Landscape geometry

We have visitors for the weekend so, on a grey damp and breezy morning, I took them for a long walk down the Redlake Valley and onto the climb up Caer Caradoc (Chapel Lawn) hillfort. From the ramparts of the eastern entrance you look down on some intriguing geometric shapes in the fields below. There are three circles and a structure which, to my eyes, vaguely resembles da Vinci's Vitruvian man. These are all made from hedging or young trees. The upper circle appears to have been grazed or mown. What purpose, if any, do the shapes serve? I would ask someone but, as usual in these hills, we don't meet a soul in our two and a half hour walk (without the spaniel). So, answers on a postcard......