In
geology a
bed is the smallest division of a
geologic formation or
stratigraphic rock series marked by well-defined divisional planes (bedding planes) separating it from layers above and below. A bed is the smallest
lithostratigraphic unit, usually ranging in thickness from a centimetre to several metres and distinguishable from beds above and below it. Beds can be differentiated in various ways, including rock or
mineral type and
particle size. The term is generally applied to
sedimentary strata, but may also be used for
volcanic flows or
ash layers.