There's a type of stone circle known as the Four-Poster which is in the class description, but I think is actually a rectangle, do you want to include those in the 'have any stone circles been found since 1976' question. Here's a link to the best stone circle that's been found since 1976. http://www.themodernantiquarian.com/site/7525 . It's huge but all the stones are flat and mainly under turf.