 Northumbria 's southern frontier with Mercia ran across England , from the Humber in the east , following the River Ouse and the River Don , to the Mersey in the west . Some archaeological evidence , the Roman Rig dyke , near modern Sheffield , appears to show that it was a defended border , with large earthworks set back from the frontier . The Nico Ditch , to the south of modern Manchester , has been cited in this context , though it has also been argued that it was simply a boundary marker without fortifications . In the far north , the evidence is less clear , and it appears that authority lay with sub @-@ kings , perhaps including native British rulers . The family of Ecgfrith 's general Berht may have been one such dynasty of under @-@ kings . 
