Isles. Part of Upper Teesdale near the Cow Green Reservoir is designated a National Nature Reserve; it contains the unique Teesdale Violet and the blue Spring Gentian as well as more common Pennine flowers such as rockrose, spring sandwort, mountain pansy, bird's-eye primrose and butterwort. Hay meadows in the valley above High Force, some now carefully cultivated to ensure this, contain an extremely rich variety of flowering plants including globe flower, wood cranesbill and Early Purple Orchid. On the south bank of the Tees near High Force can be seen the largest surviving juniper wood in England. Geography and history Over ledges in the Whin Sill fall the famous waterfalls of High Force and Low Force and the cataract of Cauldron Snout. The Tees then reaches and passes between Barnard Castle and Startforth, thereafter passing to the south of Darlington, through the borough of Stockton-on-Tees and north of Middlesbrough, reaching the North Sea between the boroughs of Hartlepool and Redcar & Cleveland. The Skerne runs through Darlington while the Leven starts and runs between Yarm and Ingleby Barwick in the borough of Stockton-on-Tees then through Stokesley then Great Ayton in the Hambleton District borough. West Teesdale's principal town and most populous settlement is Barnard Castle, a historic market town that is home to the renowned Bowes Museum. It also includes the small town of Middleton-in-Teesdale and a number of villages, including Mickleton, Eggleston, Romaldkirk and Cotherstone. Middleton was a lead-mining centre, and plentiful traces of this industry can be seen round the adjoining slopes and side-valleys. On the south side of Teesdale looms the Bronze Age burial site of Kirkcarrion. East Teesdale has been described as being the area between the a point west of Darlington and Teesmouth, where the Tees flows through flatter country, which is also known also as Tees Valley. Middlesbrough is the most populous town in the valley as a whole. The valley runs roughly parallel to Weardale (including Bishop Auckland and Sunderland), Durham Dales on the northern side and south is Swaledale (including Richmond), Yorkshire Dales to the west and the Cleveland Hills, North York Moors to the east. Teesdale gave its name to the former Teesdale district of County Durham, although the south side of Teesdale lies within the historic county boundaries of the North Riding of Yorkshire. Formerly the Startforth Rural District, it was transferred to County Durham for administrative and ceremonial purposes