Stratigraphy is the branch of geology which studies rock layers (strata) and layering (stratification).
Stratigraphy deals primarily with sedimentary rocks but also embraces layered igneous rocks where layers result from successive lava flows.
A common goal of stratigraphic studies is the interpretation of sequences of rock strata, thus understanding the time relationships involved, and correlating units of the sequence with rock strata elsewhere. Nicholas Steno described four principles of stratigraphy in the seventeenth century, including the law of superposition which states that states that, in undeformed stratigraphic sequences, the oldest strata will be at the bottom of the sequence. These ideas still underpin modern stratigraphy which is governed by The International Commission on Stratigraphy. Its primary objective is to precisely define global units (systems, series, and stages) of the International Chronostratigraphic Chart that, in turn, are the basis for the units (periods, epochs, and age) of the International Geologic Time Scale.
Stratigraphy has application in many scientific fields, including archaeology, palaeontology and in the search for natural resources. This succinct and accessible introduction to stratigraphy will prove helpful to students and amateur geologists alike.

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