The valley of the Humber gives access to the great central basin of the Trent. Up this fruitful basin, at a somewhat later date, apparently, than the settlement of Deira and Lincolnshire, scattered bodies of English colonists, under petty leaders whose names have been forgotten, seem to have pushed their way forward through the broad lowlands towards Derby, Nottingham, and Leicester. They bore the name of Middle English. Westward, again, other settlers raised their capital at Lichfield. These formed the advanced guard of the English against the Welsh, and hence their country was generally known as the Mark, or March, a name which was afterwards latinized into the familiar form of Mercia. The absence of all tradition as to the colonisation of this important tract, the heart of England, and afterwards one of the three dominant Anglo-Saxon states, leads one to suppose that the process was probably very gradual, and the change came about so slowly as to have left but little trace on the popular memory. At any rate, it is certain that the central ridge long formed the division between the two races; and that the Welsh at this period still occupied the whole western watershed, except in the lower portion of the Severn valley.
The Welland, the Nene, and the Great Ouse, flowing through the centre of the Fen Country, then a vast morass, studded with low and marshy islands, gave access to the districts about Peterborough, Stamford, and Cambridge. Here, too, a body of unknown settlers, the Gyrwas, seem about the same time to have planted their colonies. 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. But in 552, the English Chronicle tells us, Cynric, the West Saxon king, crossed the downs behind Winchester, and descended upon the dale at Salisbury. The Roman town occupied the square hill-fort of Old Sarum, and there Cynric put the Welsh to flight and took the stronghold by storm.