Brown coal deposits (black layers) in a rock face. Also known as lignite, these rock-like deposits are undergoing the process of coal formation, or coalification. The fossil fuel coal is formed over millions of years from vegetation accumulated in peat beds, which is compacted by burial under other material. The carbon content of the deposits gradually increases with time, and the deposits can be classified according to their carbon content. Lignite is formed directly from peat. Important lignite deposits exist in rocks which date from the Mesozoic era (245-66 million years ago) and the Tertiary period (66-2 million years ago). Photographed in Portugal.
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