[All]: Cf. DGB 115 (Reeve and Wright 2007, 149.108-10; cf. Wright 1988, 105, prophecy 19): Niueus quoque senex in niueo equo fluuium Perironis diuertet et cum candida uirga molendinum super ipsum metabitur ‘A snow-white old man on a snow-white horse will divert the river Periron and with a white rod measure out a mill on its bank’ (cf. Reeve and Wright 2007, 148). The Book of Llan Dav, c. 1150, locates Aper Periron not far from the town of Monmouth as a branch of the Cadlan (Curley 1982, 244, citing Williams 1955, xxxvi-xxxvii); some of the commentaries also place it near Monmouth (e.g. Hammer 1940, 419). The C10th Armes Prydein ‘The Prophecy of Britain’ describes a confrontation between the Welsh and the steward of an English king, possibly Æthelstan, at this location (Curley 1982, 225-6; Faletra 2008, 134). However, it is possible that Geoffrey is alluding by means of this anecdote about a snow-white old man to the rapid rise of the Cistercians, a monastic order distinguished by its members’ wearing of white robes. The order gained institutional definition with the Carta caritatis of Stephen Harding, confirmed by Pope Calixtus II in 1119 (Poole 1955, 187), and rapidly became very popular in England (Barrow 1956, 105). The earliest known reference to the adoption of white clothing by the Cistercians occurs in a letter of Peter the Venerable (1092/94–1156), abbot of Cluny, to Bernard of Clairvaux (Burton 2006, 10). The reference to a water mill may also be suggestive, as the typical Cistercian monastery straddled a mill-race, an artificial stream diverted from a nearby river to provide power for grain milling and other technologies as well as running water for domestic purposes (Hansen [n. d.], accessed 03/09/2015; cf. Bostan et al. 2012, 187; Woods 2005, 33). The Cistercians’ founding of Tintern Abbey, not far from Monmouth, occurred early enough for Geoffrey to have been able to allude to it in the separate Libellus Merlini as well as the subsequent DGB.