The most prevalent of all sedimentary rocks, shale comprises a significant portion of the Earth’s crust and can be found around the world. Deposits of shale most often occur nearby lagoons, rivers, floodplains, and beaches because the slow moving water in such areas tends to carry and accrete the silt, mud, and sediments from which the rock is formed. Thus, most shales also form in thick, expansive sheets, since the particles of these materials tend to settle into layers. Sometimes within large beds of shale, sheets of other rocks, such as limestone or sandstone, are present.