Coal from the Cretaceous of Colorado, USA. (public display, Durango & Silverton Narrow Gauge Railroad Museum, Durango, Colorado, USA)

Coal is a carbonaceous, biogenic sedimentary rock composed of the altered remains of plant material in ancient swamp environments. The precursor to coal is peat - a very loosely consolidated, highly porous, lightweight mass of plant fragments. With burial and diagenesis, the peat is altered to various ranks of coal - lignite coal, sub-bituminous coal, and bituminous coal. With very low grade metamorphism, bituminous coal becomes semi-anthracite coal and then anthracite coal. Each successive coal rank is harder, heavier, more carbon-rich, and hotter-burning.

The coal shown above appears to be a bituminous coal. It comes from a coal mine in Hay Gulch in southwestern Colorado. Coal from this locality is used to power steam locomotives of the Durango & Silverton Narrow Gauge Railroad, which operates tourist trains between those two towns in Colorado's San Juan Mountains.

Stratigraphy: Menefee Formation, Campanian Stage, upper Upper Cretaceous