borough (district), administrative and historic county of Northumberland, northeastern England, in the southeastern part of the county. It lies just northwest of the heavily industrialized metropolitan county of Tyne and Wear and borders the North Sea on the northeast. Castle Morpeth borough lies on a coarse sandstone upland—where the highest elevations reach about 700 feet (215 metres)—that descends to the east in a series of scarps to a coastal plain with low cliffs backed by sand dunes; on the west it borders the Cheviot Hills and associated moorlands. The land is generally covered by fertile unstratified glacial drift, and the onshore and offshore coastal areas are underlain by coal deposits. Mixed farming (including cattle, sheep, cereals, potatoes, and clover) prevails throughout.

Population is concentrated in the southeast at Ponteland, a residential suburb of Tyne and Wear, and at Morpeth and Lynemouth. Coal mining in the borough declined steadily during the late 20th century, and the future of the last remaining mine—working an undersea coal bed from a pit at Ellington—was uncertain. Light engineering industry exists in Morpeth, the administrative centre of both the borough and the administrative county of Northumberland. Numerous fortified manors and peel towers (small, massive, fortified towers built to protect livestock and commoners) bear witness to the English-Scots warfare in the area during the Middle Ages. A section of the east-west-trending Hadrian's Wall, built by the Romans in the 2nd century CE, crosses the borough near the village of Heddon-on-the-Wall in the extreme south. Area 239 square miles (619 square km). Pop. (2001) 49,011.