The arrival of the Celts to Ireland around 500 BC heralded the beginning of the Iron Age and the establishment of most of what would define Gaelic Irish culture for millennia; including Primitive Irish, Irish mythology, Celtic paganism and an early form of the Gaelic calendar. The ancient monuments of the Boyne Valley were assimilated into Celtic culture and mythology, with Cú Chulainn said to have been conceived at Newgrange. Furthermore, tradition states that Sláine mac Dela, of the Fir Bolg, cleared the forest at Brú na Bóinne and built the monuments, becoming the first High-King of Ireland. It was during the Celtic period that Meath was divided into 8 túatha, the primary political unit of Celtic Ireland. The túatha were independent petty kingdoms ruled by a chief who was elected by members of their extended family.