Thanks for all the theories.....lets put a geological slant on all this. The ice age was responsible for carrying several large stones along in ice flows, when the ice retreated the land defrosted, the small stuff slid away or was eroded. Larger stones were left scattered on the land. Over the years man has come along and moved these stones so he can farm easier, the simple thing to do is put them at the edge of fields or make boundrys out of them.
as it has been said rubbing posts can usually be identified by the fact that they stand in the centre of a field and certainly down here in Cornwall many are of cut granite or at least somewhere were the rock is not natural.
I saterted off this forum after my visit to the Essa stones, if they were originally a circle then they are not on a ridge and if they came from one of the barrows mentioned on early maps then why drag them half a mile or so to then leave them in a pile beside the road. Of the two standing, they do not line up with any other marks and can not be seen until you get close to them.....