne This initial phase of works supports Hedges’ opinion chaeological features they relate.