The term geologic column brings to mind rock layers, laid one on top of another, representing the geologic history of the Earth. However, this picture is only a small part of what constitutes the geologic column. In no place on Earth do preserved sediments represent the entire geologic column; sediments representing significant portions of geologic time are missing in all localities. The complete column would be an estimated 360 km (200 miles) thick, and the Earth’s crust is only about 70 km (39 miles) thick.
The geologic column is best described as a project that has gone from simple, local correlations to an ongoing correlation of all rocks on Earth by their age. This postulated column has been created through comparison and correlation of individual strata from around the world made by thousands of researchers over the last few hundred years. It has traditionally been based on the concept of relative time—this stratum is older than that one but younger than this one. Recently, however, radiometric dates have been used to reconcile the relative dates with real time. Understanding the geologic column thus requires an understanding of the project itself and the history of its creation.
It must first be noted that two closely related concepts, the geologic column and the geologic timescale, are actually separate entities. The geologic column is composed of strata with reference to their relative positions based on time correlations, while the geologic timescale is a time chart tied to various rock formations around the world. The distinction is small but significant. The commonly used terms of era, period, epoch, and age are chronologic units of the geologic timescale. Equivalent terms of the geologic column are erathem, system, series, and stage. The geologic column was the original project of geologists, while the geologic timescale became important only after the discovery of radiometric dating techniques.
The attempt to understand the geologic column undoubtedly started when the first human seriously pondered the origin of observed rock sequences. Early researchers, including Pythagoras, Xenophanes, Thales, and Aristotle, contemplated the origin of the Earth’s rocks and fossils in terms of natural causes. However, this concept was soon all but forgotten. The Judeo-Christian religious movement, with its “young Earth” interpretations, provided the dominant influence for nearly 2,000 years. During that time, the possibility of an “old Earth” was unthinkable (or at least unmentionable). This belief attributes virtually all the Earth’s sedimentary layering to a single flood event and suggests a static Earth created in its current state. This concept went virtually unchallenged, and it was the mid-1500s before anyone again seriously questioned the origin and age of the Earth’s rock formations.
Leonardo da Vinci was one of the first persons in recent history to actually research and write about the origins of sediments, but he wrote in mirror writing and kept his notes in his personal possession lest the
Church would find in them cause for persecution. It wasn’t until the 1800s, when his notes were deciphered, that the extent of his research on the geologic column was known. However, rumors of da Vinci’s research, along with his own observations and curiosity, sparked the interest of Nicholas Steno, a brilliant anatomist and the first recorded “geologist” of modern times. His 40-page publication, “Preliminary Discourse to a Dissertation on a Solid Body Contained Within a Solid,” is the first modern publication on the topic of geology and proposes the concept of older layers of sediment occurring below younger layers, later called the theory of superposition. Among other concepts put forth in the paper, he also suggested that sediments were generally laid down in horizontal layers. This later came to be known as the theory of original horizontality. Unfortunately, his work received widespread criticism, and the dissertation promised in the title of this revolutionary work was never finished.
It is well-known that this preliminary work was the inspiration for many workers over the next century, including Robert Hooke, Abraham Werner, Giovanni Arduino, and James Hutton. Arduino’s work is presented in the letter he wrote to Professor A. Vallisneri the younger on March 30, 1759. Arduino proposed a classification of Earth’s rock into four great “orders”: primary, secondary, tertiary, and quaternary. This work was a first attempt to apply systematic subdivisions to rock sequences. The subdivisions he proposed formed the foundation of the modern geologic column. Arduino’s system was used extensively to describe rock sequences throughout Europe, and even the first geologic map of North America is based on his system. Hutton’s 1785 paper, “Theory of the Earth, or an Investigation of the Laws Observable in the Composition, Dissolution and Restoration of Land Upon the Globe,” was one of the first to state that the Earth’s geologic features were the result of ongoing processes and events. As Steno’s principles of superposition and original horizontality, Arduino’s classification system, and Hutton’s principle of uniformity became more widely disseminated, the stage was set for the project known as the geologic column.
William Smith, a surveyor who worked for canal builders at the end of the 18th century, arguably contributed the concepts and publications that actually shaped the geologic column. He noticed that not only did rock layers themselves seem to correlate across broad distances but also the fossil content of the rocks correlated. This led Smith to use the concept of “faunal succession” in addition to superposition to correlate the strata and the fossils of England in a map based on a time sequence. Smith, a commoner, found himself fighting an uphill battle to get his ideas published. At one point, he ended up bankrupt and in debtor’s prison before a map publisher paid his way out of jail and eventually published his geologic map. This map, published in 1815, was the first iteration of the geologic column as we know it. With this publication, Smith single-handedly created the science of biostratigraphy and provided a means of correlating strata across large distances and across missing sections.
In 1830, Charles Lyell published the first of three volumes, collectively titled, Principles of Geology. In this work, Lyell summarized the history of the scientific works of geology to that point, expanded upon Hutton’s uniformity, and laid the foundations for future geological studies. His treatise also included detailed investigations of faunal and floral succession through time, the processes of fossilization, and the idea of the same gradual processes observed today being responsible for the features preserved in the geologic record. With these volumes, Lyell formalized the study of geology and created the framework within which the geologic column has been created.
Since then, literally thousands of researchers have contributed, and the geologic column is being constantly fine-tuned. Today, the accepted geologic column is the one certified by the International Commission on Stratigraphy and its parent organization, the International Union of Geological Sciences. Information currently used comes from a variety of sources, including lithostratigraphy, biostratigraphy, magnetostratigraphy, and radiometric dating. Technology continually allows more precise observations, and the column is constantly being remodeled to reflect the more precise information. What started out as a simple observation of time correlation in the Earth’s sediments has now grown into an effort to catalogue each rock unit on the Earth and accurately place it within the column. These correlations have, in turn, allowed discoveries of important mineral, oil, and coal deposits and have greatly enhanced our understanding of the processes and events that have shaped our world.
References:
- Cutler, A. (2003). The seashell on the mountaintop: A story of science, sainthood, and the humble genius who discovered a new history of the earth. New York: Penguin Books.
- Gradstein, F. M., Ogg, J. G., & Smith, A. G. (in press). A geologic time scale 2004. Cambridge: Cambridge University Press.
- McIntyre, D., & McKirdy, A. (2001). James Hutton: The founder of modern geology (Scot’s lives). Edinburgh: National Museums of Scotland.
- Winchester, S. (2001). The map that changed the world: William Smith and the birth of modern geology. New York: HarperCollins.
