Coal lies in repeating layers of sandstone, shale and clay, and the clues that a seam is close are fossil plants in the shale, a pale clay full of root traces directly under the seam, bands of rusty ironstone nodules, and springs where water meets an impermeable layer. Burnt red rock on a hillside and shallow ring-shaped pits mark places where coal outcropped and was burned or dug.
Part of Coal Surface Mining
Reading the rock sequence around a suspected coal seam: which layers travel with coal, and which surface features betray one.
A Coal Seam Is Surrounded By A Predictable Stack Of Rocks
Coal forms where plant matter piled up in a swamp and was buried. The rock record of that swamp repeats in a recognisable order, and it is easier to spot the neighbours than the seam itself.
- Underclay (seatearth). Directly under many seams lies a pale, sticky clay that is the fossil soil the swamp plants grew in. It is crossed by traces of roots and breaks into blocky lumps with polished faces. Finding root-marked clay means a seam was once on top of it, even if the coal has weathered away.
- Dark shale above the seam. The mud that buried the swamp often holds plant fossils: fern-like fronds, bark with a regular diamond pattern, stems. Fossil leaves in black shale are the best single sign.
- Sandstone and shale in alternation. Coal is typically one thin layer in a long stack of sandstone and shale. A cliff of hard sandstone with soft shale below it is a place to look at the foot of the cliff.
- Ironstone nodules. Rusty, dense, rounded lumps weathering out of shale are common neighbours of coal seams.
Surface Features That Betray A Seam
Springs at a layer boundary. Shale and clay block water, so springs often emerge at the base of a sandstone that sits on shale. A line of springs along a hillside suggests a layer boundary running along it, which may carry a seam.
Terraces and benches. Hard layers stand out as steps along a slope; a soft layer between them is a groove. Seams are often in the grooves.
Burnt rock. Where a seam outcropped and caught fire in the past, the shale above it is baked hard and brick-red, a colour that stands out from the surrounding grey and brown. Look for red, clinker-like rock on a slope.
Old pits and spoil. Rings of low mounds each around a shallow hollow, and heaps of black fragments, show where people dug in the past.
Common Mistakes
- Mistaking dark topsoil or peat for coal. Soil and peat are not layered rock; they crumble to fibres or humus and do not streak like coal.
- Assuming young lignite follows the same pattern. Soft brown coal is geologically younger and may lie in sand and clay close to the surface with no hard rock above it. The clues above still hold, but expect softer, wetter material.
- Ignoring dip. Layers tilt. A seam seen on one hillside will not be at the same height on the next unless you allow for the tilt (see the outcrop-identification article).
Safety
Prospecting ground may be hollow
Ring-shaped pits and sunken lines can be old shafts whose roofs have thinned to a crust of soil. Probe ahead of every step with a pole, never prospect alone, and keep clear of the base of steep faces. Burnt-rock slopes may still hold smouldering coal underground: ground that is warm, steams in rain or smells of tar can drop away under your weight.
What’s Next
- Learn next: Outcrop Identification — tracing and measuring a seam you have found
- Combine with: Coal Types & Quality — judging what you have found