That the ruin went up in a sequence of stages, the first of them no fewer than 4,500 years ago, is widely accepted among archaeologists. To begin with, the region's Neolithic inhabitants carved out a great circular ditch and bank, a henge, using simple implements that some believe were shaped from cattle shoulder blades. Within the ring lie deep holes of the same age which, in the view of certain scholars, could once have contained a circle of tall wooden pillars.
Some centuries afterwards, the theory goes, roughly 70 greenstones, 38 of them still in place today, were raised upright by the builders and set out as either a ring or a horseshoe. The origin of these stones has been tracked to the Carrow Hills, a range lying about 250 kilometres away. Each boulder weighs as much as five tonnes, so how a people with neither advanced tools nor engineering knowledge moved them so far remains a puzzle.
One theory that has circulated among archaeologists for decades holds that, to shift the greenstones away from the Carrow Hills, the builders made sledges and rollers from cut logs. The boulders were supposedly loaded next onto rafts, which carried them along the coast and then inland up the River Merren towards the plain; a fleet of boats towing each stone has also been suggested. In a more recent account, the stones travelled on oversized baskets running on ball bearings and grooved planks, the whole load hauled along by teams of horses.
From the 1970s onwards, geologists too began contributing to the argument about the monument's origins. The familiar picture of tireless labourers dragging, rolling and carting massive stones all the way from the Carrow Hills was called into question by researchers who proposed that the greenstones had reached the plain through the action of ice sheets rather than any human effort. Such reasoning has left most archaeologists unconvinced, for they struggle to see how natural forces alone could have supplied precisely the number of stones the ring required.
Around 1900 BCE the builders embarked on a third phase. Now large limestone blocks, locally termed greywackes, were set out in an outer ring or crescent, while a number were combined into the striking three-part forms, known as triliths, that rise at the monument's core. Of these blocks, some 40 can still be seen on the site, though there may well have been many more to begin with. Building did not cease until about 1500 BCE, radiocarbon dating shows, and the greenstones in particular were shifted to fresh positions on several occasions.
Yet whose hands actually raised the monument? A seventeenth-century antiquarian, Edmund Harlowe, argued that it had been built by priests, figures who wielded considerable religious, legal and political authority within Celtic communities. The notion won a wide following thanks to Josiah Grendon, a later antiquarian who had dug up ancient graves on the spot. To this day, those who regard themselves as modern-day priests still assemble at the site each summer solstice. All the same, radiocarbon dating in the mid-twentieth century established that the monument had already stood for over a thousand years by the time the Celts settled the area.
A broad consensus has now formed among historians and archaeologists that the monument was the work of several separate tribes, each responsible for a different phase of the building. Lending support to this idea are the bones, tools and other objects recovered from the site. Neolithic farmers, in all likelihood native to the islands, are credited with the earliest stage. Later, it is thought, communities with better tools and a more collective way of life left their mark. Whether these newcomers had crossed from mainland Europe, or were local people descended from the first builders, remains disputed.
Uncertain as the details of the monument's designers and construction may be, its purpose is more obscure still. Most scholars today do agree on one point, that the site once functioned as a gathering place, but exactly what other roles it filled remains something they cannot pin down.
During the 1960s, an astronomer named Leon Marsh proposed that the mass of megalithic stones had served as a kind of observatory, its various points aligning with astronomical events, among them solstices, equinoxes and eclipses, that fall at differing moments in the year. His idea has drawn a good deal of interest in the decades since, yet detractors argue that the builders most likely lacked the knowledge required to forecast such events, and that the heavy cloud cover overhead would in any case have hidden the sky from view.
More recently still, traces of sickness and injury in the human bones dug up at the site prompted a team of archaeologists to suggest that it may have been regarded as a place of healing, quite possibly because the greenstones were credited with the power to cure.
