A short timeline
The megalithic ruin 'Stonehenge' stands on the open fields of Salisbury Plain, 3 kilometers west of Amesbury, 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 part of the complex, which dates to approximately 3100-2300BCE, comprised a circular bank-and-ditch of about 100 meters 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 are stille there), 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.
Around 2100-2000BCE, a circle 33 meters in diameter comprised of originally of 30 neatly trimmed upright sandstone blocks ("sarsens"), standing on average 4 meters above the ground, about 2 meters wide, and 3 1 meter 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 the circle of sarsens was added inside, in a horseshoe shape, ten upright sarsens arranged as five pairs with a single lintel.
Around 2000-1550BCE, a horseshoe of smaller upright igneous stones without lintels, the "bluestones", evidently brought from a geological site in Wales.
Finally, around 1550-1100BCE, 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.