An experiment by researchers at University College London found that mounting huge stones on a sycamore wood sleigh and dragging it along a corrugated "road" of timbers required far less effort than expected. Their one-tonne stone moved along the silver birch track at around 3 metres every five seconds when pulled by just 10 people - about 2 kilometres per hour if pulled continually.
     The Preseli bluestones at Stonehenge are approximately double that weight, but could have been brought by just 20 people. The community living in the area during the Neolithic would have numbered several thousand.
     Doctoral student Barney Harris, who conducted the trial, said he was surprised so few people would be required to move the block: "It's true that we did the experiment on flat ground, and there would have been steep slopes to navigate when going through the Preseli Mountains, but actually this kind of system works well on rough terrain. We know that pre-industrialised societies like the Maram Naga in India still use this kind of sledge to construct huge stone monuments. And similar y-shaped sleighs have been found dating back to 2000 BCE in Japan which we know were used to move megaliths. The Chinese also used sleighs to build the Forbidden City and some of those blocks are 123 tonnes. So in comparison, these are blocks are quite small."
     Stonehenge was built during the Neolithic period, between 4,000 and 5,000 years ago. The largest standing stones - which weigh between 30 and 40 tonnes - are of local sandstone, but the smaller bluestones are from Wales, 225 kilometres away.
     Archaeologists at University College London and the University of Leicester recently found the actual stone quarries. The spotted dolerite bluestones came from Carn Goedog, and the rhyolite bluestones from Craig Rhos-y-felin. The rock outcrops form natural pillars, allowing prehistoric workers to detach each stone with minimum effort.
     Stonehenge expert Professor Mike Parker-Pearson of University College London believes the stones were part of a monument that once stood in Wales, which was dismantled and moved to Wiltshire, but even he was amazed how quickly the stones could be dragged.

Edited from The Telegraph (24 May 2016)