VISUALIZING GEOLOGY: Geologic Time and the Rock Record
Before we journey back in time, let's learn a little bit about geologic time and stratigraphy.
Earth is approximately 4.54 billion years old. Because it is difficult to comprehend such a large span of time, geologists have broken down geologic time into smaller units called eons, eras, periods, and epochs and placed them on a chart called the geologic time scale. Each of these smaller portions of time are defined by an easily recognizable change in geochemistry or biology that can be traced across the entire planet. For instance, the Devonian Period (419-359 million years ago) is known as the Age of Fishes because it marks the first appearance and diversification of fish in the fossil record. Another example would be the thin layer of iridium and spherules found at the end of the Cretaceous Period (65.5 million years ago) which lends evidence to the Chicxulub Meteor Impact contributing to the extinction of the dinosaurs.
As you can see, the geologic time scale is arranged with recent times at the top (0 years) and the beginning of Earth history at the bottom (4.54 billion years ago). Many of the periods are further broken into early, middle, and late times.
Rocks form in layers, which we can read and understand much like the pages of a book. These layers are called the rock record. One of the principles geologists use to understand what we see in the rock record is the principle of Uniformitarianism, which states that the key to understanding the past is to understand the present. In other words, we can study the sediments and rocks left behind by geologic phenomena today, then apply our observations to what we are seeing in the historic rock record.
We break the rock record into distinct groups and formations. Formations that can be found at the Earth's surface are seen on a geologic map like the one you see here. We can also map formations vertically in an image called a called a stratigraphic column. Similar to the geologic time scale, the oldest formations of a stratigraphic column are shown on the bottom and the youngest at the top.
As we travel back in time for this exhibit, we will start with the Paleozoic Era, the time when life first began to diversify on the planet. From there we will move on to younger Mesozoic and Cenozoic times until we return to present time. Each page will begin with a stratigraphic column listing the formations for each era, then will showcase Dunn-Seiler Museum fossils from time periods within that era.
At the end of the exhibit you can find diagrams of certain taxa describing life position, defining body parts, and indicating which parts of that particular creature are most often preserved in the fossil record.
Now, let's travel back in time to learn more about how the landscape of what is now Mississippi has changed throughout geologic history!