Besides the other problems, I would think if the stones were moved by glacier there would be a moraine thereabouts with lots of stuff deposited. But Salisbury Plain really is flat.
It's also interesting that the order of construction has been researched in recent years using improved technology, and it was found that the bluestones were moved from their original place within the structure.
“But Salisbury Plain really is flat.”
I had always pictured it as flat when I was young. The terrain is a lot more rolling than I had expected.
https://www.visitwiltshire.co.uk/explore/salisbury-plain
https://www.gov.uk/government/news/reservists-train-on-salisbury-plain
I would describe it more as gently rolling. I expected 'flat' when I got there, and it really wasn't.
I suppose it depends if one is coming from a perspective of western Colorado or Eastern Colorado. But while it was much flatter than north Georgia, I still wouldn't call it flat.