The road was thus opened in the rear to the upper waters of the Thames (impassable before because of the Roman population of London), as well as towards the valley of the Bath Avon.  Four years later Cynric and his son Ceawlin once more advanced as far as Barbury hill-fort, probably on a mere plundering raid.  But in 571 Cuthwulf, brother of Ceawlin, again marched northward, and “fought against the Welsh at Bedford, and took four towns, Lenbury (or Leighton Buzzard), Aylesbury, Bensington (near Dorchester in Oxfordshire), and Ensham.”  Thus the West Saxons overran the whole upper valley of the Thames from Berkshire to above Oxford, and formed a junction with the Middle Saxons to the north of London; while eastward they spread as far as the northern boundaries of Essex.  In 577 the same intruders made a still more important move.  Crossing the central watershed of England, near Chippenham, they descended upon the broken valley of the Bath Avon, and found themselves the first Englishmen who reached any of the basins which point westward towards the Atlantic seaboard.  At a doubtful place named Deorham (probably Dyrham near Bath), “Cuthwine and Ceawlin fought against the Welsh, and slew three kings, Conmail, and Condidan, and Farinmail, and took three towns from them, Gloucester, and Cirencester, and Bath.”  Thus the three great Roman cities of the lower Severn valley fell into the hands of the West Saxons, and the English for the first time stood face to face with the western sea.  Though the story of these conquests is of course recorded from mere tradition at a much later date, it still has a ring of truth, or at least of probability, about it, which is wholly wanting to the earlier legends.  If we are not certain as to the facts, we can at least accept them as symbolical of the manner in which the West Saxon power wormed its way over the upper basin of the Thames, and crept gradually along the southern valley of the Severn.

The victory of Deorham has a deeper importance of its own, however, than the mere capture of the three great Roman cities in the south-west of Britain.  By the conquest of Bath and Gloucester, the West Saxons cut off the Welsh of Devon, Cornwall, and Somerset from their brethren in the Midlands and in Wales.  This isolation of the West Welsh, as the English thenceforth called them, largely broke the power of the native resistance.  Step by step in the succeeding age the West Saxons advanced by hard fighting, but with no serious difficulty, to the Axe, to the Parret, to the Tone, to the Exe, to the Tamar, till at last the West Welsh, confined to the peninsula of Cornwall, became known merely as the Cornish men, and in the reign of AEthelstan were finally subjugated by the English, though still retaining their own language and national existence.  But in all the western regions the Celtic population was certainly spared to a far greater extent than in the east; and the position of the English might rather be described as an occupation than as a settlement in the strict sense of the word.