These trilithons (an assemblage of two uprights capped by a lintel) are 6.5 m (21 ft) high. Within the trilithons stands a slab of micaceous sandstone known as the Altar Stone.
The entire assemblage is surrounded by a circular ditch 104 m (340 ft) in diameter. On its inner side the ditch rises into a bank within which is a ring of 56 pits known as Aubrey holes (after their discoverer, the antiquarian John Aubrey) and used at a later stage as cremation burial pits. On the north-eastern side, the bank and ditch are intersected by the Avenue, a processional causeway 23 m (75 ft) wide and nearly 3 km (2 mi) long, bordered by a ditch. Near the entrance to the Avenue is the Slaughter Stone, a sarsen stone that may originally have stood upright. Almost opposite, and set within the Avenue, is the Heel Stone, which may have played a part in sightings of the sunrise at the summer solstice.
Stonehenge was built in several stages, probably beginning as a henge monument (ritual enclosure) surrounded by a bank and ditch and similar to many others in southern England. It was around 2200 BC that it took on its unique appearance, 82 bluestones being transported from the Preseli Mountains, in south-western Wales. The Altar Stone is believed to have come from a region near Milford Haven, Dyfed.
Stonehenge was undoubtedly built by a people who had widespread trade connections and who established their principal settlements in the area between 1600 and 1300 BC. Its importance is reflected by the fact that the landscape around the monument is dotted by some 400 barrows, circular mounds enclosing burials, dating from between 2000 and 1500 BC; excavation of some of these barrows has revealed rich grave goods as well as chips of bluestone similar to that found in the concentric ranges.