WEll, this isn't a circle ( http://www.themodernantiquarian.com/post/81089/broad_mea_long_cairn.html ). I never investigated my circular feature, on the ground, but simply watched it, from afar, for five summers. I didn't ask for it to be listed on the county SMR, as I couldn't work out what it is. It'll be more like three hundred metres across, and shows much better to the naked eye than to FlashEarth. But it is quite similar to what you are describing, and in a similar landscape.