The Manchester Coalfield is part of the South Lancashire Coalfield, the coal seams of which were laid down in the Carboniferous Period. Some easily accessible seams were worked on a small scale from the Middle Ages, and extensively from the beginning of the Industrial Revolution in the early 19th century until the last quarter of the 20th century. The Coal Measures lie above a bed of Millstone Grit and are interspersed with sandstones, mudstones, shales, and fireclays. The Lower Coal Measures occupy the high ground of the West Pennine Moors above Bolton and are not worked in the Manchester Coalfield. The most productive of the coal measures are the lower two thirds of the Middle Coal Measures where coal is mined from seams between the Worsley Four Foot and Arley mines. The deepest and most productive collieries were to the south of the coalfield. The coalfield is affected by the northwest to southeast aligned Pendleton Fault along the Irwell Valley and the Rossendale Valley anticline. The Coal Measures generally dip towards the south and west. Numerous other smaller faults affect the coalfield. The Upper Coal Measures are not worked in the Manchester Coalfield.