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Coal is formed from peat, which is an accumulation of decayed vegetation usually associated with swamps. The process of the transformation from peat to coal is called coalification. In the stages of coalification, peat is altered to lignite, lignite is altered to sub-bituminous coal, sub-bituminous coal is altered to bituminous coal, and bituminous coal is altered to anthracite coal. Most Kentucky coal is bituminous, but some lignite occurs in western Kentucky.

Coal is found in Pennsylvanian rocks in eastern and western Kentucky, where it is interbedded with shales, sandstones, conglomerates, and thin limestones. The Pennsylvanian Period began approximately 320 million years ago and lasted about 30 million years; it is commonly referred to as the Coal Age. Minor coal deposits are also found in Cretaceous and Tertiary rocks in the Jackson Purchase Region of Kentucky.

For more information, see Coal section.