Scientists believe they have solved the mystery of how Stone Age workers, armed only with antler picks and bone shovels, could have created the largest and most impressive prehistoric structure in Western Europe. Silbury Hill is a 120 foot high chalk mound formed in a Wiltshire valley in England around 4,500 years ago, in the middle of what is now crop circle country.

Built from chalk, it was originally a brilliant white color and was surrounded by a large shallow lake. It was formed in a valley, at the meeting point of two streams. Its summit could have been seen from nearby hill tops. David Field, a member of the archaeological field investigation unit, says, ?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.?

Scientists think the hill may have once been a sacred monument for prehistoric ceremonies. Around 500 feet wide at its base, it towers 120 fee 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.

Geophysical and ground surveys of Silbury show it was built in a spiral, and not made from a series of flat layers as they previously thought. A spiral processional walkway appears to have encircled the outside of the hill as well, providing access to the top.

It has been estimated that it would have taken 700 men working for 10 years to build it.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 a three-dimensional seismic survey to figure out how it was constructed. Small holes were drilled vertically into the hill and sound waves were used to scan its interior for cavities and loose chalk. The archeologists wanted to be extra careful because Silbury 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, says, ?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 layer by layer. But geophysical and surface surveys suggest that it was constructed in a spiral. Neolithic art of that time is characterized by the use of the spiral form. The hill is not actually circular, but has radial spines linked by straight lines, like a spider?s web.

And for some reason, it seems to ?attract? crop circles.

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