The celebrated geologist Herbert Henry Thomas first proposed in 1923 that the rocks which form the giant inner ring of the Stonehenge monument - so-called bluestones - were specifically quarried in the Preseli Hills in south-west Wales by Neolithic man, and pinpointed the tor on Carn Meini as the likely source.
     The bluestones at Stonehenge were placed there during the third phase of its construction around 2300 BCE, and it is assumed that there were about 80 of them originally, but only 43 remain. Each bluestone weigh between 2 and 4 tons.
     National Museum of Wales keeper of natural sciences Doctor Richard Bevins and his colleagues have now compared samples of rock and debris from Stonehenge with data from the Preseli site and categorically concluded the spotted dolerites in fact came from Carn Goedog, almost a mile away from the previously known source.
     As the name suggests, the spotted dolerites have highly distinctive markings created by the elements contained within, cooling at different rates in the minutes after they were spewed out of an underwater volcano 450 million years ago.
     In 2011, Doctor Bevins's team located the source of another of Stonehenge's Pembrokeshire Bluestones - the rhyolites - 3 kilometres away from the spotted dolerites at Craig Rhos y Felin. "We've located two of the sources, and there's another five or possibly six to go." Bevins hopes their findings will influence the question of how the bluestones were transported 300 kilometres to Salisbury Plain, and believes a definitive answer will come eventually.

Edited from BBC News (19 November 2013), The Guardian (20 November 2013)