The term 'layer of rock' can be described by two crossword solutions: STRATUM (7 letters) and mason (5 letters). A STRATUM is a layer of sedimentary rock that is formed from the accumulation of material over time. It is usually composed of different types of minerals and organic matter. A mason is a person who works with stone, brick, and other materials to construct buildings and other structures. They use their skills to shape and lay the stones in order to create a strong and durable structure.