Stonehenge
The first stage started around 3100 BC.  During this stage, people dug a circular ditch and a ring of 56 pits, known as Aubrey Holes.  The second stage started around 2100 BC. During this stage, the Stonehenge builders brought huge pillars of rocks from Southwestern Wales and erected them into concentric circles around the centre of the site. This double circle was never completed, and it was dismantled during the third period of construction. The final stage probably ended before 1500 BC.  During this period, the monument was remodeled. Its builders erected a circle of 30 upright stones, weighing up to 50 tons each, capped by ring of stone lintels. These enclosed a horseshoe-shaped formation of five pairs of upright stones, each pair capped with a stone lintel.