Coal is a combustible, rocklike substance formed from plant remains that have undergone physical and chemical changes through geological processes. In early geological times a warm, moist climate prevailed and swamps covered large parts of the world. Ferns, reeds, mosses, and other plants grew in great numbers, sometimes reaching as much as 200 feet in height. After the plants died are sank to the bottom of the swamps, water and new layers of plant material sealed them off from the air, preventing them from decaying completely. Some of this mass was covered by the sea, which deposited layers of inorganic sediment, sand, and dirt, forming the overburden now found over coal seams. Thus, coal lies in giant subterranean sandwiches, shallow or deep, flat or pitched. Most coal beds are broad and thin, and are within 3,000 feet of the surface.