Stonehenge’s sarsens, of which the largest weighs more than 40 tons and rises 24 feet, were likely sourced from quarries 25 miles north of Salisbury Plain and transported with the help of sledges and ropes; they may even have already been scattered in the immediate vicinity when the monument’s Neolithic architects first broke ground there. The smaller bluestones, on the other hand, have been traced all the way to the Preseli Hills in Wales, some 200 miles away from Stonehenge. How, then, did prehistoric builders without sophisticated tools or engineering haul these boulders, which weigh up to 4 tons, over such a great distance?
According to one longstanding theory, Stonehenge’s builders fashioned sledges and rollers out of tree trunks to lug the bluestones from the Preseli Hills. They then transferred the boulders onto rafts and floated them first along the Welsh coast and then up the River Avon toward Salisbury Plain; alternatively, they may have towed each stone with a fleet of vessels. More recent hypotheses have them transporting the bluestones with supersized wicker baskets or a combination of ball bearings, long grooved planks and teams of oxen.
As early as the 1970s, geologists have been adding their voices to the debate over how Stonehenge came into being. Challenging the classic image of industrious Neolithic builders pushing, carting, rolling or hauling the craggy bluestones from faraway Wales, some scientists have suggested that glaciers, not humans, did most of the heavy lifting. The globe is dotted with giant rocks known as glacial erratics that were carried over long distances by moving ice floes. Perhaps Stonehenge’s mammoth slabs were snatched from the Preseli Hills by glaciers during one of the Ice Ages and deposited a stone’s throw away—at least comparatively—from Salisbury Plain. Most archaeologists have remained cool toward the glacial theory, however, wondering how the forces of nature could possibly have delivered the exact number of stones needed to complete the circle.