In geology, a key bed (syn marker bed) is a widespread sedimentary layer that formed at a single time, such that it is useful for geologic correlations and dating over a large area. Examples of key beds are massive ashfalls, such as those produced by nearby normal volcanic eruptions, and far away in supervolcanic eruptions, as well as tills deposited by continental glaciers, and the global iridium layer deposited at the K–T boundary.