11. The area is also known for its passage grave discovered in the 1880s. 12. The hill is particularly important for the two possible passage grave structures atop it. 13. These symbols are also commonly used in passage graves found in Ireland and Brittany. 14. On Zealand the chamber of a passage grave on Dysselodden is quite the reverse. 15. Passage graves consisted of a central burial chamber, with a long passageway to the entrance.16. Other notable passage graves are Knowth and Dowth, also in the Boyne Valley near Drogheda. 17. Heapstown Cairn was constructed c . 3000 BC and is believed to enclose a passage grave . 18. There is a kerb of limestone blocks around the edge and probably a passage grave underneath. 19. Later versions featured larger chambers, passage graves , which could be re-used multiple times. 20. The mound is described as a Linkardstown-type cist but may be a simple passage grave .
