Teesdale, former district, administrative county of Durham, northeastern England, in the southwestern part of the county. The former district lies on both sides of the River Tees. The area north of the river belongs to the historic county of Durham, and the area to the south lies in the historic county of Yorkshire. Teesdale is mostly an upland area within the northern Pennines. Its bleak basalt and limestone moorlands gradually descend eastward from an elevation of about 2,500 to 600 feet (760 to 185 metres) and border the Tees valley on both north and south. The arable, undulating plain northeast of the Tees valley and near Barnard Castle grows cereals, potatoes, and fodder crops; dairy cattle are also raised in the area.
Coal mining was never as important in Teesdale as it was in the eastern portion of the county of Durham. A Quaker-owned company mined lead deposits in the narrow upper section of the western Tees valley from about 1750 to the early 20th century; the company’s concern for the well-being of its workers can still be seen in the substantial stone buildings it erected for them at Middleton-in-Teesdale and other nearby villages. The stark, often snow-covered mountains of western Teesdale contain waterfalls, rare flowering plants, and reservoirs supplying water to heavy industries near the North Sea. Medieval remains in the area include the well-preserved Raby Castle near the town (parish) of Staindrop. The old market centre of Barnard Castle has been preserved through its official designation as a conservation area. It was in the town of Barnard Castle in 1838 that Charles Dickens was inspired to write the novel Nicholas Nickleby.