The dale is named after its principal river, the Tees, which has its source below Cross Fell (890 m (2,930 ft)) in Cumbria.[1] The upper dale is remote and high, but becomes gentler after it enters County Durham shortly downstream. The dale follows the river's south-easterly course to Barnard Castle, at which point the landscape begins to flatten into the Tees Lowlands. The Cumbrian part of Teesdale was historically divided between Cumberland and Westmorland, and the County Durham area between the former and Yorkshire.
Unusually for the Pennines, rock of igneous origin (the Whin Sill) contributes to the surface geology and scenery of Upper Teesdale. Around 295 million years ago upwelling magma spread through fissures and between strata in the earlier Carboniferous Limestonecountry rock. As it cooled (an event which is believed to have lasted 50 years) the rock contracted and caused itself to split into vertical columns. The heating of the limestone above the rock also caused it to be turned into a crumbly marble known as Sugar Limestone.[4][5]
Economic deposits in Llandovery rocks include soft shales that were previously worked to be used as slate pencils.[6]
More recently, Ice Age glacial activity shaped the valley, and much of the pre-glacial river course is now buried beneath glacial drift.
Botany
Teesdale violet
In places this impervious dolerite rock, with shallow soil above it, prevented the growth of scrub or trees: this enabled certain post-glacial Arctic / Alpine plants to survive here when elsewhere as a rule they were overgrown. The Sugar Limestone formed by thermal metamorphism of the limestone into which the Whin Sill was intruded also meets the requirements of some of these plants. Teesdale is famous among naturalists for the "Teesdale Assemblage" of plants found together here that occur widely separated in other locations, abroad or in the British Isles.[7]
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.[8] 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.[9] On the south bank of the Tees near High Force can be seen the largest surviving juniper wood in England.[10]
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