The Warwickshire Coalfield extends between Warwick and Tamworth in the English Midlands. It is about 40 km / 25 miles from north to south and its width is around half that distance. Its western margin is defined by the 'Western Boundary Fault'. In the northeast it abuts against steeply dipping shales of Cambrian age. The larger part of the outcrop at the surface consists of the Warwickshire Group of largely coal-barren red beds. Until 2013, the Daw Mill mine near Arley within the coalfield was Britain's biggest coal-producer.