Teesdale is a dale, or valley, in Northern England. The dale is in the River Tees’s drainage basin, most water flows stem from or converge into said river, including the Skerne and Leven. Upper Teesdale, more commonly just Teesdale, falls between the Durham and Yorkshire Dales. Large parts of Upper Teesdale are in the North Pennines AONB (Area of Outstanding Natural Beauty) - the second largest AONB in England and Wales. The River Tees rises below Cross Fell, the highest hill in the Pennines at , and its uppermost valley is remote and high. The local climate was scientifically classified as "Sub-Arctic" and snow has sometimes lain on Cross Fell into June (there is an alpine ski area Yad Moss). Lower Teesdale has mixed urban (Tees Valley or Teesside) and rural (Cleveland) parts. Roseberry Topping is a notable hill on the south eastern side, of which this and other adjoining hills form the northern end of the North York Moors. Newer terms have gained stronger associations with separate parts of the valley due to their use as specific political constituencies and authorities. 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. Botany 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