The Forest of Dean, situated between the rivers Severn and Wye, is formed of a raised basin of Palaeozoic rocks folded in the Variscan Orogeny, similar to the South Wales Coalfield to the west. Underlain by great thicknesses of the Old Red Sandstone, the basin is filled with Carboniferous limestones, sandstones and coal measures - all of which have contributed to the industrial history of the region. Sometimes called ‘the prettiest coalfield in Britain’, the Forest's main coal output was in the 18th, 19th and early 20th centuries, although for hundreds of years coal has been won in the area via ‘free-mines’, which could only be operated by local people known as freeminers. One or two survive today. Small-scale mining has been enabled by the exposed nature of the coalfield, with seams often outcropping at surface. Iron-ore has been extracted from labyrinthine mines or 'Scowles' in the limestone since at least Roman times. Several large quarries are still at work in the area, providing Carboniferous Limestone for road surfacing and also Pennant sandstone (see picture) - a popular building stone used notably by Thomas Telford for the Over road bridge, spanning the River Severn near Gloucester.