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.