28. The Geomorphology of the Whin Sill

The Whin Sill is a doleritic intrusion into rocks of Carboniferous age in Northern England, stretching in an arc from the Alston Block (northern Pennines) northeast to the Northumberland coast including the Farne Islands. Its thickness is normally about 30 m but it can be much thicker locally, notably in Upper Teesdale. Injected into sedimentary strata, it behaves like a particularly resistant bed among the shales and sandstones where it is found. Its general horizontal structure means that it can form impressive cliffs, notably at High Force waterfall, along Hadrian’s Wall, where it provides the foundation for the Roman fortifications above, and along the coast at places like Cullernose Point, Dunstanburgh and Bamburgh castles, and the Farne Islands. The Whin Sill does not control the first-order structure of the landscape in Northern England but rather provides a distinctive second-order element to hillslope forms with impressive cliffs dominating local vistas.