File:Looking towards Harworth pit - geograph.org.uk - 1350430.jpg

English: Looking towards Harworth pit The view over the landscape between Firbeck and Harworth. This was taken on the colliery spoil heap giving good views all round.

This area in the Upper Carboniferous (around 360 MYA) was a delta front. The mountains of the north (Scotland) were eroding fast and the deposits formed a delta (like the Himalayas and India today). The delta was rich in plant life which over years died and were deposited. Sea levels changed over time so the peaty matter was placed into aerobic conditions and decomposition stopped. The change in sea levels meant successive coal beds (sea meant different deposits). So you would find gravel, coarse sand, medium sand, fine sand, silt, sea earth then coal. The cycle would repeat itself with a sea level rise. This cycle is called a cyclothem.

As the delta moved south it invaded on the Coral limestones, corals need clear seas so the sediment killed them. This is why you get limestone, millstone grit then coal. They are all at the surface because of a massive antiform which folded the rocks into an arch shape. The left hand limb eroded away leaving Limestone at Castleton, then millstone grit around Stannage edge, after that surface mines and further east are deep mines. There is even deeper coal under Lincoln and under the North sea.