Geologic History of the Midwestern US: the Big Picture

Geologic history is the key to this Guide and to understanding the story recorded in the rocks of the Midwest. By knowing more about the geologic history of our area, you can better understand the type of rocks that are in your own backyard and why they are there. We will look at the history of the Midwest as it unfolds: as a series of major events over the past one billion years that created and shaped the area. These events will act as the framework for the topics to follow and will shed light on why our region looks the way it does!

Geologic Time

How did geologists come up with the timeline for the history of the Earth? Over the course of many years and through the combined work of geologists around the world, the geologic time scale was developed (Figure 1.1). No rock record in any one place contains the complete sequence of rocks from Precambrian to present. Geology as a science grew as geologists studied individual sections of rock. Gradually, geologists discovered evolutionary successions of fossils that helped them determine the relative ages of groups of rocks. Rock units were then correlated with similarly aged rock units from around the world. The names you see for the different periods on the geologic time scale have diverse origins. Time periods were named after dominant rock types, geography, mountain ranges, and even ancient tribes like the Silurese of England and Wales, from which the “Silurian period” was derived.

The geologic time scale is an important tool used to portray the history of the Earth—a standard timeline used to describe the age of rocks and fossils, and the events that formed them. It spans Earth’s entire history and is separated into four principle divisions.

Figure 1.1: The Geologic Time Scale (spacing of units not to scale).

Figure 1.1: The Geologic Time Scale (spacing of units not to scale).

About the Time Scale:

The time scale in The TeacherFriendly Guides™ follows that of the International Commission on Stratigraphy (ICS). The Tertiary period, though it was officially phased out in 2008 by the ICS, remains on the scale in the Guides, since “Tertiary” is present extensively in past literature. In contrast, the Carboniferous and Pennsylvanian & amp; Mississippian periods all enjoy official status, with the latter pair being more commonly used in the US.

The first of these, the Precambrian, extends from about 4.6 billion years ago to 541 million years ago. Little is known about this time period since very few fossils or unaltered rocks have survived. What few clues exist indicate that life first appeared on the planet some 3.9 billion years ago in the form of single-celled organisms.


The second division, the Paleozoic, extends from 541 to 252 million years ago. Fossil evidence shows that during this time period, life evolved in the oceans and gradually colonized the land.

The third division, the Mesozoic (from 252 to 66 million years ago), is also called the “age of reptiles” since dinosaurs and other reptiles dominated both marine and terrestrial ecosystems. It is also noteworthy that during this time the last of the Earth’s major supercontinents, Pangaea, formed and later broke up, producing the Earth’s current geography.

The last and current division, the Cenozoic, extends from the extinction of the dinosaurs, nearly 66 million years ago, to the present. With the demise of the dinosaurs, mammals became dominant and, subsequently, more diverse and highly developed. We humans don’t come into the picture until the last 2 million years. To get some perspective on this, if the entire geologic time scale were reduced to 24 hours, we wouldn’t come onto the stage until 2 seconds before midnight!

Figure 1.2: The layers of the Earth include the rigid crust of the lithosphere, which is constantly moving over the plastically flowing asthenosphere.

Figure 1.2: The layers of the Earth include the rigid crust of the lithosphere, which is constantly moving over the plastically flowing asthenosphere.

The Earth is dynamic, consisting of constantly moving plates that are made of rigid continental and oceanic lithosphere overlying a churning, plastically flowing asthenosphere (Figure 1.2). These plates are slowly pulling apart, colliding, or sliding past one another with great force, creating strings of volcanic islands, new ocean floor, earthquakes, and mountains. The continents are likewise continuously shifting position relative to each other. This not only shapes the land, but also affects the distribution of rocks and minerals, natural resources, climate, and life. Scientists can reconstruct what the ancient Earth might have looked like by studying rocks, fossils, and other geologic features.

Continental and Oceanic Crust

The lithosphere has two types of crust: continental and oceanic. Continental crust is less dense but significantly thicker than oceanic crust. The higher density of the oceanic crust means that when continental crust collides with oceanic crust, the more dense oceanic crust will be dragged (or subducted) under the buoyant continental crust. Although mountains are created at these oceanic/ continental crust collisions due to the compression of the two plates, much taller ranges are produced by continental/ continental collisions. When two buoyant continental crusts collide, there is nowhere for the crust to go but up! The modern Himalayas, at the collision site of the Asian and Indian plates, are a good example of very tall mountains formed by a collision between two continental crusts.