Just a thought but Paul Nash was obsessed with painting Wittenham Clumps, which presumably is a tree circle, so he must have felt the similarity with stone circles....
and completely off topic but Nigel and foxes brought this to mind, 'pillow mounds' which are to do with rabbits...explanation here;
"Pillow mounds are probably a late medieval development. They are low stone mounds which were once covered with earth and used as artificial rabbit warrens. It seems strange to us now that rabbits should have been regarded in the middle ages as valuable animals, to be looked after and cosseted. Rabbits were brought into this country by the Normans. They are originally from southern Europe, and it took them a long time to adjust to our cold, wet climate. But they were such useful animals: their meat was delicate and well-flavoured, the soft fur made fine leather and warm linings for winter clothes, and their stomachs could be used to rennet cheese."
So if anyone comes across low rectangular mounds in hillforts, they're probably rabbit related ;)