You may need to register with National Geographic to read this but it is interesting and gives an insight into how Diana Gabaldon chose to write about a Scottish circle with magical powers.
http://news.nationalgeographic.com/news/2014/08/140808-outlander-scotland-orkney-islands-stonehenge-neolithic/
In this piece she also writes about her visit to Orkney.
Something is not right. This message is just to keep things from messing up down the road