The county of Beaumont-sur-Oise lay to the north of Paris, south of the county of Beauvais, east of the Vexin and west of the county of Valois, although its precise geographical boundaries are uncertain. Lépinois suggests that the county corresponded to the pagus Camliacensis [le Chambliois] and le pays de Thelle, which together became the ecclesiastical doyenné of Beaumont[6]. The first mention of a count of Beaumont-sur-Oise is dated to 1022, although it is not known when the county was formed. Douet-d´Arcq suggests that it represented a partition of the county of Beauvais, which was granted by Eudes II Comte de Blois to Roger Bishop of Beauvais in exchange for the county of Sancerre in [1013] not long before the appearance of the first comte de Beaumont in the primary sources[7]. Earlier charters dated between 627 and 861 name the pagus Camliacensis, and its main town Chambly, located in the southern part of the future county of Beaumont. Among these charters, "Theudaldus comes" donated property "sitas in pago Camliacense" to the abbey of Saint-Denis by charter dated 799[8].
Thibaut d'Ully Seigneur de Méru, a descendant of Hugues [I] Vicomte de Beaumont (see below), sold the county of Beaumont-sur-Oise to Philippe II King of France in 1223[9]. Philippe IV "le Hardi" King of France granted the county to his son Louis by charter dated 1285[10].
IVES [I] (-12 Apr 1059). Comte de Beaumont. “Comes Ivo de Bello monte, Ebo miles, Guarinus miles Parisius, Almaricus miles de Monteforte” witnessed the charter dated 1022 under which Robert II King of France confirmed the possessions of the abbey of Coulombs[11]. “Ivo...Belmontensis castri comes” donated property to la Trinité de Fécamp by charter dated to after 1027, subscribed by “Ivonis comitis, [J]osfredi filii eius, Ivonis clerici eius filii sui fratris, Alberici eius filii”[12]. “…Ivonis comitis…” witnessed the charter dated 1028 under which Robert II King of France confirmed the possessions of the abbey of Saint-Mesmin de Micy[13]. He founded Saint-Léonor de Beaumont 12 Oct 1029[14]. "Comes Ivo cum filio meo simili nomine…clerico et canonico…cum mea propria conjuge Emma et ceteris omnibus meis liberis" donated "castrum Confluentie" to Saint-Wandrigisel by charter dated 1039, signed by "Ivo…comes…Ivo clericus…comes Goffredus, Albericus, Gelduini vicecomitis…Odonis filii supradicti comitis, Gosberti fratris eiusdem Odonis"[15]. “…Yvonis Comitis Bellomontis…” witnessed the charter dated 20 May 1043 under which Henri I King of France confirmed the possessions of the abbey of Saint-Maur-des-Fossés[16]. “…Walterius comes Pontisariensis, Willelmus comes Corboilensis, Ivo comes Bellomontensis, Walerannus comes Melledensis” are named among those present at the opening of the reliquary of Saint-Denis, dated 9 Jun 1053[17]. The Necrologium Bellimontense records the death “II Id Apr” of “Ivo comes fundator ecclesie S. Leonorii”[18]. m EMMA, daughter of --- (-21 Jul ----). "Comes Ivo cum filio meo simili nomine…clerico et canonico…cum mea propria conjuge Emma et ceteris omnibus meis liberis" donated "castrum Confluentie" to Saint-Wandrigisel by charter dated 1039, signed by "Ivo…comes…Ivo clericus…comes Goffredus, Albericus, Gelduini vicecomitis…Odonis filii supradicti comitis, Gosberti fratris eiusdem Odonis"[19]. The Necrologium Bellimontense records the death “XII Kal Aug” of “Emma comitissa”[20]. Ives [I] & his wife had [five] children