Rock strata are layers of material laid down by natural forces. Strata consist of similar material and may extend over hundreds of thousands of square kilometers of the Earth's surface. Strata are typically seen as bands of different colored or differently structured material exposed in cliffs, road cuts, quarries, and river banks. Individual bands (plural: strata, singular: stratum) may vary in thickness from a few mm to a kilometer or more. Each band represents a specfic mode of deposition -- river silt, beach sand, coal swamp, sand dune, lava bed, etc.
Geologists study rock strata and categorize them by material in the beds. Each distinct layer is usually assigned a "formation" name usually based on a town, river, mountain, or region where the formation is exposed and available for study. For example the Burgess Shale is a thick exposure of dark, occasionally, fossiliferous shales exposed high in the Canadian Rockies near Burgess Pass. Slight distinctions in material in a formation may be described as "members" or sometimes "beds." Formations are collected into "groups." Groups may be collected into "series."