Many ditched structures are discovered through aerial surveys. Unfortunately they are difficult to date owing to the absence of object finds on the ground. These structures can only be studied through typology. Some of them are weIl known, such a the mediaeval strong houses, mottes... and their outbuildings and the neolithic sites (fairly rare). Native farms as defined by Roger Agache are few (or absent) and the majority of them, excavated or not, cannot be related to any known type.