"It was one of those kind of serendipitous occasions - a Eureka moment," says Professor Richard Bevins. Picking over a set of 15 sample stones from Stonehenge passed on to him by a former colleague, the experienced geologist was asked to make a quick observation on the source of rock believed to have been brought from west Wales some 5,000 years ago.

"It was one of those kind of serendipitous occasions - a Eureka moment," says Professor Richard Bevins.

"It was one of those kind of serendipitous occasions - a Eureka moment," says Professor Richard Bevins.