The most famous Neolithic monument, Stonehenge is located 13 km northwest of Salisbury, in Wiltshire, England. Stonehenge was built in several phases on a sacred site on the Salisbury Plain. Its construction began in prehistoric times around 3100 BC. In form Stonehenge is a series of concentric rings of standing stones around an altar stone at the center. The first ring has a horseshoe plan of originally five trilithons, each of two upright stones supporting a single colossal lintel. Beyond these was first a circle of smaller uprights, sacred "blue" stones, transported from South Wales, and then an outer, enclosing circle of sandstone monoliths 13.5 feet high, which support what was once a continuous lintel. Beyond this a circle of small, movable "marker stones" were set in pits and farther out, a landscaped trench separated the site from the surrounding land. A long avenue marked by uprights sets up an axis, identified by the Heel Stone, a large stone with a pointed top (see picture on right). The construction was highly accurate for the period. The engineering required for transporting, shaping, raising and connecting the stones and the accuracy of their positioning according to astronomical phenomena is remarkable evidence of the knowledge and skills of Stonehenge's makers. The Stonehenge that visitors see today is considerably ruined many of its stones having been pilfered by medieval and early modern builders and its general architecture has also been subjected to centuries of weathering. Why Stonehenge was built is unknown, though it probably was constructed as a place of worship of some kind. Notions that it was built as a temple for Druids or Romans are unsound, because neither was in the area until long after Stonehenge was last constructed. Early in the 20th century, the English astronomer Sir Joseph Norman Lockyer demonstrated that the northeast axis aligned with the sunrise at the summer solstice, leading other scholars to speculate that the builders were sun worshipers. In 1963 an American astronomer, Gerald Hawkins, suggested that Stonehenge was a complicated computer for predicting lunar and solar eclipses. These speculations, however, have been severely criticized by most Stonehenge archaeologists.