central mound of the Boyne Valley passage grave cemetery, the circular cairn in which the cruciform burial chamber is sited having a diameter of over 100 metres. Knowth and Dowth are of comparable size. There is no in situ evidence for earlier activity at the site, save for the spotfinds of flint tools left by Mesolithic hunters. The passage tombs were constructed beginning in around 3,300 BC and work stopped around 2,900 BC. The three largest tombs of Newgrange, Knowth and Dowth may have been constructed to be visible from each other and from northern and southern approaches along the River Boyne, as part of a scheme to "bind the previously disparate elements of the extended passage tomb cemetery into a more clearly defined prehistoric numinous precinct". The area continued to be used for habitation and ritual purposes until the early Bronze Age, during which a number of embanked, pit and wooden post circles (collectively referred to as "henges") were built. Artifacts from the later Bronze Age are comparatively inconspicuous: some cist and ring ditch burials and burnt mounds. For the Iron Age there is only evidence of sporadic activity, such as burials near Knowth and at Rosnaree. Valuable items from the Roman period such as coins and jewelry were found as votive offerings near Newgrange. Numerous other enclosure and megalith sites have been identified within the river bend and have been given simple letter designations, such as the M Enclosures. In addition to the three large tombs, several other ceremonial sites constitute the complex including: Cloghalea Henge Townleyhall passage grave Monknewtown henge and ritual pond Newgrange cursus Astronomical alignments Each of the three main megalith sites have significant archaeoastronomical significance. Newgrange and Dowth have winter solstice solar alignments, while Knowth is oriented towards the spring and autumn equinox. In addition, the immediate environs of the main sites have been investigated for other possible alignments. The layout and design of the Brú na Bóinne complex across the valley has also been studied for astronomical significance. Visitor centre All access to Newgrange and Knowth is by guided tour only, with tours beginning at the Visitor Centre, opened in 1997 in Donore, County Meath. The tourist visitor centre is located on the south side of the river Boyne, and the historical site is located on the north side of the river and is accessed via a shuttle with a tour guide. Public