Imprints of leaves and stems of plants are often found in the roof of a coal seam – that is, in the rocks lying immediately above the seam, and occasionally a tree stump is found in this position. In the floor, or rocks just below the seam, dark worm-like markings are seen which prove to be the remains of tree roots. When a very thin slice of coal is examined under the microscope it is seen to contain fragments of plants. When a chemist analyses a piece of coal he finds that it is a sort of altered wood.

These facts leave no doubt that coal is made up of plant material but to understand how Britain's coal seams were formed over 200 million years ago, we must first study something that is happening at the present day.

In some parts of the world there are densely forested swamps, one of the best-known of which is the Dismal Swamp in America. This is a low-lying area, only a few feet above sea level, covering an area of about 1,500 sq. miles in the States of Virginia and North Carolina. Trees are still growing in the swamp, but those that have died have formed a layer of decaying vegetable material – leaves, branches and trunks – about seven feet thick.

The same thing is happening in swamps on the delta of the Ganges in India, and here borings have shown that there are several buried layers of plant material, changed to a kind of peat, separated by layers of sand and mud. Obviously each layer of peat was formed at the surface. The land then sank below the water and sand and mud carried down by the river were laid on top until the water became shallow enough for trees to grow again and form another layer of peat. Similar forest swamps are also found in the deltas of other tropical rivers.

With these happenings of the present day in our minds we can now go back and picture how Britain's coal seams were formed 250 million years ago. At this time Britain had not its present shape. Instead, the area where Great Britain and Ireland now stands was largely a vast shallow estuary or lagoon. Hilly land lay northwards from the middle of Scotland, westwards from Ireland, and southwards from England's present south coast, and from these hills great rivers flowed into the estuary. The whole area was sinking slowly, at such a rate that for most of the time the sand, mud and clay deposited by the rivers on the bottom of the estuary kept the depth of water fairly constant.

From time to time, however, there was a pause in the sinking, so that the sand and mud nearly filled up the shallow estuary and turned it into a great swamp. In this swamp trees and giant ferns began to grow, and encouraged by the warm damp climate of those days they soon formed dense forests like the one shown at the bottom of this page. The forest swamp condition lasted for many centuries, during which the trees as they died formed a thick sludge of partly decayed vegetable matter, forming a kind of peat.

Eventually, sinking began again, the forests were overwhelmed by water, and the rivers again poured layer upon layer of sand and mud over the area. But from time to time the sinking was checked the water became shallow, new forests grew up, and new layers of peat were formed.

After millions of years these conditions came to an end. The areas of land and sea changed, and although thousands of feet of rocks were formed on top of the layers of peat, sand and clay, they never again (at least in Britain) contained any peat layers from buried forest swamps. Owing to the enormous pressures of the overlying rocks, as well as to a slight rise in temperature due to being buried so deep, the peat layers became gradually changed into coal. The other layers also became hardened, the sand being changed to hard sandstone, and the clay to shale.

When colliery shafts are sunk to reach the coal seams they pass through the different layers of rock (called 'strata', which simply means 'layers'). The illustration shows the rocks passed through by three shafts a few miles apart. Notice that the seams are at different depths in each shaft, and also that they vary in thickness; Seam A gets steadily thinner from the first to the third shaft.