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.
The battle was promptly followed by the Saxon occupation of three cities: Cirencester (Corinium), probably a provincial capital in the Roman period; Gloucester (Glevum), a former legionary fortress and a colonia; and Bath (Aquae Sulis), a renowned pagan religious centre and spa-city.
The battle is also considered by some to be decisive since it drove a land wedge between the Britons of what was to become Wales and those in the south-west peninsula.
It has, however, been objected that, though the battle may have had an impact on large-scale movements, the passage of Welsh-speaking individuals was evidently not impossible, since a Welsh genealogy appears to record that, in the 7th century, the descendants of kings of Pengwern founded a dynasty in the Glastonbury region.