The Whin Sill or Great Whin Sill is a tabular layer of the igneous rock dolerite in County Durham, Northumberland and Cumbria in the northeast of England. It lies partly in the North Pennines Area of Outstanding Natural Beauty and partly in Northumberland National Park and stretches from Teesdale northwards towards Berwick. It is one of the key natural features of the North Pennines. A major outcrop is at the High Force waterfall in Teesdale. Bamburgh Castle, Dunstanburgh Castle, Lindisfarne Castle and stretches of Hadrian's Wall all strategically take advantage of high, rocky cliff lines formed by the sill. The Whin Sill complex is usually divided into three components: Holy Island Sill, Alnwick Sill and the Hadrian's Wall-Pennines Sill, which were created by separate magma flows, but at about the same time. The Little Whin Sill is an associated formation to the south, in Weardale. Derivation of name Much of the early study of geology began in the British Isles, whence much of the terminology is derived. Quarrymen of Northern England used the term 'sill' to describe a more or less horizontal body of rock. 'Whin' was applied to dark, hard rocks. As the intrusive igneous origin of the Whin Sill was determined in the 19th century, the term 'sill' was adopted by geologists for concordant, tabular intrusive bodies. Geology Towards the close of Carboniferous Period and in the early Permian, some 295 million years ago (Ma), crustal extension caused by movement of the Earth's tectonic plates during the Variscan orogeny allowed the emplacement of igneous intrusions of magma across much of northern England of a suite of tholeiitic dolerite intrusions. These were largely concordant with the strata of the existing country rock. On cooling, these crystallised and solidified to form the Great Whin Sill. It is dated at around 301- 294 million years old, thus spanning the Carboniferous/Permian boundary at 299 Ma. It underlies much of south and east Northumberland and the Durham Coalfield. Its maximum known thickness of around 70 metres occurs in the North Pennines. Surface and subsurface records of the Great Whin reveal it is not always concordant over wide areas and often rises and falls in the stratigraphical succession in marked leaps and gentle transgressions different levels. Studies of the petrology of the dolerites of the Whin Sill complex have revealed significant differences between the Little Whin Sill and the Great Whin Sill. The Little