Earlier this month I found myself driving though the hills of southern Illinois, and decided to take an extra time to spend some quality time with the topographic relief and exposed rock that us residents of western Tennessee don’t see very often. I spent the morning rambling though Giant City State Park south of Carbondale, and didn’t want to leave! I wandered around the Giant City Nature Trail, which is only officially 1.5 miles but probably more like 3 if you take all the side-trails. After that I stopped by the short Devil’s Standtable trail on a whim, and I’m glad I did! It follows the base of a gorgeous cliff line, and has ample opportunities for scrambling and climbing.
Southern Illinois is a island of rugged terrain in a state where the elevation rarely exceeds “rolling hills”. It barely managed to escape the Wisconsonian glaciation which bulldozed Illinois north of Highway 13, creating prime cornfield habitat north of the highway and a maze of deep valleys, tall sandstone cliffs, and forested ridges south of Carbondale and Harrisburg. All in all, this section feels more like a misplaced chunk of the Ozarks.
It occurred to me, while eating my lunch on top of a stray sandstone boulder, that the landscape look like a miniature version of the Cumberland Plateau 300 miles to the east. (You can see where those rocks come from in my Red River Gorge Post) These sandstone outcrops had the same iron bands, fragmented blocky weathering structure, and selection of trees clinging tightly to their cliffs. This made me wonder – are these rocks part of the same depositional unit as the ones on the Cumberland Plateau, or do they have a different source but just happen to look similar?
Callan Bentley did a great breakdown of the geologic features of the sandstone of the park over at the AGU blog. It turns out that the lacy weathering structure that I had been calling “honeycomb” has the snazzy official name of “tafoni”! I don’t think I can beat Callan’s beautiful photographs and blow-by-blow of depositional features, but I couldn’t find a source for my origins question and set out to research it.
It turns out that the rocks I was admiring, clambering on, and using as a picnic bench were part of the Chesterian formation of sedimentary rocks laid down during the second half of the Mississippian period roughly 359 to 323 million years ago. Therefore, they’re roughly the same age as the rocks on the Cumberland Plateau which were laid down during the Carboniferous era (which is divided the Mississippian period followed by the Pennsylvanian Period).
The rocks in Giant City State Park are on the western edge of what’s called the Illinois Basin, a regional depression in the crust exacerbated by large amounts of sediment deposited when the region was covered by a shallow sea until the Late Pennsylvanian period ~300 million years ago. This same sea also covered the area now occupied by the Cumberland Plateau, and its waves crashed against the shores of the vast deltas carrying sediment down from the rising Acadian orogen of the current Appalachian mountains. The rocks in the Red River Gorge of eastern Kentucky aren’t in the same “family” as the rocks in Giant City State Park, in the sense that they don’t share a sediment source, but instead are classmates from different origins maturing together through geologic time.
There was a bit of a speed bump that separated the Illinois Basin from the Appalachian Basin – the Cincinnati Arch. Now, as we get acquainted with this speed bump, there’s something to keep in mind. Although the older Ordovician rocks at its core might indicate that it’s an anticline (a convex fold in sedimentary layers), this cross section has a trick up its sleeve.
The Cincinnati Arch isn’t a fold structure, but a strip of crust that maintained its original elevation while the regions to the east and west sagged under the weight of sediment that was piling onto them. I couldn’t find primary resources resource to mention why exactly the Cincinnati arch was able to avoid being down-warped by sediments as its eastern and western neighbors were, unfortunately. (If you know more about that, please contact me!)
That cross-section sparked my interest, so I set out to find a paleogeographic map of North America during the Mississippian period when the Illinois Basin and Cumberland Plateau sandstones were being dumped into place. This map shows a reconstruction of the Early Mississippian, so sea levels would have been a bit lower in the mid- to late- Mississippian period when the Chesterian rocks were laid down. About 1/3 to 1/2 of northern Illinois would have been above the waves. I fancied it up in CorelDraw to show the arch, basins, and approximate directions of sediment transport into the basins…
Looking at the map, you notice that the arrows pointed to the Appalachian basin come from a mountainous region, and the ones pointing towards Illinois come from part of the land surface that’s relatively flat. This is reflected in the depth of sediment with each basin – the sandstone layers in Illinois are thinner than those in eastern Kentucky. Conversely, because the Illinois basin contained a slightly deeper sea with less sediment input it has thicker layers of limestone in the Carboniferous deposits than the Appalachian basin.
The limestone of the Illinois basin is a major economic resource in the area. Driving down Highway 51 leaving the park (sigh…) I passed the Anna Quarry Company, which has been profiting from that ancient Carboniferous sea since 1865. Their 200-acre, 450-foot deep original mining pit was abandoned when it became economically unfeasable (let’s just say, it now has 350 feet of water in it) but they continue to mine new pits on the property to provide raw materials for aggregate, asphalt, concrete, and agricultural lime.
It was a beautiful sunny drive back to Memphis, but I need to drag some friends back to Jackson Falls in this same area to climb soon!
Giant City State Park website: https://www.dnr.illinois.gov/Parks/Pages/GiantCity.aspx
breakdown of geological formations: http://blogs.agu.org/mountainbeltway/2011/09/20/giant-city/
Chesterian sandstone, which includes the rock at the park: http://isgs.illinois.edu/ilstrat/index.php/Chesterian_Series
A retro but generally accurate backstory of the geology of southern Illinois: https://www.ideals.illinois.edu/handle/2142/42765
A more approachable take on the ancient landscapes of Illinonis: https://www.isgs.illinois.edu/outreach/geology-resources/build-illinois-last-500-million-years
Source of KY cross section: https://www.uky.edu/KGS/geoky/beneath.htm
Source of paleogeographic Mississippian map: http://deeptimemaps.com/wp-content/uploads/2016/05/NAM_key-345Ma_EMiss.png
Source for information on Cincinnati Arch: http://ncad.net/Gp/Erla/ErlaGeol.htm
Blog post about Cincinnati Arch: http://historyoftheearthcalendar.blogspot.com/2014/03/march-10-cincinnati-arch.html
Bedrock Geology of Illinois Map: http://isgs.illinois.edu/sites/isgs/files/maps/statewide/imap14-front.pdf
Bedrock Geology of Illinois Strategraphic Sequence: http://isgs.illinois.edu/sites/isgs/files/maps/statewide/imap14-back.pdf