Beds are the layers of sedimentary rocks that are distinctly different from overlying and underlying subsequent beds of different sedimentary rocks. Layers of beds are called strata. They are formed from sedimentary rocks being deposited on the Earth's solid surface over a long periods of time. The strata are layered in the same order that they were deposited, permitting discrimination as to which beds are younger and which ones are older (the Law of Superposition). The structure of a bed is determined by its bedding plane. 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.
In a quarry, bedding is a term used for a structure occurring in granite and similar massive rocks that allows them to split in well-defined planes horizontally or parallel to the land surface. Other kinds of beds are cross beds and graded beds. Cross beds are not layered horizontally and are formed by a combination of local deposition on the inclined surfaces of ripples or dunes, and local erosion. Graded beds shows a gradual change in grain or clast sizes from one side of the bed to the other. A normal grading occurs where there are larger grain sizes on the older side, while an inverse grading is where there are smaller grain sizes on the older side. By knowing the type of beds, geologists can determine the relative ages of the rocks.
A bed is the smallest lithostratigraphic unit, usually ranging in thickness from a centimeter to several meters and distinguishable from beds above and below it. The thickness of the bed is determined by the time period involving the deposition of the rocks.
In geotechnical engineering a bedding plane often forms a discontinuity that may have a large influence on the mechanical behaviour (strength, deformation, etc.) of soil and rock masses in, for example, tunnel, foundation, or slope construction.
There are geologic principles that the beds normally follow. Even though there can be cases where the principles do not apply mostly due to faults, they are true for most cases.