Aerial photography of the zone situated between the Vilaine, Mayenne and Loire rivers began in 1987 as part of programme of field prospection. It was continued by completing the sites and monuments record and co-financed by the Ministry of Culture and the Conseils généraux of Ille-et-Vilaine, Mayenne, Maine-et-Loire and Loire-Atlantique. The drought conditions which have prevailed since 1989 have permitted several hundred unknown archaeological sites to be located. These are to be found essentially il the valleys of the Seiche and the Oudon. A wide variety of ditched enclosures make up the major part of the discoveries (settlements, cemeteries, tracks or field systems). This evidence of human occupation places eastern Armorica within the framework of North Eastern Europe. The originality of the structures resides in their morphological aspect : omnipresent quadrilateral enclosures, nearly square, with wide ditches around which are organised a series of ditches and which appears to represent the first occupation of the site associated to a complex ditch network which could relate to an initial settlement on the site.