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.
The entire assemblage is surrounded by a circular ditch 104 m (340 ft) in diameter. On its inner side the ditch rises into a bank within which is a ring of 56 pits known as Aubrey holes (after their discoverer, the antiquarian John Aubrey) and used at a later stage as cremation burial pits. On the north-eastern side, the bank and ditch are intersected by the Avenue, a processional causeway 23 m (75 ft) wide and nearly 3 km (2 mi) long, bordered by a ditch. Near the entrance to the Avenue is the Slaughter Stone, a sarsen stone that may originally have stood upright. Almost opposite, and set within the Avenue, is the Heel Stone, which may have played a part in sightings of the sunrise at the summer solstice.
History of Stonehenge