The extraction and burning of coal have been fundamental aspects of the development of the UK over the past 300 years and throughout recent history they have shaped the social landscape of the country. However, with its economic decline in the 20th century, and now a proven link to human-driven climate change, the coal industry (at least in the UK) will soon be resigned to history. The archaeological investigation, recording and research into coal extraction is therefore becoming more and more important.
Coal mining in the UK has been identified as early as the Roman period; however, the nature of mining makes archaeological investigation difficult. Later extraction often removes all trace of earlier workings, and the physical remains are actually voids within the geology, whether a large hole from opencast mining, a series of ‘bell-pits’, or shafts leading to underground galleries. The waste and soil that often fills these voids can provide clues, but infilling can happen many years after the workings were first in use.
In some areas, medieval and later ‘bell-pits’, and their associated waste tips, are visible at the surface. Bell pits comprise a narrow shaft, usually less than 30 feet (9.1m) deep, sunk into the coal seam, with material brought to the surface either by a simple windlass or a ladder and buckets. Later bell pits were assisted by the use of horse-powered winding gear. The bell-shape was created by extracting coal outwards from the shaft until it became unsafe due to the possibility of collapse. To continue mining, a new shaft would be dug and the process repeated. Often the finished pit was used to dispose of the spoil from the new one. Therefore, extraction by bell pit is often typified by closely-spaced shafts.
Bell-pitting is impractical once all the available shallow coal has been worked out. Deeper coal was extracted using shafts that led to a series of horizontal galleries and cross passages. A technique called ‘pillar-and-stall’ mining was commonly used during the 18th and early 19th centuries, in which some coal was left in place to support the mine ceiling. Later mining used the ‘long wall’ technique, and increasing mechanisation enabled extraction from deeper seams.
In terms of archaeological investigation, it is obviously dangerous and often impossible to investigate the deeper below-ground remains except where they are exposed during modern large-scale opencast mining. Shallow workings and surface features are the usual coal-mining remains encountered during archaeological investigations. These can include the tops of shafts, drainage features, buildings and sometimes the remnants of the machinery and tools used.
Geophysical survey techniques can be very effective at detecting mining activity. Numerous geophysical survey methods can be used to identify if there are buried voids associated with former tunnels, especially if they are located deep within the stratigraphy. In shallow geophysical investigations, we are often looking for the mine shaft or entrance, or other structural remains. In this instance there is unlikely to be a void. Instead we are looking for backfilled deposits that have distinctly different physical properties to the surrounding soil. Such deposits can include mine waste, rubble and other debris. In magnetic survey datasets, mine shafts often appear as well-defined sub-circular anomalies. The magnetic strength of these anomalies can be useful in considering the potential materials used to backfill the shaft. For example, when content with a very high magnetic susceptibility is present, the shaft appears as a bipolar anomaly composed of a central circular area of positive magnetic values haloed by an area of negative readings. Alternatively, if the backfill is composed of material with similar magnetic properties to the geology, then it may just appear as an area of discretely enhanced magnetic readings, if it is visible at all.
Between 2001 and 2016, NAA undertook archaeological investigations associated with the Thorpe Park mixed-use development at Austhorpe, located to the east of Leeds in West Yorkshire. The coalfields of West Yorkshire were exploited over a long period of time, and the scars of the industry were still apparent within the vicinity of the development. Through geophysical survey, we were able to suggest the extent and distribution of mining activity, and subsequent soil stripping exposed a large number of shafts, both isolated and in clusters, associated drainage features, part of a waggonway, and buildings and features associated with the late 19th-century Ellen Pit.
Due to the potentially dangerous and unstable nature of the infilled shafts, archaeological investigation was limited; however, machine excavation of a sample of shafts was undertaken under archaeological direction. The depth of these was limited to about 7m, the maximum reach of the mechanical excavator, making identification of potential bell-pits difficult. During the project, strip-mining of intact coal deposits in two areas provided an opportunity to investigate the below ground remains at much greater depths.
A wide variety of artefacts was recovered from the infilled shafts. However, the majority were fairly modern, including fragments of glass bottles, paint cans, broken plates and tea cups and shoes. Some of the shafts did produce only 18th or 19th-century finds and were likely to be evidence of early mining. One of these earlier shafts contained the skeleton of a horse at a depth of c.4m. Specialist study of the bones proved that it was unlikely to have been a pony, donkey or mule. Radiocarbon dating from one of the horse bones produced an age of death between calAD1525-1802 (89.7% confidence), with 68.2% probability of a date within the range calAD1645-1796. The radiocarbon date range accorded well with the 18th-century potsherds recovered from the same shaft and provides a realistic date for backfilling.