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.