The term 'rock stratum' is a geological term used to describe a layer of sedimentary rock that has been formed over time. The crossword solution for this term is 'BED, ' which is a three-letter word. A BED of rock stratum is a layer of sedimentary rock that has been formed by the accumulation of sediment over time. This sediment can be composed of various materials, such as sand, silt, clay, and organic matter. The sediment is then compressed and cemented together to form a solid rock layer.