The last revellers seem to have cleared up scrupulously after the final party at Marden Henge some 4,500 years ago.

They scoured the rectangular building and the smart white chalk platform on top of the earth bank, with its spectacular view towards the river Avon in one direction, and the hills from which the giant sarsen stones were brought to Stonehenge in the other.

All traces of the feast – the pig bones, the ashes and the burnt stones from the barbecue that cooked them, the broken pots and bowls – were swept neatly into a dump to one side. A few precious offerings, including an exquisitely worked flint arrowhead, were carefully laid on the clean chalk. Then they covered the whole surface with a thin layer of clay, stamped it flat, and left. Forever.

In the past fortnight, English Heritage archaeologists have peeled back the thin layer of turf covering the site, which has somehow escaped being ploughed for more than 4,000 years. They were astounded to find the undisturbed original surface just as the prehistoric Britons left it.

"We're gobsmacked really," said site director Jim Leary.

Giles Woodhouse, a volunteer digger who must return next week to his day job as a lieutenant colonel in the army bound for Germany and then Afghanistan, has been crouched over the rubbish dump day after day, his black labrador Padma sighing at his side. He has been teasing the soil away from bone, stone and pottery so perfectly preserved it could have been buried last year.

"It gives one a bit of a shiver down the backbone to realise the last man to touch these died 4,500 years ago," he said. His finds, still emerging from the soil, will rewrite the history of the site.

Marden in Wiltshire has been puzzling archaeologists for centuries. It is set almost exactly half way between two of the most famous and tourist-choked sites in Britain, Stonehenge and Avebury, but it is far larger than either. The ragged oval of outer earth banks at Marden, completed by a bend of the Avon, enclose more than 14 hectares, compared with 11.5 hectares at Avebury, where the banks surround an entire modern village.

Famously – to its comparatively few devotees and visitors, that is – it is the biggest henge in Britain that isn't there, surrounding one of the biggest artificial hills in Britain, which isn't there either.

This is the first excavation since Geoffrey Wainwright, former chief archaeologist at English Heritage, explored one small corner of the site in 1969. What stunned the archaeologists when they started work three weeks ago was just how much is left.

Once your eye is in you can see it: the sweep of the ditches, the belt of trees hiding some of the earth bank, which still rises to three metres in some places, the stain in the grass marking the lost barrow and its massive surrounding moat, and the wholly unexpected discovery – the second, smaller henge, so close to the modern houses that the roots of two trees at the foot of a back garden are actually growing into its bank.

The neolithic buildings were not where others have looked for them, on the level in the centre of the henges, but on top of the bank.

"We've all been looking in the wrong place," Leary said, "there will have to be a major rethink about other henges. And it's actually almost terrifying how close to the surface the finds were – there's also going to have to be a major review of our management plans for other sites."

The only known image of Hatfield Barrow – an early 18th century map in the archives of the landowner, Corpus Christi College in Oxford – shows the artificial hill as a jaunty little sandcastle sporting a cockade of trees. It once rose to a height of almost 15 metres, half the height of Silbury near Avebury.

The two antiquarians who burrowed like rabbits through scores of Wiltshire earthworks in the early 19th century, Sir Richard Colt Hoare and William Cunnington, punched a massive shaft through Hatfield Barrow in 1807. Their scrappy records torment the modern archaeologists, including references to animal bones, burned wood, and "two small parcels of burned human bones".

They left the shaft open, possibly intending to return in another season, and the mound collapsed. This is a phenomenon Leary knows well, having led the rescue excavation before the engineering works to stabilise Silbury, which was also left riddled with slowly collapsing holes by Georgian and later diggers.

The farmer at Marden filled in the moat, which an 18th century naturalist recorded as fed by a natural spring and never dry even in the hottest summer, and sold the collapsed hillock as top soil. Leary's massive trench has uncovered barely a trace of hill or moat.

If the hill disappointed, the excavations at one of the original entrances and at the small henge certainly do not. They are revealing what appears to be a broad gravelled ceremonial road leading towards the river. Discovering undisturbed neolithic surfaces and building platforms on this scale counts as a discovery of international importance.

There is no evidence of permanent occupation of the dwellings or the site as a whole. As in the work led by Professor Mike Parker Pearson at Durrington Walls, 20 miles away (he couldn't resist coming over to help dig, and some of his former students had the pleasure of giving him orders) the implication is of people gathering for seasonal rituals and feasting, and maybe a work camp.

"A completely artificial division has been made in the past between domestic and religious, recreation and ritual," Leary said. "We're going to have to rethink all that. It's not one thing or the other, it's everything mixed in together."

If it wasn't a village, or a temple, or a farm, or a cemetery, what was Marden for? Leary suspects the answer may be emerging in stone working tools, and flakes of sarsen, turning up all over the site. If you were going to drag sarsens the size of double decker buses from their original site to Stonehenge, he said, the obvious route is straight through a natural gap in the hilly landscape, which would take them through Marden.

The evidence that Marden was a sort of builder's yard for the most famous prehistoric monument in the world may have been in the mud under the boots of Leary's puzzled predecessors.

So why did the site's temporary occupants leave? Maybe with Stonehenge complete, the sarsens shaped into the giant trilithons that still fill the hordes of modern visitors with awe, their job was done. They tidied up nicely, turned out the lights, and left.