The excavation aims to date the arrival of the bluestones
The first excavation of the stones at Stonehenge in Wiltshire for 40 years is under way.
The two-week project will focus on the smaller bluestones that make up part of the world-famous prehistoric monument.
It aims to date more precisely the double bluestone circle, the first stone structure built on the site.
Dr Simon Thurley, chief executive of English Heritage, said: "The bluestones hold the key to understanding the purpose and meaning of Stonehenge."
There is now no visible trace of the original setting of this circle. What visitors see now are freestanding bluestones re-erected later.
Archaeologists tried to date the circle in the 1990s and estimated that it was first erected at around 2,550BC but no precise dating has yet been found.
A trench measuring around 3.5 metres by 2.5 metres will be dug in a previously excavated area on the south-eastern quadrant of the double stone circle with the hope of retrieving fragments of the original bluestone pillars.
The excavation is being led by Stonehenge academics Professor Tim Darvill of Bournemouth University and Professor Geoffrey Wainwright, president of the Society of Antiquaries.