It looks like a fairy ring! 
"In Britain, fairy rings are known as hag tracks, in the belief that they are created by the dancing feet of witches.
Because fairies are associated with Magic, fairy rings have magical superstitions attached to them. It is said that if one stands in the center of a fairy ring under a full moon and makes a wish, the wish will come true. If one wishes to see and hear the fairies, who often are beyond the awareness of the five senses, one can run around a fairy ring nine times under a full moon.
Fairy rings are still associated with natural magic and are used by contemporary witches as sites for meetings and seasonal festivals." 