Stonehenge is a prehistoric monument located in the English county of Wiltshire, about 3.2 kilometres (2.0 mi) west of Amesbury and 13 kilometres (8.1 mi) north of Salisbury. One of the most famous sites in the world, Stonehenge is composed of earthworks surrounding a circular setting of large standing stones and sits at the centre of the densest complex of Neolithic and Bronze Age monuments in England, including several hundred burial mounds. Archaeologists had believed that the iconic stone monument was erected around 2500 BC, as described in the chronology below. However one recent theory has suggested that the first stones were not erected until 2400-2200 BC, whilst another suggests that bluestones may have been erected at the site as early as 3000 BC (see phase 1 below). The surrounding circular earth bank and ditch, which constitute the earliest phase of the monument, have been dated to about 3100 BC. The site and its surroundings were added to the UNESCO's list of World Heritage Sites in 1986 in a co-listing with Avebury henge monument, and it is also a legally protected Scheduled Ancient Monument.