Stonehenge is a Neolithic site located near Amesbury in Wiltshire, England, about 13 kilometers northwest of Salisbury. It is composed of a circular set of large stones erected, known as megaliths. The stones of Stonehenge have their current alignment with the reconstruction work in the first half of the twentieth century. The site was added to the UNESCO World Heritage List in 1986. In addition to being a mass tourism destination, Stonehenge is currently a pilgrimage site for many followers of Celticism, Wicca, and other neopagan religions, and was a theater of a free music festival between 1972 and 1984; in 1985 this festival was banned by the British government because of the violent confrontation between the police and some participants who became known as the Battle of Beanfield. The stone of the altar: a block of five meters of green sandstone. The main stones are made up of an extremely hard form of silica sandstone, which is naturally about thirty miles further north, on the Marlborough Downs. The internal structure, known as Bluestone Horseshoe, is made up of much smaller stones, which weighing on average four tons. These stones were extracted from the Presele Mountains, in southwest Wales. They are mainly painful but include examples of riolite, sandstone and volcanic calcareous ash. The heel stone once known as Friar's Heaven, Friar's Heel. A popular tale, which can not be dated until the 17th century, thus explains the origins of the name of this stone. (The devil bought the stones from a woman in Ireland, wrapped her and brought them to Salisbury's plain. One of the stones fell in the Avon River, the others were brought to the plain. The Devil then shouted, No one will ever discover how these stones came "A friar replied," This is what you believe! "then the devil cast one of the stones against the friar and struck him on a heel. The stone came into the ground and is still there. The largest stones in gneiss were cut off by a 30-km-long hill from the archaeological site, likely to be transported through slides slipping on wooden rolls, drawn by leather strings from dozens of men. The stones that made up the vertical elements were first dragged to a hole in the ground and then slid into the hole with the help of a lever system resting on a "castle" of logs. The stone was then placed vertically by pulling it with ropes, and the hole was filled with stones. Once the vertical stones had been raised, the architrave was added, raising it at a time by building a timber plank and using levers. The first study to observe and understand the monument was conducted around 1640 by John Aubrey. He proclaimed Stonehenge's work of the Druids: this idea was widely spread by William Stukeley. Aubrey also made the first designs with site measurements, which allowed a better analysis of its shape and meaning. Thanks to this work, he was able to attribute an astronomical or calendar function to the arrangement of the stones. Architect John Wood undertook the first true accurate survey of Stonehenge in 1740. However, Wood's interpretation of the monument, considered a place of pagan rituals, was strongly criticized by Stukeley, who considered the Druids not as pagans but as biblical patriarchs. Stukeley's convictions underwent the influences of one of his most illustrious masters: Isaac Newton; the latter, albeit marginally, dealt with the arrangement of the Stonehenge stones which he would deliberately recall, the non-geocentric configuration of the solar system and the laws of gravitation.