The Stonehenge bluestones are the shorter stones at the monument, not the later, larger sarsen trilithons. They are not native to Salisbury Plain where Stonehenge stands, and as long ago as 1923 the eminent petrographer, Herbert Thomas, was able to identify their source generally as the Preseli hills of southwest Wales, roughly 200 miles distant.
The specific source area on Mynydd Preseli is in and around Carn Menyn, an extensive complex of rock outcrops that is itself situated amidst a range of other named and unnamed carns or outcrops (known in the plural as Carn Meini). From current knowledge, it seems that the bluestones came from Carn Menyn and adjacent outcrops, while other Preseli rocks came from the slopes around them.
“Bluestone” is the common name for spotted dolerite, an igneous rock that looks blue when broken and is spotted with small pellets of feldspar and other minerals that got into the molten matrix when the rocks were forming geological ages ago.
Interestingly, the 12th-century historian, Geoffrey of Monmouth, indicated by means of a story concerning Merlin that the stones of Stonehenge had come from “Ireland” (for which read the far west of Britain – Wales and Ireland had definite connections in prehistory). Many modern historians deride Geoffrey of Monmouth’s histories as a collection of myths, yet the question remains: how did Geoffrey, back in the 12th century, know where the bluestones originated?
The use of the bluestones at Stonehenge transformed it into a unique monument – and even though the site underwent many phases, the bluestones remained from the first to last, though sometimes in varying settings. Much archaeological debate has been expended on how the bluestones arrived at Stonehenge – whether by human effort, floating the stones (each weighing several tons apiece) across water and dragging them across land, or whether they were deposited on Salisbury Plain naturally by glacial action. Although a few archeologists still think a glacial explanation is tenable, most think the bluestones were brought by human transportation. For one thing, glacial movement in the region does not support the transport of glacial erratics in the required manner, and there is also evidence of human action: the are bluestones arranged at the monument with their outer ring setting (inside the later sarsen outer ring) containing the range of Preseli stones (dolerite, rhyolite, tuff) and the inner horseshoe setting of bluestones (inside the horseshoe of massive sarsen trilithons) of spotted dolerite, mimicking the natural arrangement of the rock types in the Preseli landscape, with the spotted dolerite of Carn Menyn at the centre and other rock types, including more scattered spotted dolerite blocks, around its fringe.
There has been less debate about why Neolithic people should have wanted to move the stones such a long distance. This is an issue both S.P.A.C.E.S. and the Landscape and Perception project are trying to address – see A Stone Age Holy Land?.