The most famous and splendidly decorated of the Irish passage graves, part of the Boyne Valley cemetery, in Meath County. The kidney-shaped mound, dated to c 3100 BC, is over 100 meters in diameter and 13 meters high. The cairn itself was carefully made of alternate layers of stones and turf. A kerb of large stones carved with wavy lines, lozenges, triangles, etc. encloses the base of the mound. On either side of the entrance the green kerbstones were topped by a retaining wall of white quartz. Some distance from the original base of the mound is a surrounding circle of free-standing stones. The burial chamber, cruciform in plan, is roofed by corbelling and has three subsidiary cells; the tomb has a very long passage, 19 meters in length, and built of orthostats. Midwinter sunrise shines through an opening above the door to illuminate the central chamber, the clearest example of an astronomical orientation recorded from a European prehistoric monument. Many stones of both chamber and passage carry pecked designs including an unusual triple spiral. Excavation has shown that the upper surfaces of the capstones had drainage channels, as well as art which would have been invisible once the overlying cairn had been built. Traces of cremation burials were found in the cells of the chamber, and soil from a habitation site, possibly close to the tomb, had been used to pack the interstices of the passage roof. There are two radiocarbon dates around 3200 BC and the site was reoccupied after the tomb-builders had left it and the cairn had begun to slump by a group which used Late Neolithic and Beaker pottery.