The prehistoric monument known as Stonehenge includes a circular arrangement of massive, upright stones surrounded by a large circular earthen embankment. It was built between about 3100 and 1500 bc and is located about 8 miles (13 kilometers) northwest of Salisbury, in southern England. There are hundreds of similar structures throughout Britain. Stonehenge and the nearby circular monument called Avebury were designated as UNESCO World Heritage sites in 1986.
Stonehenge is believed to have been a place of worship of some kind. Many explanations have been offered as to why the enormous monument was built, some speculating that it was a type of astronomical clock for predicting eclipses or a temple for sky worship. These ideas have never been proven, however, and the specific reasons for its construction remain unknown. An earlier notion connected Stonehenge to the Druids, a caste of Celtic priests, but it is now known that the monument was built long before Druids reached the area.
Today only ruins of Stonehenge remain. In medieval and early modern times people took stones from the monument to use in construction projects, and the stones that remain have been subjected to thousands of years of weathering. The modern scientific understanding of Stonehenge is based on archaeological excavations conducted since 1919, and especially since 1950. The excavations revealed that there were three main periods of building, now called Stonehenge I, II, and III.
The first stage of construction began in about 3100 bc during the late Neolithic period. Using deer antlers as picks, the builders dug a circular ditch some 320 feet (98 meters) in diameter. The ditch was about 20 feet (6 meters) wide and 4.5 to 7 feet (1.4 to 2 meters) deep. A ring of 56 shallow pits, now known as Aubrey Holes, were dug just beyond the inside bank of the ditch, but they were refilled very soon after. Two parallel entry stones were also erected. One of these, today called the Slaughter Stone, still stands. Stonehenge I was used for about 500 years and then abandoned.
During the second period, in probably about 2100 bc, about 80 huge pillars of rock were brought from what is now southwestern Wales. The pillars were of a type of rock called bluestone and weighed up to 4 tons each. They were erected in what were to be two concentric circles around the center of the site. The double circle was never completed and was dismantled during the following period. A large approach to the double circle, now called the Avenue, was also built. The Avenue and the entranceway to the double circle were aligned with the sunrise at the summer solstice. Their placement was one of the most sophisticated accomplishments of that age and provides evidence that early people used astronomy.
The monument was remodeled again beginning in about 2000 bc. A circle of 30 upright stones—sandstone blocks each up to 30 feet (9 meters) long and 50 tons in weight—was erected and capped by a ring of stone lintels. They enclosed a horseshoe-shaped formation of five pairs of stone uprights, each pair capped with a stone lintel. Subsequent changes involved adding, removing, and rearranging stones that had been used during the second period. This final phase of building probably ended before 1500 bc. Today only 17 of the massive upright stones still stand.