It would be nice to think that neolithic engineers applied something akin to our approach to the problem of how to get a large stone from Wales to the Salisbury Plain and to have applied the same plan to several large stones - just enough, and not too many. But do you really think they planned and then implemented the feat like a modern day construction management exercise? Do you really think they designed swinging beams, lifting levers and boats straddling jetties rising and falling on the tide in harmony with the forces involved? They had no paper, no telephones, no maps, no CPM theory. It would be nice to think that they could have done these things, but I doubt that it happened like that.
If it was done like that the land would be littered with stones which arrived at the right places as well as stones which were abandoned all over the country when efforts went awry. I think the people who hauled these stones took advantage of a particular circumstance when permanent ice conditions made it possible and this coincided with a large (slave) labour force being available.
Sometime around whenever it was (7,000 BC?) there was a very cold spell - a mini Ice Age - when dragging/pushing large stones around the country was a relatively easy matter.
Rod Northway (M)