Geology of Illinois (No, it’s not an oxymoron)


I learned most of what I know about geology, geophysics, and hydrogeology in Illinois, starting with the basics at a small school in the suburbs of Chicago with a small handful of hardworking and dedicated earth-science faculty.

Especially for scientists on the coasts, the suburbs of Chicago might seem like one of the least interesting places to learn geology in the country, maybe the world. I recently had a non-scientist friend visit who had just moved from Los Angeles. They, like me, had taken an intro geology course in Illinois, and was much more taken by the dramatic geology of the West Coast then by the subtle science of Midwest geology. To drive the point home that California has waaaaaay more geological things then central Illinois, my friend asked somewhat sarcastically, “So…how do you even do geology in the middle of nowhere?”

Well, aside from the fact that Illinois is not the middle of nowhere…

  1. the middle of nowhere is often the best place to do geology,
  2. that is part of why I do geophysics, not geology (you get to see underground that way),
  3. quarries are like geologic oases, and
  4. this is actually not really the middle of nowhere (geologically speaking).

Since I have begun working in Illinois, I have been able to see all sorts of geologic goings-on (and there is much yet to see). Illinois–in spite of its blah geologic reputation–has a lot going for it, featuring geologic phenomena you wouldn’t be able to find (maybe) anywhere else in the world.

So at the risk of building up Illinois geology too much (and of sounding too much like a Buzzfeed article), here’s a quick rundown of some of the fun Geological highlights in Illinois:

The Largest freshwater system in the world …

Great Lakes Winter
The Great Lakes in the Winter. Image Source: NOAA

Yes, I am talking about the Great Lakes. Although Illinois only contains part of Lake Michigan, that area of Lake Michigan (the southern end) is one of the most dynamic of any part of the Great Lakes. Look at the image above. Notice that the ice stops at just about the Illinois-Indiana state line. Winds blow south down Lake Michigan and create currents, mesmerizingly imaged by the National Oceanic and Atmospheric Administration (NOAA) here. Though the winds and currents change, ice and sand tend to get pushed down to the South end of the Lake. This process creates a beautiful ridge and swale beach complex north of Chicago into Wisconsin, and just over the border in Indiana and Southern are some of the largest and most picturesque sand dunes in the U.S. and the largest freshwater dunes in the world (a few pictures I’ve taken while doing field work along Lake Michigan are below) .

If you count Lake Michigan and Lake Huron as one lake (a la an old NOAA report), it is the largest freshwater lake in the world. If you just include Lake Michigan, it is the largest lake entirely contained within one country, at least by surface area. (Lake Baikal in Russia has a larger volume and has to be one of the scariest lakes in the world, aside from Lake Nyos or really any meromictic lake). These inland seas are

1743 map of British Empire in America, centered on Illinois and the lake formerly known as Lake Illinois. Image Source: Library of Congress

(literally) impressive depressions left by humongous glaciers in the Wisconsinan Glacial period. The Third Coast Atlas has some beautiful images, infographics, and charts about the Great Lakes that I would recommend to all cartophiles in the Great Lakes Region. My favorite is the one that shows that the coasts of the Great Lakes are longer than both the West Coast and East Coast. And they call us the Third Coast (or even the Fourth Coast? What?!!?)

Oh, and did I mention Lake Michigan was originally called Lake Illinois? You have four lakes, Michigan, share the love!

In addition to the interesting geological history of the Great Lakes, the political and legal history is equally interesting. Stay tuned for next week’s post, “The Lakes and the Law of the Sea” for more on that. [That post is now live here]

The second most hazardous seismic area in the continental U.S….

2014 U.S. Geological Survey (USGS) National Seismic Hazard Map. Image Source: USGS

and the most active east of the Rockies. Illinois is one of only 12 of the 48 lower states with the highest level of Earthquake hazard, as determined by the USGS. The New Madrid Seismic Zone produces small earthquakes quite often, and every once in a while, produces a very a large one. The most prominent earthquake(s) in this zone occurred in 1811-1812. It is in the top 20 earthquakes in U.S. history (top 10 if you don’t include Alaska). The estimated magnitude of this earthquake was 7.5-7.9, and it is said to have awoken President James Madison in Washington, DC. Because the crust in the eastern half of the U.S. is less fractured, older, and stiffer, the area of impact of a relatively small earthquake is much larger than in the West.

