Geology of Gloucestershire - Forest of Dean

Forest of Dean

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 coal-field 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.

The scenic Wye Valley defines the western edge of Dean as well as the England/Wales border. Impressive cliffs have been cut by the river in lower Dolomitic sections of the Carboniferous Limestone, most notably at the popular ‘Symonds Yat’ viewpoint, which affords one of the most famous views in England. Other ‘Karst’ features such as caves, rock pillars and sink holes abound in this area.

On the north-east edge of Dean the Silurian period is represented by the distinctive inlier of May Hill, which rises to some 970 feet, crowned with a pine plantation. This isolated hill lies on the north-south Malvern fault line and affords superb views from its summit of the Severn Vale, with the Cotswolds forming the horizon beyond. A high degree of faulting complicates the geology of the May Hill area, causing outcrops of Wenlock limestone and Downton Castle sandstone on its flanks, as well as a possibly Precambrian section of the core of the hill exposed at Huntley quarry. The hilltop is of Llandovery age May Hill sandstone.

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