coal formation

Coal is basically old dead plants. 300-400 million years ago some dead plants fell into a swamp and started to become buried in the swamp. With the absence of oxygen the dead plants began to form peat rather than just rot away. Through time, heat and pressure the peat turns to lignite, the lignite turns to bituminous coal, and the bituminous coal turns to anthracite coal. From one stage to another the carbon concentration and heat value continues to go up.