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The wave-cut Carboniferous limestone reef of Overton Mere (Walks 15 and 17)

These stable conditions were interrupted around 320 million years ago by earth movements caused by approaching continents from the west and south. The compressive forces within the earth’s crust caused the nearby landmass to be forced upwards and the increased rate of erosion flooded the limestone sea with sediments of sand, shale and mud from the river deltas.

This transition from limestone is marked by a coarse sandstone known as millstone grit, originally laid down by fast-flowing rivers. In its lower layers the gritstone contains massive white quartz conglomerates and sandstones, within which there are very pure bands of over 99 per cent quartz that were once worked for firebrick.

The next succession, the Coal Measures, originated in a widespread system of river deltas close to sea level, upon which grew lush tropical forests of giant mosses, horsetails and ferns that eventually became the coal. The Measures consist of sandstones, shales and coals arranged in a repeated sequence, as the forests flourished for a time, were inundated and buried by mud and sand as sea levels rose, and then developed once more on the river delta shales as the sea retreated.