English: Map showing British river names of Celtic etymology, after Kenneth Hurlstone Jackson, 'British River Names', in Language and History in Early Britain: A Chronological Survey of the Brittonic Languages, First to Twelfth Century A.D., Edinburgh University Publications, Language and Literature, 4 (Edinburgh: Edinburgh University Press, 1953), p. 220. According to Jackson, in Area I, Celtic names are rare and confined to large and medium-sized rivers; the area 'corresponds fairly closely with the extent of primary English settlement down to about the first half of the sixth century'. Area II has a higher proportion of Celtic river names and seems to correlate with Anglo-Saxon settlement in the second half of the sixth century. In Area III, 'Brittonic river names are especially common, including often those of mere streams'. The area corresponds to Anglo-Saxon settlement from the mid-sixth century into the eighth. In Area IV, Brittonic remained the dominant language 'till at least the Norman Conquest' and river names are overwhelmingly Celtic.