Stratum

In geology and related fields, 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 called bedding surfaces or bedding planes.

Before a special guide called the International Stratigraphic Guide was published, people used different meanings for the word "stratum." Sometimes it meant a single layer, sometimes many layers together, or even very thin layers. 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 lies on top of another layer. These layers are separated by surfaces called bedding planes. When many layers stack together, they can form large areas covering hundreds of thousands of square kilometers of the Earth's surface. You can often see these layers as bands of different colors or textures 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 be formed in different ways, such as from river silt, beach sand, coal swamps, sand dunes, or lava beds.

Types

Geologists have identified several types of layers in rocks and sediments. A bed is a single layer that looks different from the layers above and below it. Only beds that are special enough to help scientists study and map rocks get formal names.

Other types include a flow, which is 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 and easy-to-spot layer that helps scientists match layers in different places.