The faults and seismic zone seem to be the result of a rift that failed to split completely, a process that began at the end of the Precambrian Eon (see more on that in my post on the Precambrian in Illinois: 20,000 feet under the Trees). Perhaps the most famous rift in the world is the East African Rift, where the density of volcanoes is only matched by hotspots in the Ocean.

The fourth longest river in the world…

Mississippi River Pathways
Distribution of Alluvial Deposits at Wabash/Mississippi Confluence. Image Source: U.S. Army Corps of Engineers 

…and at places, more than a mile across in Illinois. That’s right, I’m talking about Big Muddy…well, not that Big Muddy. The Big “Big Muddy.” You know, Ol’ Man River. Father of Waters. The Great River…


Besides draining 41% of the contiguous U.S., it comprises the entire western border of Illinois. This section of the river has inspired some of the best of American Literature and scientific art (image above), and it has been the lifeblood of much of the nation’s shipping and agriculture. It creates epic sandbars, “palisades” (see interactive image below), floods, and oxbow lakes unlike any other place in the country, even to the point of creating some odd geopolitical boundaries, such as the Kentucky Bend (I go into more detail on that run of the Mississippi River in the West Tennessee section of this post on Tennessee geography and geology). It was the lifeblood (and namesake) of one of the greatest pre-columbian civilizations in the Americas, the Mississipian Culture.

The largest pre-Columbian city in the current U.S…

Artist rendition of Cahokia at its height

…and one of the most mysterious as well.

Ok, so, this may seem like it is more archaeology than geology (it is), but I’m including it because I do shallow geophysics, and many of the methods we use here at the ISGS are more or less the same as what scientists at the Illinois State Archaeological Survey use. In fact, there have been several ISGS scientists who have helped archaeologists study this site.

Anyway, the city built on mounds–Cahokia (AKA “North America’s Lost Medieval City“)–is one of the wonders of the Americas, and a UNESCO cultural world heritage site (one of only nine in the U.S.). When Europeans arrived in the 17th century at the Cahokia mounds, the city had already been abandoned for hundreds of years. Home to up to 30,000 people in 1050 A.D., it was the largest city north of the Rio Grande until Philadelphia surpassed it…over 600 years later! (The largest city in the world at the time was only about 400,000.) The society there is thought to have been a metropolitan people, composed of citizenry from as far away as current-day Mexico and Ohio.

The Mississippian culture is ancient, and mound-building in this culture is a tradition older than Abraham (yeah, the biblical one). The largest mound at Cahokia–Monk’s Mound— although not as tall as the Great Pyramid, had more or less the same footprint, and had a building 100 ft long on the top. Considering there is little bedrock near the surface in that area of Illinois, the fact that a structure that large could have been built is astounding.

One of the most important aquifers in the U.S…

Map of the Mahomet Aquifer overlaid on Illinois Counties. Image Source: Illinois State Geological Survey

…and one of the largest under glacial deposits, the Mahomet Aquifer is the treasure of Central Illinois. It is an important sole-source aquifer (meaning it provides more than 50% of drinking water to a given population) that traces the route of an ancient river valley. It provides water to nearly 900,000 people, and is considered one of the most important aquifers in the United States. There is a cool map here showing some of the important aquifers in the U.S. Note: this map does the Mahomet discredit, because it shades entire areas of “aquifer systems,” whereas the Mahomet is merely one, big, more-or-less unbroken aquifer.

I will talk more about the Mahomet Aquifer in a later post.

The Entire Glacial Spectrum

Glacial landforms forming
Glacial Landforms. Image Source: Illinois State Geological Survey

From pro-glacial lakes to outwash plains, Illinois features the entire spectrum of continental glaciation (the furthest southern extent of glaciation in North America is named after Illinois, after all). In Illinois you can start your travel at an isostatic depression (i.e., Lake Michigan) where glaciers were so large (and/or the crust weak) that the ice literally bent the crust of the earth, a bruise from which the earth is still recovering (see isostatic equilibrium diagram).

