Coal Measures In the United Kingdom, ‘Coal Measures’ refer to layers of rock specifically from a time that geologists call the Upper Carboniferous period. The Coal Measures were deposited about 310 million years ago, and these layers of rock contain many coal seams. At this time in the Earth’s history, the rocks that make up the modern United Kingdom were part of a larger landmass and were situated on the equator. This meant that the climate was very hot. During previous geological periods, more complex plants had evolved, and the Carboniferous saw the development of the first large primitive trees. Large populations of plants started to grow in wet coastal areas, and around large rivers. The late Carboniferous period saw frequent changes in sea level. These cycles of rising and falling sea levels deposited cyclical patterns of sand and silt destined to be different rock formations; shale from deep water; siltstone from lakes or lagoons; sandstones from river deltas; and coal representing the forests and swamps. These layers appear in cycles as the sea level changed throughout the period, meaning coal appears in many bands, known as seams. In the UK, these Coal Measures outcrop in many places including Fife, Midlothian, Lanarkshire and Ayrshire in Scotland; Northumberland, Yorkshire, Lancashire, Nottinghamshire and the East Midlands, Shropshire, Staffordshire and the West Midlands in England; and in Glamorgan and Pembrokeshire in South Wales. Across the Pennines, in Yorkshire and Lancashire, the rocks are folded into the ‘Pennine anticline’. This means that the rocks in Yorkshire dip to the east, and those in Lancashire dip to the west. The apex of this fold has long since been eroded away, but as you go further east in Yorkshire, the coal lies deeper beneath the surface and is overlain by thick layers of younger rocks. This area is known as the ‘concealed coalfield’.