A publication of the Archaeological Institute of America
Geophysical surveys at Rathcroghan, Ireland, seat of the pre-Christian kings of Connacht, have revealed the presence of archaeological features extending well beyond the 300-foot-diameter mound at the center of the site. Two pillar stones, four barrows, and an ancient road were also known in the vicinity, and a shallow, arcuate depression was visible in aerial photographs 500 feet to the south
Scientists from the National University of Ireland, Galway, have taken nearly 150,000 magnetic gradiometry readings in the 30 acres around the great mound. These measurements were used to generate a map with lines of positive and negative magnetic gradient that reflect subsurface features. Parallel, alternating, positive and negative lines may show the buried remains of "ridge-and-furrow" cultivation, possibly medieval, though no surface or historical evidence of tilling survives. Two parallel lines about 25 feet apart leading away from the mound to the southeast may represent an ancient road.
The map also showed that the arcuate depression to the south was one segment of a circular enclosure some 1,200 feet across surrounding the great mound and other monuments in the vicinity. Another circle, 100 feet in diameter, within the larger circle and just northeast of the mound probably represents the buried foundation trench of a large wooden structure. Atop the mound itself were six partly overlapping circles with diameters between 75 and 130 feet, possibly the foundation trenches of buildings or enclosures built and rebuilt on the same spot over several generations.
Though the nature of the features shown on the map can only be verified by excavation, there is already enough information to draw parallels between Rathcroghan and the other major pre-Christian royal centers of Tara, Knockaulin, and Navan Fort. These sites all have ritual enclosures similar to the 1,200-foot one at Rathcroghan, and excavations there have revealed large timber structures similar in plan and scale to the ones at Rathcroghan.