MAGNETIC SURVEY AT NAVAN FORT, NORTHERN IRELAND

Navan Fort, located in County Armagh, played a central role in the Irish Epics and history. Known in the old tongue as Emain Macha, it was the earliest capital of Ulster and the seat of King Conchobar and his champion Cu Chulain. Navan Fort is an 18 acre enclosure on a hilltop marked by a surrounding bank and ditch. At Navan there is much that is unusual. For example, the bank and ditch are in a reverse order, with the ditch being on the inside of the bank. This would have offered a disadvantage to the defenders! Two mounds, excavated during the 1960s, dominate the hilltop. One, which contained two burials, is surrounded by a shallow ditch and a low ring of mounded earth. The second mound is 45m in diameter and six meters high. Excavations showed that in 98 BC (precisely dated through dendrochronology) a gigantic structure 40m across was constructed, supported by hundreds of wooden beams. Soon after it was built the structure was filled with limestone rocks, burned, and covered with several meters of earth to form the mound visible today.

In 1994-5 Dr. Kenneth L. Kvamme, now of the Department of Anthropology, further explored this enigmatic site using a geophysical exploration method known as proton precession magnetometry. In 1994 this work was part of a team that included Dr. Elizabeth A. Ambos of California State University at Long Beach, who conducted a ground penetrating radar survey, and Dr. Daniel O. Larson, the project team leader, also of CSU-LB. In 1995 Dr. Larson performed additional survey with a Cesium vapor magnetometor. This work was sponsored by the Department of the Environment of Northern Ireland, the Queen's University, Belfast, and the site's museum, the fabulous Navan Center.

Proton Precession Magnetometry

Proton magnetometry is an established geophysical prospection method that records variation in the strength of the earth's magnetic field across a region of study. Past human disturbances to the natural geological makeup of a region can subtly alter the magnitude of this field. For example, an excavated ditch that was subsequently filled with rubbish or sediments can yield a magnetic contrast with the surrounding geological matrix because the content of the fill may be somewhat more or less magnetic. Similarly, a buried stone wall or cobbled roadway may be more or less magnetic than the surrounding earth depending on the native properties of the stone employed. By taking a magnetometer and recording the strength of the magnetic field systematically over a surface (e.g., every meter), it is possible to create a map of significant changes in the field, termed "anomalies," some of which may be cultural in origin.

GIS and Magnetometry

Computer processing was performed by a Geographical Information System (GIS), a program specifically designed for the handling and manipulation of spatially distributed data. The GIS allowed (1) the tiling of the 14+ survey blocks into a single image or map, (2) linear interpolation of the one meter data down to 50 cm (allowing less "blocky-looking" and therefore more interpretable output), and (3) the use of image processing techniques to correct or balance the brightness and contrast of the various survey blocks relative to each other, further enhancing their visual interpretation. The basic data are portrayed with white indicating very high (positive) magnetic readings, black showing very low (negative) values, and various grays indicating intermediate measurements. Insets offer local contrast improvements of selected features.

Computer-generated Shadows

The subtle nature of many culturally-derived magnetic anomalies makes it appropriate to explore GIS techniques that can bring out features which may not be initially apparent in the raw data. One such method is to place an artificial, computer-generated light source above the magnetic surface to illuminate those portions that face it, and to cast shadows over regions facing other directions. By placing the light source at low angles and in different places, one can explore very weak patterns in the magnetic surface through their shadows.

Interpretations

Beginning with recent cultural features, two field boundaries are apparent. One is a striking continuous feature running east- west. The second is indicated by a series of strong magnetic highs and lows (caused by bits of iron in the ground). Within the fields is clear evidence of nineteenth century potato planting beds, some 18 in all.

Turning to features of greater archaeological interest, most striking in its size and in the magnitude of its readings is part of the wide earthen "ring" that surrounds the small mound and ditch visible on the surface (far right). Perhaps the most obvious feature in the magnetic imagery is a "double-ring" approximately 30m in diameter. This feature is a circular enclosure similar in form to those encountered in the nearby excavations, but it may have additional significance owing to its larger size and the fact that it lies at the geographical center of the Navan hilltop. The twin rings may represent two slots or sockets (within which vertical timbers were placed) stemming from two episodes of building, a circumstance that closely parallels findings in the excavations where round houses were rebuilt as many as eight times. This feature is completely invisible on the surface and is probably of great significance to an understanding of this site.

Finally, there is the faintest suggestion of several small circular and rectangular features. Their magnetic contrast with surrounding areas is very weak and they may simply be artifacts of noise in the potato planting beds. On the other hand, they may be the first indication of additional habitation structures in this part of the site. One possibility for the more rectangular features is a medieval habitation of the 14th century which is known to have existed near the Mounds of Navan. This is part of the frustration, and promise, of ground-based geophysics. Although we must ultimately rely on excavation to resolve and verify these findings, magnetometry has made a strong case for further excavations and will provide important guidance in their placement.

References

• A Proton Magnetometry Survey at Navan Fort. Emania No. 14:83-88 (1996).

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(last updated: 2/99)