Stratigraphy is they classification of different layers or layering of sedimentary deposits, and in sedimentary or layered volcanic rocks. This field is important to understanding the geological history and forms the basis for classification of rocks into distinct units that can be easily mapped. Stratigraphy is therefore the key to understanding the structure of the bedrock in time and space.

Stratigraphy can can be divided into several subfields:

Lithiostratigraphy is the classification of rock units on the basis of their physical and mineralogical properties and relationships to other, surrounding rocks.  A lithiostratigraphic unit comprises a sedimentary, igneous or metamorphic-equivalent layer that is defined on the basis of the layer's composition and position in a succession of layers. The basic principle is that younger layers are deposited over older layers.

Biostratigraphy is used to divide layers or successions of layers in units (biozone) based on the presence of one or more fossils that are characteristic of the zone. This classification may therefore only be used when it is possible to identify such fossils. The method is based on the assumption that the fossils that are found across succession of layers will reflect how the evolution through time has led to changes in life forms.

Chronological Stratigraphy deals the classification a rock type based on date of its formation.     The purpose of this is to have a common international system to classify rocks in accordance with an established geologic time scale (geochronological units).  Link to

Magnetiostratigraphy deals with the measurable variation on remanent (inherited) magnetic properties. Changes in the Earth's polarity during the deposition of sediments a tool used along with other stratigraphic methods to determine the age of a deposit.