Geologists call individual layers of sedimentary rocks beds, whereas everal of them together are called strata.
Sedimentary rocks are formed when sediment is deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension. The sediment is often formed when weathering and erosion break down a rock into loose material in a source area. Strata are layers of rocks that are laid down as sediment, often in the sea, and are slowly changed by pressure, heat and chemical actions into rocks.