Diagram showing isosatic rebound

You can travel through the Chain o’ Lakes region, featuring “kettle” lakes and all sorts of glacial geomorphology which were formed as glaciers were melting and breaking up into large blocks of ice. From there, travel south through a series of various glacial landforms, formed in separate episodes under separate conditions (as noted in my previous two-part post “How to Build a Moraine”, you can see the “skid marks” of many of these episodes just by looking at a topographical map). As you travel further south, you see the last of the outwash plains and in southern Illinois, you catch a glimpse underneath the blanket of the glaciers, where bedrock is near or at the surface and beautiful sandstone vistas abound. (see below)

Oil fields (and other mine-able resources)

Illinois Basin Oilfields. Image Source: Illinois State Geological Survey

Unfortunately, the study of bedrock in Illinois often does not get the same glory as the study of Quaternary (kwa-TERN-uh-ree) glacial deposits these days. But the bedrock is where some of the more economically valuable natural resources in the state are located, and there is yet a vibrant community of mineral, petroleum, and coal geologists doing great work in Illinois and the Illinois Basin.

Though Illinois is not known for being a high production state, in 2016 (last date for which data is available), it was the 15th highest producing state in the U.S. Most of the oil production in Illinois comes from an area known as the “hydrocarbon kitchen.” In this area, carbon-carrying shales were at the right pressure and temperature for the right amount of time to produce oil. There is also a fair amount of coal, and the second largest coal power plant in the U.S. (the Gibson Generating Station, previously called Public Service Indiana) is across the Wabash River from Mt. Carmel, Illinois in Indiana, and runs largely off coal produced in Illinois.

It has also had adverse environmental effects on residents in Illinois, particularly in 2001 when a “plume inversion” at the plant caused sulfuric acid to waft into Mt. Carmel, IL. Pretty crazy stuff.

A whole, whole bunch of dead sea creatures


Another way of saying this may be that Illinois has a lot of skeletons in its closet, but that might hit a little too close to home, politically speaking.  Limestone, dolomite, and other calcareous rock is in the bedrock pretty much through all of Illinois. It is made up largely of the bones of reef animals, and–as any first year geology student could tell you–these types of rock fizz when you drop hydrochloric acid on them (see GIF, it’s pretty fun). Limestone fizzIn fact, Limestone and other calcareous stone are very sensitive to chemical erosion, creating karst landscapes in the bedrock in a few parts of the state (diagram above). This process is also what forms the stalactites and stalagmites that make caves look so other-worldly. This chemical erosion is the process that formed the great cave systems in the world, including Mammoth Caves, a mere 100 miles from Illinois’ own Cave in Rock State Park. Cave in Rock features a giant cave along the Ohio River, which you can see in the interactive image below. Who knows how many of these ancient cave systems are hidden beneath dozens and even hundreds of feet of glacial drift?

But besides the cool underground features, limestone hints at a former tropical paradise in present-day Illinois, a topic I will get into in later posts. Sunny, shallow, equatorial seas able to support reefs must have been the norm for a long, long time in what is now Illinois. Gives a new perspective on a state that is known for snowy winters and unpredictable weather.

In short, there is a lot of Illinois Geology that the flat plains and the seemingly endless corn desert belie (overly?). Getting below the surface, or even just taking a closer look at the surface, you begin to see some interesting “Geology Underfoot.” (Speaking of which, more can be found in a book with that title – Geology Underfoot in Illinois).

As George Orwell wrote (in a very different context), “To see what is in front of one’s nose needs a constant struggle.” Seeing the beauty of the geology in the Illinois is one such struggle, but one that is worth the work.


One comment

  1. That’s pretty cool stuff friend. As much as I loathe Illinois government, the state is really fabulous and full of interesting things. I love science, so thanks for sharing this content. Good stuff.


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