There is an entry for the bank on the Royal Commission on the Ancient and Historical Monuments of Scotland website at http://canmore.rcahms.gov.uk/en/site/3448/details/sanday+mires , although it is inconclusive as to what it was for.
Uploaded to Geograph by Claire Pegrum on 11 June 2013
Photo © Claire Pegrum, 11 June 2013. Licensed for reuse under this Creative Commons licence