Stonehenge in Britain was not built in a single project. Its ditch, stone circles, and other structures were created in different phases, and further prehistoric remains surround the site.
Some stones came from distant regions. Its relationship with astronomical alignments is one subject of research, while specific ceremonies and uses remain partly unknown.
Stonehenge 1 (c. 3100 BC)
The first monument consisted of a circular bank and ditch enclosure made of Late Cretaceous (Santonian Age) Seaford chalk, measuring about 360 feet (110 m) in diameter, with a large entrance to the north east and a smaller one to the south. It stood in open grassland on a slightly sloping spot. The builders placed the bones of deer and oxen in the bottom of the ditch, as well as some worked flint tools. The bones were considerably older than the antler picks used to dig the ditch, and the people who buried them had looked after them for some time prior to burial. The ditch was continuous but had been dug in sections, like the ditches of the earlier causewayed enclosures in the area. The chalk dug from the ditch was piled up to form the bank.
Stonehenge 2 (c. 2900 BC)
The second phase of construction occurred approximately between 2900 and 2600 BC. The number of postholes dating to the early third millennium BC suggests that some form of timber structure was built within the enclosure during this period. Further standing timbers were placed at the northeast entrance, and a parallel alignment of posts ran inwards from the southern entrance. The postholes are smaller than the Aubrey Holes, being only around 16 inches (0.4 m) in diameter, and are much less regularly spaced. The bank was purposely reduced in height, and the ditch continued to silt up.
At least twenty-five of the Aubrey Holes are known to have contained later, intrusive, cremation burials dating to the two centuries after the monument's inception. It seems that whatever the holes' original function was, it became a funerary one during Phase two. Thirty further cremations were placed in the enclosure's ditch and at other points within the monument, mostly in the eastern half. Stonehenge is therefore interpreted as functioning as an enclosed cremation cemetery at this time, the earliest known cremation cemetery in the British Isles. Fragments of unburnt human bone have also been found in the ditch-fill. Dating evidence is provided by the late Neolithic grooved ware pottery found in association with the features of this phase.
Stonehenge 3 II (2600 BC to 2400 BC)
During the next major phase of activity, 30 enormous Oligocene–Miocene sarsen stones (shown grey on the plan) were brought to the site. They came from a quarry around 16 miles (26 km) north of Stonehenge, in West Woods, Wiltshire. The stones were dressed and fashioned with mortise and tenon joints before 30 sarsens were erected in a circle of standing stones approximately 98 feet (30 m) in diameter, with a ring of 30 lintel stones resting on top. The lintels were fitted to one another using tongue and groove joints – a woodworking method, again. Each standing stone was around 13.5 feet (4.11 m) high, 7.0 feet (2.13 m) wide, and 3.5 feet (1.06 m) deep, weighing around 26 tons.
Each had clearly been worked with the final visual effect in mind: The orthostats widen slightly towards the top to keep their perspective constant when viewed from the ground, while the lintel stones curve slightly to continue the circular appearance of the earlier monument.
The inward-facing surfaces of the stones are smoother and more finely worked than the outer surfaces. The average thickness of the stones is 3.6 feet (1.1 m) and the average distance between them is 3.3 feet (1 m). A total of 75 stones would have been needed to complete the circle (60 stones) and the trilithon horseshoe (15 stones). It was thought the ring might have been left incomplete, but an exceptionally dry summer in 2013 revealed patches of parched grass which may correspond to the location of missing sarsens. The lintel stones are each around 10 feet (3.2 m) long, 3.3 feet (1 m) wide and 2.6 feet (0.8 m) thick. The tops of the lintels are 16 feet (4.9 m) above the ground.