"Also, maybe the ancient boundaries, marked by the circles, held for longer than we thought in these remote northern parts"
Synchronicitously I was reading "Orcadiana" by Gregor Lamb in the Orkney Room yesterday, and he made the same claim for tunship and parish boundaries using brown-field brochs (i.e. those that are a continuing occupation rather than a green-field settlement) as central or water-marginal markers, the tunships being generally surrounded by delineating dykes.