Your nice plan of Crichton Souterrain, Paulus, made me look at Martin's notes about the site. Then I realised, well if it includes stone with Roman carvings, then well it's got to be later than Roman has it not. But I am right in thinking that some souterrains are genuinely Iron Age? or is it all wishful thinking?
some photos of the carved stones here
http://www.rcahms.gov.uk/pls/portal/NEWCANMORE.NEWCANDIG_COLL_LIST_IMAGES.show?p_arg_names=pnumlink&p_arg_values=54797&p_arg_names=pcat&p_arg_values=I&p_arg_names=puid&p_arg_values=59566