hi

Dartmoor is covered in stuff (technical term there) - could it be the cairns in the woods at Bellever Tor? There are several circles/cairns/cists.

http://www.themodernantiquarian.com/site/638/bellever.html

If it isn't Bellever, try looking at the nearby sites to it, as it sounds like it's in the right area at least.

cheers

sam