The most famous prehistoric structure in Europe, possibly the world, Stonehenge stands on Salisbury Plain, an area rich in monuments such as long barrows and round barrows. It draws visitors from all over the world but viewing is restricted and it is difficult to get a sense of the grandeur of the place amongst all of the tourists.
   There have been many timescales attached to the building of the site although it is generally held to have been completed in  three identifiable phases over a 1500 year period starting perhaps around 3000BC. The exact sequence of these phases and indeed their sub phases has changed over the years as new evidence from excavation has come to light and absolute dating techniques such as radiocarbon C14 have been applied. The following sequences are based on those proposed by Cleal, Walker and Montague whose work was published in 1995 and accepted by many as currently the most complete picture of construction at the site. Interpretations of exactly what each of these successive changes meant to the builders however are open to conjecture.

Stonehenge Phase 1 (2950-2900BC)
Begun in the late Neolithic, a circular bank nearly 2 metres high and 6 metres wide and with an internal diameter of 85 metres was built with chalk quarried from an outer ditch, the bright white fresh chalk contrasting vividly against the surrounding grassland. The ditch would have been hacked out of the chalk with antler picks and appears to have been cut in individual sections, perhaps echoing the earlier style of monuments known as causewayed enclosures. However here these sections were then joined together to produce a continuous trench about 7 metres wide and 2 metres deep with a causeway or entrance about 4 metres wide to the south and another of 10 metres wide towards the northeast. It is this earthwork structure that give the name 'henge' to other similar monuments elsewhere in the country but unusually at Stonehenge the bank is internal and the ditch external, elsewhere henges have internal ditches and external banks. Stonehenge does have a small external counterscarp bank around the outer edge of the ditch but this appears to only surround the north and eastern sections and may only have been a low mound about 2 or 3 metres in width. The entire diameter of the earthworks was hence about 115 metres. Excavations around the southern entrance revealed a pair of ox jaw bones had been placed in the ditch on either side of the causeway along with an ox skull further to the west of the entrance and a deer tibia to the east. Radiocarbon dating shows that these remains predate the digging of the earthwork by as much as several hundred years suggesting they had been preserved as venerated objects or had perhaps been taken from collections of bones from the preexisting long barrows of Salisbury Plain.
   Also during this phase of construction a ring of fifty-six pits were cut about 5 metres away from the inner edge of the bank. These have been named the 'Aubrey Holes' after their discoverer John Aubrey who originally noticed five circular depressions in 1666 although it was not until the 1920's that their full extent was confirmed and recorded. There is some debate as to whether these pits ever held wooden posts or stones that were later removed.

Stonehenge Phase 2 (2900-2600BC)
It appears that there was a hiatus in construction at the site at the beginning of Phase 2, the ditch was allowed to silt up and if there were timber posts in the Aubrey Holes then these were either removed or rotted away. Later there was some deliberate backfilling of the ditch and the height of the bank was also reduced somewhat. It was during this phase that excavations have revealed a series of timber posts and structures were erected, although what these post actually represented is unclear. The posts are clustered in three areas, the northeast entrance, the southern entrance and the central area of the henge. Roughly in line with the ditch the causeway of the northeastern entrance shows evidence of between seven and nine rows of posts set along a southwest-northeast alignment, and it has been suggested that they formed setting out lines for the observation of the major and minor northern moonrises, another set of four posts that stood about 15 metres northeast could also have been involved with following the movements of the moon. An alternative explanation is that these posts formed the lines of a series of fences allowing or blocking entrance into the central area of the henge. The use of posts to form an entrance corridor does seem to occur near the southern entrance through the earthwork. Here the roughly south-north rectangular arrangement which could have been fenced meets several rows of posts set at a perpendicular angle which may represent further fences or temporary screens. What it was in the central area that was hidden from view is difficult to determine, timber buildings are possible but the many post holes found here do not seem to form any recognisable structures and disturbance of the ground during the subsequent stone building episodes means that whatever these post holes originally represented remains a mystery.
   Excavation of thirty-four of the Aubrey Holes has revealed that twenty-five of them contained cremation remains that date from this phase of the site indicating that the holes were still visible features in the landscape. Fragments of pottery show that these deposits belonged to people associated with the Grooved Ware culture and were mostly found towards the eastern side of the monument, other small pits with cremated remains placed in them have been uncovered in the bottom of the ditch and just within the inside of the bank, again these are nearly all found on the eastern side of the earthwork.

Stonehenge Phase 3 (2600-1600BC)
It is in this third stage of construction spanning the late Neolithic and early Bronze Age that the stones for which the site is so famous finally arrive. As is so often the case at Stonehenge the exact sequences and timescales are subject to debate and reassessment as more evidence comes to light.
