Folks interesting in the general topic of aerial archaeology, crop marks of ancient structures and sites, etc., should check out Prehistoric Britain from the Air, by Darvill, NOT by Bord (there are 2 totally different books with the same name).
Tons of pictures showing crop marks of any number of oddities and curiosities. If you are near one of these sites and have seen the pictures, you can sometimes see the marks lightly from the ground yourself. More generally, it makes you look at slight color/tone differences in archeological or other crop areas a lot differently.
Thanks!