Fossil fuels are the remains of once-living organisms that were preserved and altered as a result of geologic forces. Significant differencs exist in the formation of coal from that of oil and natural gas. the oil ases in Coal many Coal was formed from plant material that had been subjected n, they to heat and pressure. Freshwater swamps covered many regions of the Earth 300 million years ago. Conditions in these favored extremely rapid plant growth, in large lations of plant material. Because this plant material collected under water, decay was inhibited, and a spongy mass of organic material formed. It is thought that the chemical nature of these ancient plants and the lack of many kinds of decay organisms at that time also contributed to the accumulation of plant material. Today we see deposits of plant materials, known as peat, being formed in bogs. Due to geologic changes in the Earth, some of these organic deposits were submerged by seas. The plant material that had col- lected in the swamps was then covered by sediment. The weight of the sediment on top of the deposit compressed it and heat from the Earth caused the evaporation of water and other volatile com- pounds. Thus, the original plant material was transformed into coal. Depending on the amount of time the organic matter has been subjected to geologic processes, several different grades of coal are produced(See 9.3.) Figure 9.4 describes the district 015 button of world coal deposits.