A complex basin of coal known as the Bristol Coalfield underlies the City of Bristol and South Gloucestershire at its northern end whilst the southern portion underlies northern parts of Somerset including Radstock, Midsomer Norton and Nailsea.
The extraction of coal occurs at a very early date within the coalfield mainly due to the occurrence of seams at or near the surface. Although there is some evidence for Roman exploitation, the economic potential of the resource becomes important to the region in the later medieval and early post-medieval periods. By the end of the 19th century competition from the mines of South Wales had caused a large decline in the extraction of coal from the Bristol region which was difficult to extract due to the complex nature of the geology.
The extraction of coal over a long period has left a legacy of features that may be hazardous to development within the region. These include crop workings, bell pits and shafts many of which are undocumented and cannot be recognised from the ground surface. Crop workings are some of the earliest features and consist of simple linear quarries following coal seams dug to a depth of around 5 metres and backfilled as they progress. Bell pits and early shafts may have been dug to a depth of 30 metres or more and may be only a metre or so in diameter at the ground surface. Later shafts may descend several hundred metres and have associated industrial buildings.
Geophysical survey, in particular magnetometry, can be effective in locating features associated with mining and is particularly useful where documentary evidence is lacking. Evidence from the surveys carried out in the region indicate a pattern of early industrial activity perhaps not fully recognised as playing an important part in the industrial revolution of south western England.