That was the title of a book but in this instance the enigma is outlined at http://phys.org/print311926419.html … in a paper in the Journal of Archaeological Science. It claims, as noted some weeks ago, that the bluestones came from a different quarry than previously thought. In fact, they were quarried on the other side of the Prescelli Mountains – and this creates a problem. It has been assumed they were transported by sea. If so they would have had to be hauled over the hills and down to the sea shore. Why would they have done that when there was suitable dolerite on the side of the hills that avoided such an exercise in futility. Clearly, there are two options. The stones were brought by a land route to Wiltshire – or they are erratics. The same applies to the rhyolites at Stonehenge – these were quarried not far from the new dolerite source.
See also Planet Earth Online – which is published and funded by NERC – see http://planetearth.nerc.ac.uk
The story is also at http://news.yahoo.com/source-stonehenge-bluestone-rocks-identified-14343… .. which adds some information on the minerology in the different dolerite seams (sent in by member W Thompson).