Fitz, to the wall builder, could chopping these circles in 'arf be a way of desecrating their druidicalousness ?......kinda "I can't be arsed movin' these stones you good for nowt druid, but this wall will put an end to all your goat sacrificing shenanigans, now get to church, or be forever damned" ......... sorta fing!

Georges point about RA is a goodun, down here a number of humungous erratics with RA on form part of boundary walls, heres a few I can think of:

http://www.themodernantiquarian.com/post/27635 Skyreholme
http://www.themodernantiquarian.com/post/41437 Rivock
http://www.themodernantiquarian.com/post/44174 Guisecliff

I'd go for these CnR'd rocks just being prominent bits on the landscape to stick your wall on, but who knows.