Coal was formed several times at different geological periods in the long history of earth. Most of the coals were formed in Penn sylvaanian period. That was the period from 250 million years ago to 3.5millions years ago.
Coal is the remains of plants and trees mostly giant ferns of swamps or marshy land. These plants got buried into swamp water and protected from rotting. Due to the action of bacteria some parts of plant body (wood) changed into gas and escaped leaving carbon. This process had repeated for many years and the carbon layer became thick and buried under the rock layers during the geological change. Most of the water squeezed out leaving carbon due to the pressure of rock layers formed above and it then hardened to form coal in the long run.
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