STONEHENGE: PLANS, DESCRIPTION, AND THEORIES by W M Flinder Petrie with an update by Gerald S Hawkins.
A fascinating book with this excellent map produced in 1966.
Click to enlarge
Hawkins is intrigued by the semi-circular depression just to the east of the North Barrow.
It shows up in this photograph from the early sixties as a dark mark. He assures the reader it isn't a Fairy Ring and wonders what it is? I do as well, as I haven't seen it mentioned anywhere else.