310 million years ago the area that was to eventually become Great Britain lay across the equator. The great delta that had been built in northern England in Millstone Grit times was now largely covered by plants which grew rapidly in the hot, wet conditions.

The forests teemed with insect life and the rivers and swamps were populated by fish, bivalves and large amphibians. Dying plants fell into stagnant water where they accumulated to form thick layers of peat.

Large areas of forest were repeatedly flooded by either the sea or rivers, forming vast lagoons. Mud, then sand, covered the peat layers as the lagoons slowly silted up and the forests grew again. This happened many times, slowly converting the buried peat to coal as it was compressed by the weight of overlying sediment. Over millions of years several thousand metres of mudstone, sandstone and coal were deposited in repeating units called cyclothems.

While coal only formed about 5% of the total thickness of the Coal Measure sediments its impact on the region has been enormous.

Coal Measures cyclothems