Coal originated from ancient plants that flourished in swamp-like environments millions of years ago.  In Virginia, coal was formed mainly during the Carboniferous period of the Earth's history, 280 to 360 million years ago.  There are also coalbeds in Virginia that were formed during the Triassic period, 205 to 245 million years ago.  By comparison, most of the coals in the western U.S. were formed during the Cretaceous period, 70 to 140 million years ago, and the Tertiary period, 2 to 70 million years ago.
 
 

            At the time in Earth's history when the Appalachian coals were accumulating, there was no Atlantic Ocean.  What was to become the North American continent was situated near the equator, slowly drifting northward.  This large mountainous landmass was east of our present-day east-coast and a vast, shallow sea stretched to the west, submerging even the Great Plains.  The influence of the marine sea decreased and the water became brackish as freshwater influence from the eastern highlands increased because of extensive deltaic plains developing from the erosion of these highlands.  Extensive "swamps" covered much of the coastal lowlands that lay along the western margin of these highlands.  The perennially wet, tropical climate at that time was particularly conductive to coal formation.

            Leaves, stems, spores, free trunks, branches, plant roots, resins, charred wood from swamp fires, other organic material, and mineral (inorganic) matter were deposited within the swampy basins.  Thick deposits of partially decomposed vegetal debris (peat) accumulated under the cover of stagnant water in these basins.  Over time this peat was buried by layers of sediment from deltaic and coastal processes or by an occasional rising of sea level.  Continued burial and the influence of the earth's thermal gradient subjected the peat to pressure and heat.  Eventually, the heat and various chemical and physical changes transformed the peat into coal.  This process is called coalification.  A very high volume reduction occurs in the transformation of peat to coal.  Although coal can range in thickness from less than one inch to more than 100 feet, it has been estimated that 3-10' of compacted vegetal matter are necessary to form each foot of coal.  The extensive accumulation of thick coals involve slow rates of basin subsidence; this implies a minimal change of sea level (or a "stand-still") and an equable tropical climate.  The final composition of the coal depends upon the original ratio of organic to inorganic matter found in the parent peat.