Phase 3i
The first stones to be set up within the henge earthwork are thought to be the bluestones (Phase 3i). The original layout appears to have been a pair of concentric semicircles with an average diameter of 25 metres in the centre of the monument with an opening towards the southwest. This setting has been determined by the excavation of two sets of stones pits known as the 'Q' and 'R' holes. This setting was however only short lived with the stones then being removed and the holes backfilled with chalk rubble. Possibly contemporary with this bluestone setting is the erection of the Altar Stone to the southwest of the semicircle which may have acted as a focal point of activity for the setting. This brings us to the most contentious issue surrounding Stonehenge, how these stones got here. About eighty bluestones were used at the site and the rock for them came from different areas around the Preseli mountains in southwest Wales (some possibly from Carn Meini) over 135 miles away. They consists mainly of dolerite and some of rhyolite, both types of volcanic rock with the stones weighing up to 4 tons each. One theory is that they would have been hauled from the mountains to Milford Haven and then loaded onto rafts and brought along the Welsh coast to the Severn estuary, then along the Bristol Avon and the River Frome, via the Wiltshire Avon before being offloaded to the banks of the river. From here they were brought overland to Stonehenge - a total journey distance of 250 miles. The Altar Stone, a block of micaceous sandstone, is thought to have originated from the Brecon Beacons area of south Wales. The opposing theory is that a mixture of stone from west Wales was moved eastwards by glaciation during one of the Ice Ages to be deposited around the Salisbury Plain area as the ice sheets retreated. The builders of Stonehenge then simply used sources of stone that were most readily available, the bluestone that already existed in the Salisbury Plain area. Both theories have faults and it is unlikely that a solution to the problem will be agreed upon for some time.
   Also dating from this phase were four sarsen blocks known as the Station Stones which were erected about 5 metres within the henge bank. Laid out in an almost perfect rectangle only two of these stones now remain but it appears that they were designed to align with the midsummer sunrise, midwinter sunset and the major northern moonset although lines drawn through these stones and a missing stone or post towards the east of the site suggest that other alignments may also have been intended.
Phase 3ii-3v
After the dismantling of the bluestone arc the stones for the sarsen circle and trilithons were brought from the Marlborough Downs, 25 miles to the north (Phase 3ii). Roughly cut to shape before moving, these huge blocks weighing between 6 and 50 tons each may have been placed on sledges and dragged over rollers or temporary trackways to Stonehenge. It is estimated that it would have taken a team of 1000 men seven weeks to move each stone, the whole mammoth task requiring over ten years to complete. Once at the site, the rough sarsen blocks would have been shaped by a team of masons, another task that would have taken several years. To create the Sarsen Circle pits of about 1.5 metres deep were cut in a 30 metre diameter circle and the blocks tipped into them then pulled upright and their tops leveled, but with a protruding tenon to receive the lintels. These lintels had mortises on their undersides to fit the uprights and tongue-and-groove joints to bind them to their neighbours as well as a slight curve along their outer edges. The Sarsen Circle dates from around 2500BC and consisted of thirty uprights each weighing about 25 tons and standing 4 metres in height supporting thirty lintels each weighing 6 tons and forming a ring whose top was nearly 5 metres above ground level. Today only seventeen of the uprights remain standing along with six lintels still in place which fortunately includes the pair of uprights to the northeast that were set slightly further apart than the other circle stones and formed the grand entrance into the inner sanctified area. Within the circle's interior were raised the five huge sarsen trilithons in a horseshoe setting whose open end facing towards the northeast, each trilithon consisting of a pair of uprights whose tops where also cut with a single tenon to secure the mortised lintel. The horseshoe was graduated in height towards the southwest with the northeast opposing pair standing just over 6 metres tall, the inner pair 6.5 metres and the southwest or 'Great Trilithon' towering to a massive 7.3 metres high in total. It is the uprights of the Great Trilithon that are the largest and heaviest stones on the site weighing in at a staggering 50 tons. Sadly today only three of the trilithons stand complete along with a single upright from the remaining pair.
   Although the earlier bluestone setting had been removed it is thought the Altar Stone was re-erected slightly to the northeast near the centre point of the circle within the sarsen horseshoe. The bluestones were now erected in an uncertain arrangement somewhere within the Sarsen Circle (Phase 3iii) before being removed and sixty of the stones placed in circular formation known as the Bluestone Circle between the trilithons and the sarsen circle (Phase 3iv). Within the horseshoe of trilithons a further twenty-three carefully shaped and dressed bluestones were initially set up in oval before those towards the northeast were removed to form the 'Bluestone Horseshoe', the stones graded in height towards the southwest echoing the layout of the larger sarsen trilithons (Phase 3v). Only six stones of this horseshoe remain standing plus eleven stones from the bluestone circle although just over twenty other bluestones remain as either stumps or fallen or broken stone fragments.
