Geologic Column

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. [Page 1062]

Source: © iStockphoto/Paige Foster.

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[Page 1063]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. His40-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.

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