Most of the surviving structure is composed of earthworks, known as the dykes, consisting of a massive ditch and external bank henge. Although the henge is not perfectly circular, it has a diameter of about . The only known comparable sites of similar date are only a quarter of the size of Avebury. The ditch alone was 21 metres (69 ft) wide and 11 metres (36 ft) deep, with a sample from its primary fill carbon dated to 3300 - 2630 BC (4300+/-90).