The Lancashire Coalfield in North West England was one of the most important British coalfields. Its coal seams were formed from the vegetation of tropical swampy forests in the Carboniferous period over 300 million years ago. The pits on the coalfield were at their most productive in 1907 when more than 26 million tons of coal were produced. By 1967 just 21 collieries remained. Parkside Colliery in Newton-le-Willows, the last deep mine to be sunk on the coalfield, was closed in 1993.