As soon as Brennius heard that what he had asked for had been refused and that his own blood-brother was marching against him in this way, he went forward to meet Belinus in a forest called Calaterium, determined to do battle with him there.... In the end, the Britons were victorious. The Norwegians fled to their ships with their lines of battle slashed to pieces. [III.3]

The forest of CALATERIUM first appears in Geoffrey as the site of a battle between the brothers Brennius and Belinus. Its next appearance also concerns royal brothers, as it is the site where the dutiful Elidurus encounters his brother Archgallo, whom he will subsequently restore to the throne:

When Elidurus had been King for some five years, he came upon his deposed brother one day when he was hunting in the Forest of Calaterium. Archgallo had wandered about through certain of the neighbouring kingdoms, seeking help so that he might recover his lost honour. He had found no support there and, coming to the point where he could no longer bear the poverty which had overtaken him, he had returned to Britain with a retinue reduced to ten knights. He was travelling through the above-named Forest, seeking those whom he had in earlier times called his friends, when his brother Elidurus came upon him unexpectedly. [III.17]

The forest also appears in Merlin's prophecies:

Next a Heron shall emerge from the Forest of Calaterium and fly round the island for two whole years. By its cry in the night it will call all winged creatures together and assemble in its company every genus of bird. They will swoop down on to the fields which men have cultivated and devour every kind of harvest. A famine will attack the people and an appalling death-rate will follow the famine. [VII.4]

Tatlock associates this forest with the region Calatria, mentioned by Aelred of Rievaulx and associated with the Carse of Falkirk, a marshy region "near where the Firth of Forth narrows and becomes passable" (p. 18).