Coal is a form of amorphous carbon chiefly found beneath the earth, from whence it must be mined if it is to be used. It is chiefly used as a fuel, since burning it produces large amounts of heat energy. Coal is a fossil fuel, which means that it is composed of the remains of biological organisms, which have decayed and become coal over time through geological processes. Fossils of plant material are frequently found in coal.
Coals seams are typically tens of centimeters to tens of meters thick and can extend for tens of kilometers. Often a layer of coal will split somewhere and become two separate layers separated by inorganic sediment. Occasionally there will be one split opening in one direction and another split opening in another direction, resulting in a sort of z-shaped seam of coal. Since coal results from the accumulation of organic matter over a large area, seam splits can be understood in a number of ways. If the organic material is accumulating in a flat area and tectonic motion causes the area to tilt, then the lower region may be covered with sediment, after which the accumulation begins again. The split will appear where the new surface intersects the old one. Alternatively, the region where the organic material is accumulating may have a slight tilt to begin with. A rise in sea level could cause the lower regions to be buried under sediments while the upper regions remain exposed. A split results if the conditions allow the accumulation to begin again over the exposed old material as well as the new sediment.