In 2014, geographers from the University of Kansas developed a method for deducing “flatness” by simulating what would be seen by standing in any particular spot within a state, rotating 360 degrees, and then classifying the landscape into four flatness categories. They performed this study because a common perception among Americans was that Kansas is the flattest state. However, it doesn’t even rank in the top 5. The flattest state is Florida, owing to its Coastal Plain setting with no mountains. Yes, Illinois ranks second, followed by North Dakota, Minnesota, Louisiana, and Delaware. Kansas ranks seventh, followed by Texas, Nevada, and Indiana. It might also be a perception that the least flat state might be a western state occupied by the Rocky Mountains, such as Colorado, Montana, or Utah. However, while mountainous, these states also contain vast areas of relatively flat plains. In fact, the three least-flat states are all in the eastern part of the country: West Virginia, Pennsylvania, and Kentucky.
Suffice to say that geology dictates the degree of flatness and whether an area is flat, hilly, or something in between. Our landscape has been acted upon by physical processes that have been ongoing for billions of years, culminating in our present landscape. These include the obvious formation and eruption of volcanoes and episodes of mountain building, as well as continental drift and the collision of tectonic plates. However, here in Illinois, the main “culprits” have been glaciers, rivers and streams, wind, and weathering, the latter four of which are still occurring today and continue changing our landscape. We might add human modification of the landscape as well. Illinois ranks second only to Iowa in the percentage of its landscape that has been altered from its pre-settlement condition (most notably by agriculture and urbanization).
Illinois does have a pre-glacial landscape buried beneath a few hundred feet of glacial sediment. That primarily bedrock landscape “pops out” beyond the glacial boundary in southern Illinois, extreme west-central Illinois, and the northwest corner centering on Jo Daviess County. This landscape was complete with hills, valleys, rivers, and streams. Once the glaciers commenced their journey into Illinois about 800,000 years ago, the bedrock valleys started to fill with sediment, and the hills became buried. In east-central Illinois, the bedrock valley that now contains the Mahomet aquifer and its over 300 feet of sediment is a testament to that process.
The glaciers advanced and retreated several times across the state, eroding portions of older landscapes, redepositing sediment elsewhere, but also preserving other portions. Therefore, a legacy landscape was left behind when the last glacier left the state about 16,000 years ago. Glacial moraines are many of the highest points in Illinois counties, and the lowest points are generally in river and stream valleys. However, the highest point in Illinois is a bedrock exposure in Jo Daviess County at an elevation of 1,235 feet. The lowest elevation at 280 feet is at the confluence of the Ohio and Mississippi Rivers at Illinois’ southern tip.
Getting back to flatness, Douglas County is the flattest with a relief (distance between the highest and lowest points) of just 100 feet and an average slope of just 0.4%. Small wonder that the Embarrass River is so sluggish as it traverses the county that flooding is a common occurrence. The greatest relief of 680 feet is in Union County, but the county with the greatest average slope of 4.25% is Jo Daviess County. Both of these counties are outside the area of glaciation.
Glaciation and moraines, coupled with river and stream valleys, are particularly responsible for the high relief of Champaign, DeKalb, Kane, LaSalle, McHenry, McLean, Ogle, Winnebago, and Woodford counties. The larger moraines (~50 feet high) are quite obvious while driving along Illinois’ highways because they are often tree-covered, owing to their slopes being less than favorable for row crop agriculture. However, while Illinois’ glacial landscape is indeed flat, it remains subtle in its flatness. Bikers and runners know this quite well when traveling up the gradual incline of a not-so-prominent moraine and getting an assist in doing down. Essentially, every dip and rise along an Illinois road is due to either a positive glacial feature such as a moraine or a river/stream valley that is carved into the landscape. While the surface may appear expressionless to many, the underlying subsurface geology containing that record of multiple glacial advances and retreats and buried landscapes atop an even older bedrock surface is far from boring.