Thanks, useful to see that context.

It's worth mentioning that the Ordnance Survey didn't employ any archaeologists until the 1920s, and "stone" on a map in non-antiquity typeface means just that, a stone. It didn't have to be standing, or have anything to indicate artificial placement. It was just a stone big enough to warrant someone noticing it. I'm sure there were guidelines about how big a stone had to be to show, but I'm also fairly certain that different surveyors/draughtsmen interpreted the guidelines slightly flexibly!

None of which is to dismiss the idea that there could have been circles, just to point out that in itself a "stone" appearing on a 19th century 6 inch map is a pretty unreliable basis for a theory without something else to support it.