A single layer of sedimentary rock or soil that formed during a specific time period and under particular conditions. Multiple strata stack on top of each other to create the layered appearance seen in cliffs and rock formations.
From Latin 'stratum' meaning something spread or laid down, from 'sternere' meaning to spread or strew. The geological usage developed in the 1600s when scientists began systematically studying rock layers and realized each represented a distinct period in Earth's history.
Each stratum is like a page in Earth's autobiography! Geologists can read these rock layers like a timeline - a red stratum might indicate an ancient desert environment, while a black one could show where an ancient swamp created coal deposits. The famous White Cliffs of Dover are made of chalk strata formed from billions of tiny marine organisms that lived in warm seas 100 million years ago.
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