At a later date they coalesced with the Mercians.

However, the comparative scarcity of villages bearing the English clan names throughout all these regions suggests the probability that Mercia, Middle England, and the Fen Country were not by any means so densely colonised as the coast districts; and independent Welsh communities long held out among the isolated dry tracts of the fens as robbers and outlaws.
In the south, the advance of the West Saxons had been checked in 520, according to the legend, by the prowess of Arthur, king of the Devonshire Welsh.

As Mr.Guest acutely notes, some special cause must have been at work to make the Britons resist here so desperately as to maintain for half a century a weak frontier within little more than twenty miles of Winchester, the West Saxon capital.

He suggests that the great choir of Ambrosius at Amesbury was probably the chief Christian monastery of Britain, and that the Welshman may here have been fighting for all that was most sacred to him on earth.

Moreover, just behind stood the mysterious national monument of Stonehenge, the honoured tomb of some Celtic or still earlier aboriginal chief.