I recently began working on a project that is in partnership with the Illinois State Water Survey, a sister survey of the Geological Survey in Illinois’ Prairie Research Institute. The purpose of this project is to create a hydrologic model of the entire state of Illinois, from bedrock to surface. I’ll probably be sharing more about this project in the future. For now, let’s just say that Microsoft Access has been a powerful – albeit fickle – frenemy.
Because most of Illinois was recently (geologically speaking) covered in a mammoth sheet of ice, there are huge deposits of unconsolidated material on top of bedrock. Glaciers (technically, ice sheets) covered the face of the earth from St. Louis over the north pole past Moscow – a Noachian flood of the northern latitudes.
Glaciers (or ice sheets), as they move, carry with them the pieces of earth they have shaved off. These pieces can be as fine as clay, or larger than trucks. There is even a “wild” name for the large boulders that are deposited in a foreign terrane: erratics. Some of the most famous of these are located in England and are called the “Norber Erratics.”
Normally, the pieces of earth that glaciers carry with them are not large and dramatic. It is more often clay, sand and gravel, which can be deposited differently depending on what is going on with the glacier at that time.
As a glacier moves, morphs, or melts, all the sand and clay and gravel and boulders that were stuck in the ice drop to the earth, covering it sort of like snow blankets a porch. Check out this beautiful youtube video showing a time-lapse of snow falling on the ground if you want an analogous understanding of how Illinois (and much of the upper Midwest) got to be so flat – the snow starts at about 0:30.
Notice as the snow level increases, the often large relief between objects is flattened – it doesn’t matter the topography – footsteps and even the chair are obscured by the end. Even though much of the bedrock topography of Illinois is comparable to the hills and mountains of Kentucky or Tennessee, much of it has been obscured by the sediment that was dropped by glaciers thousands of years ago (in a much different way than snow, but you get the idea). You can actually see this a fairly clearly even within Illinois. Take a look at the elevation relief maps of Illinois to the right and below. You can see the extent of the largest glaciation ends with a large terminal moraine somewhere south of Shelbyville and Effingham (directly east of the confluence of the Illinois and Mississippi River). South of that, the drift gradually declines until the rough texture of bedrock begins to poke through the surface again around Marion near the south of the state: we have now reached pre-glacial Illinois.
Don’t believe me – Check out this interactive 360-degree image of Illinois’ version of Garden of the Gods in the Shawnee National Forest (southeast of Harrisburg):
Besides the smoothing out of these and other spectacular geologic features, glaciers leave other tell-tale “skid-marks” behind. Just like skid marks on a road can tell you a lot about what was happening with a car a day, week, or even a month ago, glacial features (moraines, kettles/”potholes,” drumlins, kames, eskers, etc.) on the surface tell you something about what was happening with the glacier thousands of years ago.
Terminal moraines may be the most visible of these, especially on aerial maps. In the maps above these are visible in the map as large “C” shaped objects coming in from the north east. Several of them can be seen, showing the skid marks of several glaciers or glacial lobes over the years.
These glacial landforms and glacial deposits make the earth look young – very young, in fact. They cover up the wrinkles of time.
But the processes shaping the Earth are dynamic. Below is a LIDAR (AKA super-high resolution) image of McLean County here in Illinois (it is where Bloomington-Normal is located).
You can see part of a very large terminal moraine that runs right through McLean County (it is the red area of higher elevation). Unsurprisingly, Moraine View State Park is right in the middle of the red area. This is a beautiful example of a glacial “skid mark”. And like skid marks on the road, the elements are already working to erase this moraine (see the yellow and green river valleys already carving through the Moraine) – the processes shaping the Earth are ever-changing.
The geologic history of Illinois is much more interesting than a cursory glance at its topography indicates. You just have to take a closer look, and sometimes – like a good geophysicist – look under the surface.
Stay tuned for part II (now posted here), we’ll go underground!
Writing in Rock 