Stonehenge, prehistoric ritual monument, situated on Salisbury Plain in south-western England and dating from the Neolithic (late Stone Age) and Bronze Age. It is the most celebrated of the megalithic monuments in England, and the most important prehistoric structure in Europe. Although its precise purpose is unknown, it is likely to have been a tribal gathering place or religious centre connected with astronomical observations. CONFIGURATION Stonehenge consists of four concentric ranges of stones. In the outermost range, large rectangular sandstone blocks (sarsen stones), 4 m (13 ft) high above the ground, form a circle 33 m (108 ft) in diameter; they were originally capped with lintel stones (only a few of which remain in place today) that also formed a continuous circle. Within this outer range is a circle of smaller bluestones (consisting mainly of dolerite, a coarse basaltic rock having a bluish colour). They enclose a horseshoe-shaped arrangement of bluestones capped with lintels. These trilithons (an assemblage of two uprights capped by a lintel) are 6.5 m (21 ft) high. Within the trilithons stands a slab of micaceous sandstone known as the Altar Stone.