Sometimes a miner finds a ferny leaf or piece of wood imprinted on a piece of coal. This helps to reveal the story of how coal was formed.
Here in Britain, coal was formed at various times between 170 million to 300 million years ago. The rocks associated with the coal demonstrate that the major geological forces which have controlled the features of the earth's crust have exerted their control also on the type of coal and the way it is presented in the earth. Major forces affect rocks at depth underground, causing crumpling, bending and breaking of the beds and cause mountain chains to be formed, sea levels to rise and fall and also give rise to earthquakes. Volcanoes have also left their evidence within coal seams and associated rocks. You can see the evidence of these vast disturbances most easily along the coast in the curving strata and breaks and faults of a cliff face.
The rock sequences, which include coal seams, were first laid down as sediments washed down from upland areas by streams and trapped within lakes and sea covered basins. When these sediments filled the basin and land emerged, trees and other vegetation grew. Huge trees tumbled and were crushed; forests of saplings were engulfed or overlaid; ferns and mosses, grew through several centuries - then were swallowed or covered. Earth and rock and lava compressed trees, and other plants. They rotted and changed as bacteria attacked their remains. They were composted, condensed and compressed. More millions of years passed. They became coal.
So what is coal? It is the woody (cellulose) part of vegetation formed by time, pressure, bacteria and other agents into a carbon rich mass.
Coal seams can be found near the surface or deep underground
Types of Coal
It is easy to imagine hundreds of variations in the ways in which coal is formed. For example, any of the processes that produce coal may take place more than once - and then layer upon layer of coal will be found. Again, some coal is deeply buried in thick seams hundreds or thousands of metres underground, but elsewhere it might be found only in thin layers on or near the surface. Yet again, the nature of the 'coal' depends very largely on how the original matter was composted and compressed. Look at the table below and see how the hardest coal, anthracite, contains the most carbon and the least hydrogen and oxygen.
| Where found | %Carbon | %Hydrogen | %Oxygen | |
|---|---|---|---|---|
| Lignite | Bovey Tracey, Devon | 67 | 5 | 28 |
| Cannel | Most British coalfields | 86 | 6 | 8 |
| Bituminous | Most British coalfields | 88 | 6 | 6 |
| Anthracite | South Wales | 94 | 3 | 3 |
| As a comparison: | ||||
| Normal Woody Fibre | Anywhere | 53 | 5 | 42 |
| Peat | Eire | 60 | 6 | 34 |
Coal is found in many parts of Britain
First find your coal
Coal represents only a fraction of the rock beneath our feet so it must be prospected for. Exploration has found coal underlying large parts of Britain.
The search for new supplies of coal begins with the geologist. His studies of the underground formations of a locality enable him to say 'I think we should do some trial borings here. The geological evidence leads me to expect to find a coal seam.' So a drill bores into the ground and samples of the strata are brought to the surface from various levels. If coal is found, further samples at increasing depths prove whether the seam is a workable proposition. if it is, a vertical shaft or an inclined tunnel known as a drift is sunk from the surface and radiating from this, a series of underground tunnels are driven. Most of our mines began in this way. Some large collieries have been established in recent years as a result of exploration.
In addition to drilling boreholes, UK COAL's geophysicists conduct other surveys to discover coal reserves including seismic surveys - using sound waves to indicate the presence of coal seams.
Coal is also present beneath coastal waters and undersea mining has been carried out since the early 17th century.
Surface Mining
Surface mining is a different method for mining coal from seams near the surface. Modern earth-moving machines like giant draglines and shovels remove the overlying earth, dig the coal and restore the topsoil at great speed. The land is then rehabilitated for future use - it can, for example, be for agriculture, country park, nature reserve, golf course or industrial or residential development.