Several hundred years later, it is thought, Stonehenge’s builders hoisted an estimated 80 non-indigenous bluestones, 43 of which remain today, into standing positions and placed them in either a horseshoe or circular formation. During the third phase of construction, which took place around 2000 B.C., sarsen sandstone slabs were arranged into an outer crescent or ring; some were assembled into the iconic three-pieced structures called trilithons that stand tall in the center of Stonehenge. Some 50 sarsen stones are now visible on the site, which may once have contained many more. Radiocarbon dating suggests that work continued at Stonehenge until roughly 1600 B.C., with the bluestones in particularly being repositioned multiple times.