The Battle of Deorham occurred in 577 between the West Saxons and the Celts. Deorham is usually taken to refer to Dyrham, Wiltshire . The battle is considered by some to be decisive since it drove a land wedge between the Celts of what is now (approximately) Wales and those occupying territory to the south-west, e.g. what are (approximately) now Somerset, Devon and Cornwall.

However, passage through this area by Welsh-speaking individuals was not impossible: a genealogy of Welsh aristocrats notes that the descendants of kings who ruled in Powys founded a dynasty of rulers in the Glastonbury region in the eighth century.