A rock layer is a type of geological formation that consists of sedimentary, igneous, or metamorphic rocks. The term 'SHELF' (5 letters) refers to a flat, horizontal surface of rock that is exposed at the surface of the Earth. A STRATUM (7 letters) is a single layer of sedimentary rock that is part of a sequence of layers. STRATA (6 letters) is the plural form of STRATUM and refers to multiple layers of sedimentary rock.