A fine-grained or fine-textured sedimentary rock formed by the cementation or compaction of silt and clay (mud): "Shale has thin layers (laminations) and can be split into thin layers or small chips."

"Shale and mudstone are composed of a mixture of the finest particles of clay and silt; however, the difference between them is that shale splits into thin layers while mudstone is thickly layered and breaks into massive chunks."