1.7 How old is coal?
Not surprisingly, the distribution of coal deposits through time corresponds closely to the origin and distribution of land plants. (This is discussed further in Section 4.) Coals are commonly found in rocks from Carboniferous times onwards, Devonian coals are rare, and pre-Silurian true coals are never found. This coincides with evidence for the evolution of land plants, which first appeared in Silurian times about 400 Ma (million years) ago, colonized the land surface rapidly through the Devonian and became abundant by Carboniferous times. Not all post-Carboniferous terrestrial sediments contain coals, however. Coal-bearing rocks tend to be concentrated in rocks of specific ages in certain locations, reflecting the development of ancient mires under specific climatic conditions.