Concealed Coalfields

A concealed coalfield is one whose Coal Measures do not reach the surface. They continue underground beneath a cover of younger rocks that were laid down across their eroded top. The part where the measures crop out is the exposed coalfield, and the two are halves of one field: east Durham, the Nottinghamshire and Derbyshire field east of its outcrop, and Kent are concealed ground. The coal is the same. What differs is that nothing at the surface shows it is there, and every shaft has to pass through the cover first.

How it forms

The Coal Measures were folded and faulted at the end of the Carboniferous and then worn down; in Durham the whole of the Upper Coal Measures and part of the Middle were removed. Younger sediments were then laid across the eroded surface, resting on it unconformably. In Durham and Nottinghamshire the cover is Permian: wind-blown sand or a thin breccia at the base, then the Magnesian Limestone, laid down as the Zechstein sea spread over the worn-down land. The cover thickens eastward, away from the exposed coalfield. Dykes in the Durham Coal Measures stop at the base of the Permian, which shows how complete the break is.

Why minerals concentrate here

The cover hid the coal. Before 1820 it was generally believed that no valuable coal lay under the Permian rocks of Durham; in that year the Hetton shafts went down through the Magnesian Limestone and found the Hutton Seam as thick and as good as in the pits to the west. A concealed field cannot be followed from outcrops and has to be proved by boring. The Kent coalfield was predicted on geological grounds in 1857 and proved in 1890 by a borehole at Shakespeare Cliff, near Dover, which found fourteen seams of coal on its way to 2,274 feet. Reserves found in this way carried the northern industry through the twentieth century: workings from the Northumberland and Durham coast eventually reached almost 7.5 kilometres offshore.

Typical shape of the deposit

A concealed coalfield is the continuation of an exposed one, under cover that thickens away from the outcrop. Pleasley Colliery stands on Magnesian Limestone about two miles east of the edge of the exposed coalfield. Farther in, the coal lies deep: the Barnsley seam beneath the Vale of York was worked by the Selby mines at depths of roughly 390 to 1,040 metres. The measures were folded and eroded before the cover was laid, so their structure does not show in the gently tilted rocks above.

What this means for mining

Every pit on a concealed coalfield starts with a sinking through the cover, and the cover is often full of water. In Durham the trouble was the Yellow Sands at the base of the Permian. Only 27 feet thick at Murton, they poured 9,000 gallons a minute into the shaft in 1837; at Haswell, where they are over 100 feet thick, the first sinking was abandoned at a loss of £60,000, and in the later nineteenth century few pits were sunk in east Durham because of that risk. The answer came soon after 1900, when the water-bearing ground was frozen before sinking: at Dawdon in 1903, Horden in 1904, and then Easington and Blackhall. In Nottinghamshire the Magnesian Limestone itself is a fissured aquifer and gave very high inflows in the Pleasley shafts. A colliery on concealed ground is therefore a deep and expensive shaft, and the structure of the measures below is learned only as it is proved: at Thurcroft the shaft itself showed that a fault had displaced the seam.

Associated commodities

Coal (Coal Measures hidden beneath younger cover rocks.)

Related geology

Anticlines and Synclines, Dykes and Sills, Sedimentary Beds and Seams

Related Mining Terms & Methods

Ground Freezing (Used from 1903 to sink the east Durham shafts through the water-bearing Yellow Sands.), Magnesian Limestone (The Permian cover over the concealed coalfields of Durham and Nottinghamshire.), Shaft Sinking (Every concealed-coalfield colliery begins with a sinking through the cover rocks.)

MineArchive examples

Included only where MineArchive's own research gives a reasonable evidential basis — not every mine where this geology might plausibly apply. Showing the 10 strongest examples of 19 linked mines.

• Snowdown Colliery — Kent (well documented example)
  Worked the concealed Kent coalfield, buried beneath chalk, which made Kent's pits deep and their sinkings wet.
• Thorne Colliery — South Yorkshire (well documented example)
  Deep coal on the concealed eastern edge of the South Yorkshire coalfield, beneath a difficult unconformable cover.
• Chislet Colliery — Kent (well documented example)
  The sinkers had to consolidate the chalk cover with cement before they could get through it.
• Eppleton Colliery — Tyne and Wear (well documented example)
  The productive measures lie beneath the Magnesian Limestone and the pits had to be sunk through it.
• Haswell Colliery — County Durham (well documented example)
  Won through the Magnesian Limestone and about 20 fathoms of water-bearing quicksand beneath it in the early 1830s.
• Nailstone Colliery — Leicestershire (well documented example)
  The shafts passed through some eighty yards of water-bearing New Red Sandstone before reaching coal.
• Sandwell Park Colliery — West Midlands (well documented example)
  The first colliery to work the concealed part of the South Staffordshire coalfield, east of the boundary fault.
• Silksworth Colliery — Tyne and Wear (well documented example)
  East Durham concealed coalfield, beneath the Magnesian Limestone that made the coast-field sinkings difficult and expensive.
• Thurcroft Colliery — South Yorkshire (well documented example)
  The seam's displacement by a fault was not established until the shaft itself proved it.
• Gascoigne Wood Mine — North Yorkshire (historical documentary reference)
  The drift entry to the Selby coalfield, a deep concealed field beneath the Vale of York.

See all 19 linked mines.

Sources

1. (primary) Early Permian magmatism, Northern England — British Geological Survey (Earthwise)
2. (primary) Fuel and energy, geology and man, Northern England (British regional geology: Northern England, 5th edition, 2010) — British Geological Survey
3. (primary) Smith, D. B. and Francis, E. A. (1967). Geology of the country between Durham and West Hartlepool (Memoir for Sheet 27), Chapter 4: Intrusive igneous rocks - the Hett Dyke — British Geological Survey
4. (secondary) Geological — Pleasley Colliery (Friends of Pleasley Pit)
5. (secondary) Kent Coalfield — Wikipedia
6. (secondary) Selby Coalfield — Wikipedia

Record created: 1 October 2026 · Last researched: 1 October 2026