   The central area of Stonehenge was now in the layout we see it today but other stones stand, or stood around the area of the northeastern entrance that also date from Phase 3 although they cannot be precisely placed within any of the five sub phases above. The most obvious of these stones is the Heel Stone which stands 20 metres beyond the entrance , a rough unshaped sarsen nearly 5 metres tall that now leans inwards towards the circle. Excavations have revealed that another stone once stood 2 metres to the northwest and together this pair of stones formed a portal through which the midsummer sun could be observed from the centre of Stonehenge as contrary to popular belief the sunrise occurs just to the left of the Heel Stone and not above it at the summer solstice. For some reason the second stone was then removed with a ditch being cut around the remaining Heel Stone. Just next to the entrance bank a fallen sarsen known as the Slaughter Stone is the sole remainder of an alignment of two or three uprights that once stood across the entrance. A plan of Stonehenge drawn by Aubrey in 1666 shows a pair of stones standing within this entrance but sixty years later William Stukeley recorded only the Slaughter Stone which by now had been toppled and its neighbour removed.
   One other major component of the Stonehenge landscape is also believed to date from the third phase, the Stonehenge Avenue is thought to have been built at some point during this time although again where it fits in with the chronology is unclear. Starting on the banks of the River Avon a mile and a quarter southeast of Stonehenge it travels northwest, swings round to the west and then makes a final turn in the depression of Stonehenge Bottom before making its final 500 metre approach up the slight slope to the northeast entrance of the henge. This final section of the Avenue has the same axis as the stones i.e. it forms a corridor or alignment with the midsummer sunrise. Although the Heel Stone is the only stone located within the Avenue's banks and ditches William Stukeley noticed the traces of lines of stone holes which suggested that a double row of stones also once ran along the inside of the earthwork, perhaps fifty stones in all. The results of geophysics surveys in 1979-1980 would seem to support this view.
Phase 3vi
Although no stones were added after Phase 3v there was a final bout of construction within the henge earthworks. Sometime between 1800-1600BC a double ring of pits were cut into the earth and chalk outside of the main stone setting. These have been termed the 'Z' and 'Y' holes and formed rather ragged and inaccurately laid out circles of 38 metres and 54 metres diameter respectively. There appears to be no evidence that these pits ever held stones although it has been suggested that they were designed to hold two rings of bluestones, perhaps the plan was abandoned as Stonehenge fell out of use finally bringing to an end 1500 years of building and remodeling at the site. In 1953 Professor Richard Atkinson who was carrying out excavations at the site made a remarkable discovery of a dagger carved into the face of one of the trilithon uprights (stone 53) and beside it about a dozen faint carvings of axe heads similar in design to Bronze Age flanged axes that are known to date from this period in Stonehenge's history (about 1800BC). Other markings have since been found on other stones, particularly on one of the uprights of the sarsen circle which has possibly two dozen axe heads carved into it and perhaps the carvings on these stones were amongst some of the final acts that took place on the site before the sun finally set on prehistoric activity at the site. Over the 3500 years since then the stones fell into a long period of gradual decay and dilapidation caused by the elements, damage caused by accident as well as deliberate human destruction and the robbing of stones for building material elsewhere.

What was Stonehenge built for?
There have of course been many theories put forward over the years to explain the purpose of Stonehenge, from the plausible to the downright eccentric. It appears that during the building of the henge earthworks in Phase 1 the northeast entrance causeway was deliberately aligned to observe the rising of the moon around the major standstill in its 18.6 year cycle. This concentration on the moon seems to have continued into Phase 2 when lines of posts were set up in the entrance to allow move accurate observations. Later during this phase the Aubrey Holes and the ditch and bank on the eastern side of the earthwork begin to be used for the burial of cremation remains. It is unclear whether the use of the site as a cremation cemetery associated with the Grooved Ware culture was by incomers to the area or by the local population who had adopted the new ideas and beliefs. The beginning of Phase 3 marks a shift in both the ritual and physical alignments of Stonehenge. Whereas the henge causeway has an alignment of about 46.5 degrees from north the stones set up within it are aligned slightly further east at just under 50 degrees - the direction of the sunrise on the solstice, the longest day of the year. This association of Stonehenge with the rising sun probably continued throughout the remainder of its use in prehistory and continues in modern times with the opening of the site to thousands of visitors on the morning of the summer solstice. It seems that Stonehenge never had just one use, as the years passed the people and the rituals enacted within the earthwork and later within area enclosed by the stones changed as the Neolithic moved slowly into the early Bronze Age. What is left to us today gives only a tantalizing glimpse of what may have happened at one of the worlds most enigmatic ancient sites.