A STRATUM is a LAYER of material, often sedimentary rock, that is found in the Earth's crust. It is typically composed of a variety of materials such as minerals, fossils, and organic matter. The term 'STRATUM' is derived from the Latin word for 'LAYER'. Strata are used to study the geological history of an area, as they can provide information about the environment in which the rocks were formed. They can also be used to identify different types of rocks and minerals.