but the latest theory is it is a second hand IKEA monument
http://www.theguardian.com/uk-news/2015/dec/07/stonehenge-first-erected-in-wales-secondhand-monument
“It’s the Ikea of Neolithic monument building. The nice thing about these particular outcrops is that the rock has formed 480 million years ago as pillars. So prehistoric people don’t have to go in there and bash away … All they have to do is get wedges into the cracks. You wet the wedge, it swells and the stone pops off the rock.”
Maybe it did take 500 years to drag the stones along to Salisbury Plain, the coach service to Wales is only marginally faster.
My theory is they were taken by force, another thing for the Welsh to bitch about, along with the flooded valleys.