In about 2500 BC the stones were set up in the centre of the monument. Two types of stone are used at Stonehenge – the larger sarsens and the smaller ‘bluestones’. The sarsens were erected in two concentric arrangements – an inner horseshoe and an outer circle – and the bluestones were set up between them in a double arc. (For more about how the stones were shaped and erected and the different geologies of the stones, see Building Stonehenge.)
Probably at the same time that the stones were being set up in the centre of the monument, the sarsens close to the entrance were raised, together with the four Station Stones on the periphery.
About 200 or 300 years later the central bluestones were rearranged to form a circle and inner oval (which was again later altered to form a horseshoe). The earthwork Avenue was also built at this time, connecting Stonehenge with the river Avon.
One of the last prehistoric activities at Stonehenge was the digging around the stone settings of two rings of concentric pits, the so-called Y and Z holes, radiocarbon dated by antlers within them to between 1800 and 1500 BC. They may have been intended for a rearrangement of the stones that was never completed.