see http://en.wikipedia.org/wiki/Lochmaben_Stone
The two boulders shown as clochmabenstones / 1+2,are all that remains of a large stone circle which once existed at this location.From my researching,it is my belief that this is the location of what the late 3rd and 4th century Romans referred to as the "LOCUS MAPONI".By the later period of Roman occupation in the north of england and southern Scotland,justice was seen to be done over "legal" disputes,by the equivalent of a circuit judge.A Senior Roman official,backed presumably by a reasonable display of Roman weaponry and might,would arrive 3 monthly at given locations in the buffer zone of Hadrian's Wall,to administer justice in local disputes.One such place is the Locus Maponi,as mentioned on the Ravenna Cosmography of the 7th century.
see also http://www.facesofarthur.org.uk/articles/guestdan14.htm which gives a fascinating insight into a re-appraisal of Arthurian legend and the Locus Maponi.