The mystery of why Stonehenge was built on the unremarkable chalk plateau of Salisbury Plain may have finally been solved.

An expert claims two of Stonehenge's largest stones had been in place at the site for millions of years before Neolithic people built the monument.

Their coincidental alignment with the sunrise and sunset on the longest and shortest days of the year prompted ancient people to construct Stonehenge around them.

Mike Pitts specialises in British pre-history and is one of a small number of scientists who have excavated on the site of the ancient monument.

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In a paper published in the journal British Archaeology, the freelance archaeologist describes uncovering a pit, around six metres (20 feet) in diameter, besides the heel stone in 1979.

The heel stone is 75 metres (250 feet) from the centre of the stone circle, weighs around 60 tonnes and has not been shaped or dressed, unlike the other sarsens.

It is the point at which the sun rises and falls below the horizon at midsummer and midwinter, from the perspective of those looking towards it from inside Stonehenge.

Mr Pitts believes the hole, rather than being a socket dug for a missing standing stone, was once home to huge heel stone.

A second undressed stone in the centre of the circle lines up with the heel stone and sun at the winter and summer solstice.

This rock, known as stone 16, also has a pit next to it, suggesting it too originated at the site of Stonehenge.

Speaking to The Times, Mr Pitts said: "The assumption used to be that all the sarsens at Stonehenge had come from the Marlborough Downs more than 20 miles away.

"The idea has since been growing that some may be local and the heel stone came out of that big pit.

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"If you are going to move something that large you would dress it before you move it, to get rid of some of the bulk. That suggests it has not been moved very far.

"It makes sense that the heel stone has always been more or less where it is now, half-buried."

Sarsen is a layer of sandstone that formed millions of years ago above the chalk layer on Salisbury Plain.

During the various ice ages, permafrost repeatedly froze and thawed this chalk layer, shattering the sarsen.

Over millennia, these stones sank below the surface, leaving a few fragmented rocks jutting out.

These stones, of varying sizes, can be found across Salisbury Plain and the Marlborough Downs in Wiltshire, as well as in Kent and in smaller quantities in Berkshire, Essex, Oxfordshire, Dorset and Hampshire.

The act of building Stonehenge may have been as important a ceremony to its ancient creators as the use of the finished stone circle, experts claimed in March.

Construction of the 5,000-year-old monument drew people together from all over the country to drink and get to know one another in large ceremonial feasts.

Work on Stonehenge could have been used to show outsiders the power of the small community building it, researchers at English Heritage said.

The theory may explain why some of the Wiltshire site's stones were transported more than a hundred miles (160km) from a quarry in south Wales.

Susan Greaney, a senior historian at English Heritage, said: "In contemporary Western culture, we are always striving to make things as easy and quick as possible, but we believe that for the builders of Stonehenge this may not have been the case.

"Drawing a large number of people from far and wide to take part in the process of building was potentially a powerful tool in demonstrating the strength of the community to outsiders.

"Being able to welcome and reward these people who had travelled far, perhaps as a kind of pilgrimage, with ceremonial feasts, could be a further expression of the power and position of the community."

The theory follows English Heritage's recent discovery of feasting at the nearby Neolithic Durrington Walls settlement, also found in Wiltshire.

According to the charity's historians, this attracted people from across the country to help build the Neolithic monument.

The discovery pushed English Heritage to look again at theories of how Stonehenge was built, concluding that building the monument was important ceremonially and cause for celebration.

Ms Greaney said the new theory may explain a mystery surrounding the impressive distances some of Stonehenge's monoliths were carried.

The large standing stones at the monument are made of local sandstone, but the smaller ones, known as "bluestones", come from a quarry in south Wales.

Stonehenge's architects would have had to shift the huge rocks 140 miles (225km) from what is now Pembrokeshire Coast National Park to the monument's build site.

Ms Greaney said: "As soon as you abandon modern preconceptions which assume Neolithic people would have sought the most efficient way of building Stonehenge, questions like why the bluestones were brought from so far away - the Preseli Hills of south Wales - don't seem quite so perplexing."

She added that the idea of "stone-pulling ceremonies", in which people celebrate moving monoliths by hand, is not a new one.

She said pictures from a 1915 stone-pulling ceremony on Nias, Indonesia, showed people in ceremonial dress 'revelling' in the task and taking part in feasts and dances.

Stonehenge is one of the most prominent prehistoric monuments in Britain.

The monument that can be seen today is the final stage of a project that spanned 1,500 years.

Stonehenge was donated to the nation's heritage collection in 1918 by owners Cecil and Mary Chubb.

Mr Chubb had bought the then-neglected monument on impulse at an auction three years earlier having been sent there by his wife to bid for a set of dining room chairs.

HOW WAS STONEHENGE BUILT?

Stonehenge is one of the most prominent prehistoric monuments in Britain. The Stonehenge that can be seen today is the final stage that was completed about 3,500 years ago.

According to the monument's website, Stonehenge was built in four stages:

First stage: The first version of Stonehenge was a large earthwork or Henge, comprising a ditch, bank and the Aubrey holes, all probably built around 3100 BC.

The Aubrey holes are round pits in the chalk, about one metre (3.3 feet) wide and deep, with steep sides and flat bottoms.

They form a circle about 86.6 metres (284 feet) in diameter.

Excavations revealed cremated human bones in some of the chalk filling, but the holes themselves were likely not made to be used as graves, but as part of a religious ceremony.

After this first stage, Stonehenge was abandoned and left untouched for more than 1,000 years.

Second stage: The second and most dramatic stage of Stonehenge started around 2150 years BC, when about 82 bluestones from the Preseli mountains in south-west Wales were transported to the site. It's thought that the stones, some of which weigh four tonnes each, were dragged on rollers and sledges to the waters at Milford Haven, where they were loaded onto rafts.

They were carried on water along the south coast of Wales and up the rivers Avon and Frome, before being dragged overland again near Warminster and Wiltshire.

The final stage of the journey was mainly by water, down the river Wylye to Salisbury, then the Salisbury Avon to west Amesbury.

The journey spanned nearly 240 miles, and once at the site, the stones were set up in the centre to form an incomplete double circle.

During the same period, the original entrance was widened and a pair of Heel Stones were erected. The nearer part of the Avenue, connecting Stonehenge with the River Avon, was built aligned with the midsummer sunrise.

Third stage: The third stage of Stonehenge, which took place about 2000 years BC, saw the arrival of the sarsen stones (a type of sandstone), which were larger than the bluestones.

They were likely brought from the Marlborough Downs (40 kilometres, or 25 miles, north of Stonehenge).

The largest of the sarsen stones transported to Stonehenge weighs 50 tonnes, and transportation by water would not have been possible, so it's suspected that they were transported using sledges and ropes.

Calculations have shown that it would have taken 500 men using leather ropes to pull one stone, with an extra 100 men needed to lay the rollers in front of the sledge.

These stones were arranged in an outer circle with a continuous run of lintels - horizontal supports.

Inside the circle, five trilithons - structures consisting of two upright stones and a third across the top as a lintel - were placed in a horseshoe arrangement, which can still be seen today.

Final stage: The fourth and final stage took place just after 1500 years BC, when the smaller bluestones were rearranged in the horseshoe and circle that can be seen today.

The original number of stones in the bluestone circle was probably around 60, but these have since been removed or broken up. Some remain as stumps below ground level.