Near the end of the Carboniferous period, while the last of the Coal Measures were being formed, earth-movements began to develop. At first they were very gentle, though they were enough to upset the conditions required for forming coal seams, which are therefore rare in the upper part of the Coal Measures. Later, as the movements continued, the rocks were folded into synclines and anticlines, as well as being broken by faults. The whole area was also raised to form dry land.

As the anticlines or arch folds were pushed up, they were at the same time worn away particle by particle by frost, wind, rain and rivers, the particles (sand and clay) being collected in the hollows formed by the synclines. Over large areas the Coal Measures on the anticlines were worn right away, so that all that remained of the great sheet of Coal Measures, originally covering most of Britain, was smaller separate areas preserved in the synclines.

Eventually, the earth movements came to an end. The land sank again, and thousands of feet of other rocks (without coal seams) were formed above the Coal Measures. These rocks have since been raised again to form dry land, and have for millions of years been worn away by frosts, winds, rain and rivers.

Above is a map of Britain showing the position of the coalfields. Where the Coal Measures are still covered by the later rocks the area is called a 'concealed coalfield', because the Coal Measures cannot be seen at the surface of the ground. The coalfield in Kent, shown below, is an example of a concealed coalfield.

In other coalfields the later rocks have been worn entirely away, and the Coal Measures are now at the surface again. These areas are called 'exposed coalfields'. Several British coalfields are exposed in some parts and concealed in others.

The exposed coalfields in this country have been known for hundreds of years. They were probably discovered by accident when people were digging wells for water, and deep cellars. In the concealed coalfields the existence of coal is proved by drilling deep holes from the surface, and this method is also used to explore the deeper parts of the exposed coalfields.

From the information obtained from boreholes and shafts it is possible to build up a picture of the 'lie' of the different rock layers (including coal seams) below the surface. The illustration is a section' across the Lancashire and Yorkshire coalfields – that is, it shows what would be seen if England could be cut in two across those coalfields.

Looking at this section it is easy to imagine that the coal seams once stretched right across this part of England. As earth-movements pushed up the Pennine arch-fold or anticline, the Coal Measures over this high range were slowly worn right away, leaving separate coalfields on each side.

The seams of coal can be seen to outcrop close to the foot of the Pennines on each side. Folding and faulting have both occurred in each coalfield but they are more severe in Lancashire. The western part of Yorkshire is an exposed coalfield, and the eastern part is concealed.