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Teesdale

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Through Valley, Igneous rock and Stratum

The dale is named after its principal river, the Tees, which has its source below Cross Fell (890 m (2,930 ft)) in Cumbria. 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.

Large parts of Teesdale are within the North Pennines national landscape, and Upper Teesdale has been designated a Site of Special Scientific Interest. Parts of the local climate have been scientifically classified as "Sub-Arctic", and snow has sometimes lain on Cross Fell into June.

Geology

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 Limestone country 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.

Economic deposits in Llandovery rocks include soft shales that were previously worked to be used as slate pencils.

More recently, Ice Age glacial activity shaped the valley, and much of the pre-glacial river course is now buried beneath glacial drift.

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