Frederick George Meeson Beck, 1911 Encyclopædia Britannica, Volume 18… (1911)
“ With this exception, Watling Street, the Ouse and the Lea, continued to be the boundary between Mercia and the Danish kingdom of East Anglia down to the death of Æthelred, between 910 and 912. The government was then carried on by Æthelflaed, who built a number of fortresses, and in conjunction with her brother, King Edward the Elder, succeeded in expelling the Danes from Derby and Leicester by the year 917–18. After her death in the latter year her daughter Ælfwyn was soon deprived of the government by Edward, and Mercia was definitely annexed to Wessex. ”
