Stratum

Typically, a stratum is generally one of a number of parallel layers that lie one upon another to form enormous thicknesses of strata.

[3][4] Stacked together with other strata, individual stratum can form composite stratigraphic units that can extend over hundreds of thousands of square kilometers of the Earth's surface.

As in case of a bed, a flow should only be designated and named as a formal lithostratigraphic unit when it is distinctive, widespread, and useful for stratigraphic correlation.

A band is a thin stratum that is distinguishable by a distinctive lithology or color and is useful in correlating strata.

Finally, a key bed, also called a marker bed, is a well-defined, easily identifiable stratum or body of strata that has sufficiently distinctive characteristics, such as lithology or fossil content, to be recognized and correlated during geologic field or subsurface mapping.

Strata in Salta ( Argentina )
Goldenville strata in quarry in Bedford , Canada. These are Middle Cambrian marine sediments. This formation covers over half of Nova Scotia and is recorded as being 8,800 m (29,000 ft) thick in some areas.
The Permian through Jurassic strata in the Colorado Plateau area of southeastern Utah demonstrate the principles of stratigraphy . These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park . From top to bottom: Rounded tan domes of the Navajo Sandstone , layered red Kayenta Formation , cliff-forming, vertically jointed, red Wingate Sandstone , slope-forming, purplish Chinle Formation , layered, lighter-red Moenkopi Formation , and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area , Utah.