h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

If megalith-building is your metier, then somewhere there has to be a place that yields the perfect rock for it. The Prescelly Mountains of North Pembrokeshire provide a prime candidate.

Ordovician volcanoes made the dolerite. According to the difficult books, it’s a metamorphic igneous rock consisting of plagioclase felspar and augite. More important, it’s exceptionally hard (harder than granite) but forms very regular cleavage planes, which means that it comes out of the ground pretty much as pre-formed pillars. It also looks very fine : blue-grey, glossy and sparkling.

4000 years ago, a shady Bronze Age tribe remembered as the Beaker People dug eighty-odd columns out of the mountain, each one weighing about four tonnes. It’s supposed that they used a combination of river-barging and dragging overland using rollers to move these ‘Bluestones’ the three hundred miles to Salisbury Plain, where they erected them in a double ring to form the second phase of Stonehenge.

H H Thomas of the Geological Survey of Great Britain first demonstrated the Pembrokeshire origins of these stones in the early 1920s, and was sceptically received, mainly because of incredulity about a primitive civilisation’s ability to move such loads over long distances. To this day, some claim that Merlin’s magic was involved, which is roughly as plausible as the still-espoused theory that the bluestones were ice-age erratics – this despite the fact that Gloucestershire and Wiltshire are devoid of any other evidence of glaciatic action.

The transportation feat was human, then, and it was very remarkable, but of greater interest still is how the Beaker People came to know that the stones were there in West Wales. The perfect lithology for the builders’ purposes explain the logistical undertaking, but the political culture of Bronze Age Britain is not usually thought to be sophisticated enough to spread ideas over such distances.

The Beaker People, sadly for them, never quite finished their defining project. Perhaps the Wessex folk, architects of the third phase, arrived and saw them off. The final manifestation of the temple is dominated by the gargantuan sarsens of the interlopers, though they also redeployed about half of the original complement of bluestones as part of the lesser circles and the horseshoe formation. They also, intriguingly, buried bluestone shards in a system of outlying pits.

The great sarsens were deftly dressed, and righting them and raising their lintels remains an awe-inspiring engineering accomplishment for its time. Few would consider these titans to be particularly attractive, though. In terms of grace and form, it’s the bluestones of the second age that lend Stonehenge much of its beauty.