Avebury henge is a multi-period site dating back over 4,500 years. It is formed by a surrounding ditch and bank enclosing an area of 28.5 acres (11.5ha) and having a mean inner diameter of 1,140 feet (347m). Not all henges (which are earthen features) contain stone circles by any means, but it happens that Avebury henge encloses the largest stone circle anywhere. The giant sarsens (local sandstone rocks) forming the circle were dragged on wooden sleds from the surrounding downs, primarily from an area called the Grey Wethers. It was a colossal undertaking.
Within this great circle are the remnants of other stone settings, and there is some controversy as to their purpose and precise layout. The usually accepted concept is that there were two inner circles – one in the north half of the great stone ring, the other in the southern sector – which in turn had stone settings in their centres. The north inner circle had a ‘cove’ of which two stones still stand today, and the south inner circle had a massive stone, the Obelisk, now lost, surrounded by a rectilinear arrangement of small stones. Little survives of the north inner circle’s ring of stones; the south inner circle has fared a little better. Holes of missing stones are marked by concrete plinths erected during excavations in the 1930s.
The enclosing ditch of the Avebury henge was originally up to 33 feet (10m) deep, but it is now silted up to over half that depth. This ditch was dug out of the solid chalk with antler picks: it is estimated that 100,000 tons of chalk were removed in this way. The bank is on the outside of the ditch, which tells us that it was never intended for defensive purposes. There were four original entrances to the henge. The stones of the great circle stand on the inner lip of the ditch, forming an overall plan that is in fact far from circular. Many of these stones are lost or still buried. Indeed, much still remains buried and uninvestigated within the henge.