Stratigraphic columns are a basic tool in geology, used for everything from understanding the geologic history of a location to determining relationships between different features.

But what are stratigraphic columns really?

Stratigraphic columns are a sketch representing a sequence of actual geologic layers. Most commonly, stratigraphic columns are used for sedimentary rocks. Unless disturbed by tectonic activity, sedimentary rocks generally form on an essentially flat surface—such as from material deposited by wind on the surface of a planet, or stuff that has fallen out of suspension in water from a lake or ocean. This means that sedimentary layers typically form a sequence such that the bottommost layer is the oldest and the topmost layer is the youngest.

Each layer in the stratigraphic column is designated by symbols showing the specific (usually) sedimentary rock. Sedimentary rocks are classified by their origins: whether they are formed by physical processes (clastic) or by chemical and biological processes. Clastic sedimentary rocks are classified by the size of the grains that form them. This classification gets very complex for sedimentologists, but for our purposes we will use the chart below. For each particle size category, there are corresponding sedimentary rocks. Chemical and biological sedimentary rocks are rocks formed by chemical or biological processes.