Flexi Says: Coal formed millions of years ago when the earth was covered with huge swampy forests. When plants died, they fell into the swamp waters. Over a period of time, there was a thick layer of dead plants rotting in the swamp. After millions of years, many layers of dead plants had formed, one on top of the other. The weight of the top layers and the water and dirt packed down the lower layers of plant matter. Heat and pressure produced chemical and physical changes in the plant layers which forced out oxygen and left rich carbon deposits. Over a period of time, material that had been plants became coal. Coals are classified into three main ranks, or types: lignite, bituminous coal, and anthracite. These classifications are based on the amount of carbon, oxygen, and hydrogen present in the coal. Coal is defined as a readily combustible rock containing more than 50% by weight of carbon. In the process of transformation (coalification), peat is altered to lignite, lignite is altered to sub-bituminous, sub-bituminous coal is altered to bituminous coal, and bituminous coal is altered to anthracite.