I have recently spent quite a lot of time poring over the North York Moors in the Google Maps satellite images and I have come across a couple of circular structures which look like prehistoric ring-ditch monuments, but which are not marked on the Ordnance Survey maps. The above one is on Bilsdale East Moor near Fangdale Beck and is about 30 metres in diameter. Click on the image to see its location on Google Maps.
This one is on Scarth Wood Moor near Ingleby Arncliffe and is about 15 metres in diameter.
(I should perhaps point out that I have not inspected either of these features from the ground and, also, I have no formal training as an archaeologist. So make of this what you will.)