The coal seams worked in the South Yorkshire Coalfield lie mainly in the middle coal measures within what is now formally referred to as the Pennine Coal Measures Group. These are a series of mudstones, shales, sandstones, and coal seams laid down towards the end of the Carboniferous period between about 320 and 300 million years ago. The total depth of the strata is about 1.2 kilometres (0.75 mi).