Stratum

A stratum (pl.: strata) is a layer of rock or sediment. Each layer has special qualities that make it different from the layers above and below it. These layers are separated by visible surfaces.

Before a special guide called the International Stratigraphic Guide was published, people used different meanings for the word "stratum." Other words related to strata include substrate and substratum, which describe layers that lie under another layer.

Characteristics

A stratum is a layer of rock or dirt that sits on top of another layer. These layers are split by surfaces called bedding planes. When lots of layers pile up, they can cover huge areas of the Earth. You can often spot these layers as colorful bands in cliffs, road cuts, quarries, and river banks. These bands can be very thin, like a few millimeters, or very thick, several meters or more. Each band can form in many ways, such as from river silt, beach sand, coal swamps, sand dunes, or lava beds.

Types

Geologists have found many types of layers in rocks and sediments.

A bed is a single layer that looks different from the layers above and below it. Only special beds get formal names.

Other types include a flow, a layer of volcanic material that can be identified by its texture or composition. A band is a thin layer that stands out because of its color or material. Lastly, a key bed (or marker bed) is a very clear layer that helps scientists match layers in different places.