In the centre of the Forest of Dean basin are Carboniferous shales and sandstones with coal seams, belonging to the Coal Measures. These are underlain by older, iron-ore-bearing Carboniferous limestones, which crop out and locally form steep cliffs around the rim of the basin.
During the folding of the rock strata that moulded the synclinal basin, the western side of the Forest of Dean was tilted less steeply than the eastern side. The difference of structure between mines on each side of the basin meant that very different approaches to mineral extraction were adopted in each area.
The iron-ore mines are part of ancient natural cave systems that began their development mainly within a bed of Carboniferous limestone known locally as the Crease Limestone, fairly soon after the rock formed, some 330 million years ago. Later, about 225 million years ago at the start of the Late Triassic Epoch, the surface of the area became a hot desert, totally unlike our modern landscape both in climate and appearance.
Occasionally, torrential rain storms, far heavier and more prolonged than anything that we experience today dissolved iron minerals from the arid land surface. Massive floods of acidic, iron-rich water then entered the older cave systems, where iron-ore minerals were deposited as the water was neutralised by contact with the limestone.
Millions of years later, at about the same time that the Alps were forming elsewhere in Europe, the whole of the Forest of Dean area was uplifted again. The ancestors of major rivers, the Wye in the west and the Severn in the east, and their tributaries, eroded deep valleys through the rocks of the basin, locally cutting through the old cave systems, exposing the iron-ores that most of them now contained.
The road that now runs outside the Clearwell Caves entrance follows the now-dry valley of a former tributary to the river Wye, which probably fed underground streams until the end of the cold climatic phase associated with the last Ice Age, around 10,000 years ago.
The original source of iron mineralisation still mystifies geologists as the remaining strata above the limestone contains relatively little iron mineralisation compared to the vast quantity that became deposited into the cave systems that eventually became iron mines