Phase 3a.

Dating from phase three is imprecise, but during the early Bronze Age at some point before 1500 BCE, the original bluestone circle was removed and replaced with an inner horseshoe and an outer circle of immense sarsen trilithons. Two additional standing sarsens were added at the entrance to the enclosure.

Sarsen is a geological term referring to sandstone boulders carried by glaciation. Like the bluestones, the sarsens were likely transported from the nearby countryside. Trilithon is a term derived from the Greek tri-lithos, lit. "three stones". It refers to two upright stones linked across the top with a horizontal lintel. At Stonehenge, the trilithons are impressively large. The tallest stones, weighing over 45 tons, support enormous lintels over seven meters (22 feet) above the ground.