Shallow water that is warm and stagnant contains very little oxygen, so the leaves and stems that fall into it are sealed off from scavengers and thus preserved. These materials are first converted into peat, a low-grade fuel that contains about 50 percent carbon. If the peat remains buried and is subjected to elevated temperatures and pressures, some of its volatile components are driven off. Since carbon is not lost in this process, its relative percentage is increased and the material becomes more compact. Since coal represents the accumulation of material on the floor of ancient swamps, the coal tends to form in layers known as seams.
Seams can range in thickness from a few inches to many feet.
Usually only seams that are several feet thick are mined .