There are other chemical sedimentary rocks that follow are produced by
different processes. Coal, is an example of a chemical sedimentary rock
that is derived from plant matter that has been buried. Coal comes in 3
major forms including anthracite which is a hard coal, bituminous
which is softer, and lignite which is the softest with
recognizable plant remains.
The rich coal fields in the East Coast of the United States were
created during the Carboniferous Period (about 350-300 million
years ago) when huge swampy forests of giant ferns, reeds and mosses,
which were around grew taller than our tallest trees today. As these
plants died and fell into the swamp water, new plants grew to take their
place and when these plants died, still others grew. In time, there was
a thick layer of dead, decaying plants in the water. Decomposition,
chemical alteration, and burial pressure formed different coal types.