Mount Ambrius in Geoffrey of Monmouth’s Historia Regum Britanniae is the “mountain” on which the Cloister of Ambrius as been built and on which Stonehenge was constructed. Presumably it is supposed to have been named for the Ambrius who had founded the monatery many years ago.
However, in fact, Stonehenge is on a very slight rise that one would not even call a “hill”, much less a “mountain” and both Wace and Lawman place the abbey within Amesbury which is 3.2 kilometres (2.0 mi) east of Stonehenge in the river-valley of the River Avon.