The megalithic ruin known as Stonehenge stands on the open downland of Salisbury Plain 2 miles (3 kilometres) west of the town of Amesbury, Wiltshire, in Southern England. It is not a single structure but consists of a series of earth, timber, and stone structures which were revised and re-modelled over a period of more than 2000 years.
The earliest portion of the complex, which dates to approximately 3100-2300 BCE, comprised a circular bank-and-ditch of about 330 feet (100 metres) in diameter. Just inside the earth bank were a circle of the 56 "Aubrey holes" (now invisible on the surface). Probably dating to this time also the four "Station Stones" (only two of which survive), and on the north-east side, an earthwork Avenue which runs from the break in the bank-and-ditch was added. The now-fallen "Slaughter Stone", located at the break in the bank-and-ditch may date from this period, as may also the "Heel Stone", located further out along the Avenue.
Stonehenge IIIa (2100-2000 BCE) (image from Mohen, p. 30)
Part of the outer of sarsens (added around 2100-2000 BCE). In front of them, stones of the bluestone circle (added around 1550-1100 BCE)
Around 2100-2000 B.C.E., a circle about 108 feet (33 metres) in diameter comprised originally of 30 neatly trimmed upright sands blocks (known today as "sar "), standing on average 13 feet (4 metres) above the ground, about 6.5 feet (2 metres) wide, and 3 feet (1 metre) thick, supporting a continuous ring of sarsen lintels (held in place by tongue-and-groove joints) was constructed in the centre of the original circular bank-and-ditch. A little later was added inside the circle of sarsens, in the shape of a horseshoe, ten upright sarsens arranged as five pairs with a single lintel.
(right) Stonehenge IIIb (2000-1550 BCE) (image from Mohen, p. 31)
Two of the trilithons (added around 2100-2000 BCE). In front of them are two upright stones of the bluestone horseshoe (added around the same period)
Around 2000-1550 BCE, a horseshoe of smaller upright igneous stones without lintels, the "bluestones", evidently brought from a geological site in Wales.
Finally, around 1550-1100 BCE, a circle of smaller upright bluestones was added between the outer sarsen circle and the outer horseshoe. Also added around this time are two concentric circle of holes - the so-called 'Y' and 'Z' holes.