Great amounts of fallen organics would collect at the bottom of these anoxic swamps, forming what is termed peat. As the layers of peat and sediments grew thicker, the pressure exerted on the lowermost layers grew also, compacting the material. Subsequent heat and pressure of the overlying sediments produces chemical and physical changes in the peat. These pressures were so great they forced the hydrogen and water(along with other contaminents) out, leaving a rich carbon deposit - coal.