E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Geoffrey of Monmouth says that Cundah and Morgan, the sons of Gonorill and Regan, usurped the crown at the death of Cordeilla. The former resolved to reign alone, chased Morgan into Wales, and slew him at the foot of a hill, hence called Gla-Morgan or Glyn-Morgan, valley of Morgan. (See Spenser: Faërie Queene, ii. 10.)