Ask GeoMan...

What is a stratigraphic column?

This is going to be a long answer, so pull up a chair and get comfortable. While it's fun to look at the earth and try to figure out what has happened in a particular place, one of the most important things that geologists do is to attempt to combine these bits of local information into a regional model of earth history. This is much tougher, but in the long run well worth the effort.

Several steps/processes are involved in any attempt to unravel earth history. It all starts with the rocks. Lithology is the study of bedrock: the rocks which occur in a specific location. While this obviously includes igneous and metamorphic rocks, sedimentary rocks are very important for several reasons: many of the classical laws of geology relate to the accumulation of sediments, and the formation of sedimentary rocks; and they may contain fossils which aid in determining how old they are. Therefore sedimentary rocks can also help in our understanding of both sequence and timing.

As fate would have it, the earth works in a completely different time frame than we do, and all earth processes don't even work at the same speed. Two of the main processes which have different timing are the tectonic forces which build the mountains, and the surface processes which tear them down and transport the sediments to low energy environments where they are deposited. This often results in thick piles of similar sediments which accumulate before there is a fundamental change in the area's depositional environment. Changes in sea level provide a great example. The distribution of sediment on the seafloor is directly related to the amount of energy in the water, and the energy is directly related to the depth of the water and the distance to the beach. In general, the earth deposits big stuff (sand and gravel) near the beach, with silt and clay piling up farther offshore. As the land rises and falls in response to tectonic forces, the higher energy shore zone (the sandy beach) changes location, moving "offshore" or "onshore" in response to the resulting fluctuations in sea level. But the sand is coming in faster than the tectonic processes are causing the sea level changes, so it's common to get a thick pile of sand before water depth changes and something different is deposited. Geologists group together similar lithologies, and call these larger sedimentary sequences formations. There are some fairly simple rules on how formations are named, mostly related to where they are located and what's in them. For example, if it's all the same stuff it might be called the "Lyons Sandstone," or the "Benton Shale." If, on the other hand, there are several different lithologies within the formation, we need to use more general terminology such as the "Morrison Formation," which contains siltstone, sandstone, and limestone.

After geologists study the lithologies in a particular area and group them into formations, the next step is to work out the stratigraphy. Determining the local stratigraphy obviously requires an understanding of the rocks, but it also deals with the relationships between the rocks: the sequence and timing of events are now extremely important. So now it's time to throw in all the other stuff: faults and folds, dikes, unconformities, age relationships, and other complexities which tend to further complicate the issue. The stratigraphic information can be presented in several ways. One of the most useful is the stratigraphic column: a graphical representation of the stratigraphy of a particular area. For regional studies, geologists will study the stratigraphy of as many separate areas as they can, prepare a stratigraphic column for each, and combine them in an attempt to understand the regional geologic history of the area.


Click here to ask GeoMan a question

Return to Ask GeoMan's Index of Questions

Return to GeoMan's Home Page


You are GeoManiac number since April 1, 1997