In Geology, a stratum is one of the natural layers, or beds, that make up sedimentary rock formations. Each stratum records conditions from a particular period of deposition, such as mud settling in a lake, sand accumulating on a shore, or organic material gathering in a swamp. Poets may use strata literally to evoke a landscape’s physical history, or figuratively to suggest accumulated layers of time, memory, culture, or experience.

Stratum comes from the Latin stratum, meaning “a covering” or “something spread out.” Its plural is traditionally strata, though stratums is sometimes used in general English. In Geology, strata are especially associated with sedimentary rocks, which form in layers through deposition over long periods. Related terms include bed, a layer of sedimentary rock; layer, a more general word; and stratigraphy, the study of rock layers and their sequence. Do not confuse stratum with a social stratum, which means a level or class within society. Poetry may deliberately draw on both meanings when discussing layers of land, history, or human life. In literary writing, strata can serve as a metaphor for buried histories, inherited memories, or successive emotional experiences.