Work to answer Stonehenge’s big questions — when the ring of stones was created, and why — began in earnest yesterday with the first excavation within the site’s inner circle for nearly half a century.

Archaeologists believe that a small patch of grass between a giant standing sarsen stone and a smaller knee-high bluestone, marked out yesterday with orange twine, may provide clues.

To the prehistoric tribes of southern Britain the site on Salisbury Plain may have been a place of healing, a temple and an astronomical observatory. To modern archaeologists, Stonehenge is far more important than that. Professors Geoffrey Wainwright and Tim Darvill, who are leading the two-week dig, spent six years toiling in the stone quarries of the Preseli Hills in West Wales to earn the right to excavate in hallowed ground.

They were investigating the origins of the bluestones that were dragged more than 150 miles to form Stonehenge’s first circle, a double ring. An estimated 82 bluestones each weighing up to four tonnes were brought about 4,550 years ago.

By the time the site was abandoned more than a millennium later the bluestones had been uprooted and rearranged at least once and been overshadowed by the 5m-tall sarsens that still dominate the site. Some were removed for purposes unknown or may have been deliberately destroyed. Even in the 20th century it was possible to rent a hammer to chip a souvenir off the stones.

The excavation will delve back into Stonehenge’s earliest origins. Professor Wainwright, president of the Society of Antiquaries, said: “Until the arrival of the bluestones, Stonehenge was just one of thousands of similar sites the length and breadth of the country, from the Orkneys to Cornwall. Bringing the bluestones here set it apart. We have identified the hillside they came from in southwest Wales. We have even found bluestones that broke in two as they were being brought down the mountain still lying exactly where they were abandoned.

“But what we’ve been unable to do in the Preselis is establish exactly when they were erected.”

The hole they intend to dig will measure just 3.5 by 2.5m and will be less than 1m deep. The archaeologists will be looking for socket holes that would reveal where the bluestones stood originally. At the bottom of the holes they hope to find organic material that will give a precise date for when they were erected.

They will also be looking for stone chips known to archaeologists as the “Stonehenge layer”, which should reveal whether they were the result of stones being worked by masons or of deliberate destruction. The last dig within the circle took place in 1964. Dr Darvill, Professor of Archaeology at Bournemouth University, said: “It is an incredibly exciting moment.”

Why? The theories

— A druid temple: possible, but the druids were around at the time of the Roman conquest, long after Stonehenge had fallen into disuse

— An astronomical observatory: yet to be proven, but like many prehistoric monuments Stonehenge is aligned to the rising sun in midsummer

— A place of healing: supported by the excavation nearby of skeletons showing signs of chronic disease

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