Charcoal results from incomplete combustion of plant material. It is produced naturally by wildfire, and being relatively tough and unbiodegradable it may be transported and incorporated into a variety of sedimentary environments. Wildfire requires adequate atmospheric oxygen for the combustion of plant fuel (wood, leaf litter). Fossil charcoal from the Devonian onwards suggests that the oxygen level in the atmosphere has not fallen below 13% in this interval. Further, the extreme flammability of even wet plant fuel at oxygen levels above 35% makes this the highest figure compatible with the occurrence of terrestrial vegetation.