Stratigraphy

characteristics. Currently, many geologists use the term to describe the lithologic (rock) component of a deposition system. A geologist might describe a sequence of rock as a “fluvial facies” if he or she felt the rocks were similar to those deposited by modern river systems.

The science of stratigraphy is important for the information it tells us about the rock record and how those rocks were originally deposited. The laws and principles set forth by early scientists help modern stratigraphers to know how sediment was originally deposited, to know which rock is older, and how to use fossils to help link different rock units to the same geologic time. In my next articles I will cover some of the other basics of stratigraphy, including how to handle missing rock units, and the sometimes-confusing concept of time, rock and time-rock units.

Glossary
Facies: A body of rock with specified characteristics.
Original Continuity: Law formed by Nicolaus Steno that states that sedimentary layers normally form continuous sheets that either covered the entire earth or were bounded by solid substances.
Horizontality:w formed by Nicolaus Steno that states that sediments on a solid base must have been deposited horizontally.
Principle of Faunal Succession: William Smith first realized that distinct fossil groups, or assemblages, can be used to relate two separate rock units.
Stratigraphy: Branch of geology concerned with the description, organization, and classification of rocks.
Superposition: Law formed by Nicolaus Steno that each rock layer must have been deposited onto a solid layer, and therefore each layer is necessarily younger than the layer below it.
Uniformitarianism: Concept put forth by James Hutton and summarized by the statement, “The present is the key to the past”.

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