The term 'layer of coal' can be summarized as a geological formation consisting of a single bed or SEAM of coal. A SEAM is a layer of sedimentary rock that is thicker than a stratum and can be traced over a large area. Coal seams are typically found in sedimentary rocks such as shale, sandstone, and limestone. They are formed when organic material is compressed and heated over time. Coal seams are mined for their energy content and are used to generate electricity and heat homes.