Geophysical and ground surveys of Silbury Hill, a 120 ft high chalk mound formed in a Wiltshire valley around 4,500 years ago, suggest that it was built in a spiral fashion, and not made from a series of flat tiers like a wedding cake as previously thought.

A spiral processional walkway also appears to have encircled the hill, providing access to its flattened summit.

If the walkway is confirmed, it suggests that the hill may have once been a sacred monument for prehistoric ceremonies. Silbury Hill, which lies close to the stone circles of Avebury, Wilts, was an astonishing achievement of prehistoric Britain.

Around 500 ft wide at its base, it towers 120 ft above ground level in a grassy valley and is surrounded by a shallow, wide ditch. It was built around 2,500 BC, several hundred years before Stonehenge's standing stones were erected.

When complete, it would have been an awe-inspiring sight for Neolithic people. Built from chalk, it was originally brilliant white and surrounded by a large shallow lake. It was formed in a valley, at the meeting point of two streams. Its summit could have been overlooked from nearby hill tops.

It has been estimated that it would have taken 700 men working for 10 years to build the hill.

Some researchers have argued that it was used for rituals, others that it was a burial mound. But no human remains have been found at the hill in more than 200 years of excavations.

Last year English Heritage began the first three dimensional seismic survey of the hill to find out how it was constructed.

The archaeologists were also concerned that the hill had been damaged during earlier, cruder excavations. Their first job was to shore up a shaft from an 18th century excavation that had collapsed and examine the damage caused by other digs.

Dr Kevin Brown, the regional director of English Heritage, said: "The results of the seismic survey are very encouraging as they have shown that the hill's structure appears stable.

"The survey has revealed, however, that a small part of a tunnel constructed near the base of the hill in 1969 has suffered a roof fall."

At one time it was thought that the mound was built up slice by slice. But geophysical and surface surveys suggest that it was constructed in a spiral way. Neolithic art is characterised by a preoccupation with the spiral form.

The hill is also not circular, but has radial spines linked by straight lines, rather like a spider's web.

David Field, a member of the archaeological field investigation unit, said: "When it was newly constructed it would have been a brilliant white. As you approached it from the valley, it would have stood out against the green landscape around it.

"If you start to walk around the uppermost ledge, you end up three metres lower than where you started. There is some sort of spiral at the top. It may go to the bottom, which would make sense in construction terms."

The team suspects that there was a processional spiral pathway to the top of the mound.

It has also begun to study the results of the seismic surveys. Small bores were drilled vertically into the hill and sound waves used to scan its interior for cavities and loose chalk. The scans have shed light on how the mound was built.

Fachtna McAvoy, an English Heritage archaeologist, said: "We can see what is effectively a Neolithic building site at the base of the mound.

"The workmen were evidently struggling with wet ground conditions and churned up the land surface into a mixed layer of chalk and mud. We have also discovered that the mound when it was built was 31 metres high and that there were no long layoff periods during its construction."

English Heritage said more details from the surveys would emerge in the coming months, giving the most detailed look inside the hill.