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Stratigraphy

© Geoff Habiger

In a previous article I discussed how sediments become sedimentary rock. As more and different sediments accumulate in a specific area, the rocks that form from them become a sequence. The study of these sequences by geologists is called stratigraphy. Stratigraphy is the branch of geology concerned with the description, organization, and classification of rocks. Geologists use stratigraphy to relate different rock units that are separated over vast geographic distances, as well as to compare different rock units to the same geologic time. Over the next few articles I will cover some of the basic concepts of stratigraphy.

Many of the basic principles and laws stratigraphers use everyday were originally formulated several hundred years ago. A Danish physician named Niels Steensen (1638 – 1686) set forth the first of these basic principles. Today we refer to Steensen by his Latin name, Nicolaus Steno. Steno was the first to recognize that solid rock had not always been solid, but had originally been soft sediment. From this and other observations, he defined three basic laws.

The law of Original Horizontality states that sediments on a solid base must have been deposited horizontally. Otherwise they would have slid to a lower point. Therefore, rocks that lie at an angle must have been tilted after the sediments were lithified. The law of Original Continuity states that sedimentary layers normally form continuous sheets that either covered the entire earth or were bounded by solid substances. Therefore, similar layers of rock on either side of a valley must have been separated by erosion from their original continuous state. The law of Superposition states that each layer of sediment must have been deposited onto a solid layer, and therefore each successive layer is necessarily younger than the layer below it. When looking at an outcrop of rocks, the oldest rocks are on the bottom, the youngest at the top.

Steno’s laws form the basis of much of stratigraphy. Another early naturalist, James Hutton (1726 – 1797), observed closely the process of how sediments formed sedimentary rocks. Hutton realized that sedimentary rocks formed as the result of continual earth processes, and that if a rock formed in one manner today, it must have formed in the same manner in the past. This concept became known as uniformitarianism and can be summarized by the phrase, “The present is the key to the past.” Using this principle a geologist can look at a how rocks form in a particular environment today, such as rocks formed from the deposit of river sediments. The same geologist can then look at a sequence of rock at an outcrop and determine with good accuracy the type of environment that formed the rock layers.

The copyright of the article Stratigraphy in Everyday Geology is owned by Geoff Habiger. Permission to republish Stratigraphy in print or online must be granted by the author in writing.