The most famous Neolithic monument, Stonehenge was built in several phases on a sacred site on the Salisbury Plain. 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.