MANY PEOPLE THINK OF ILLINOIS AS ONE FLAT expanse of corn and soybean fields, and they are mostly correct. In a 2014 study, the Geographical Review found Illinois to be the second-flattest state in the contiguous United States, right behind Florida.1 How did Illinois end up so flat? In this article, I will discuss the geological record of Illinois, highlight some of the key structural features, and explain how glaciation played a major role in shaping the landscape of Illinois.The oldest rocks in Illinois come from the Precambrian basement complex. Not a great deal is known about these rocks because they are buried so deep, and there is very little economic incentive to drill to that depth. What is known comes from a small amount of extremely deep drill holes and remote sensing data. These rocks consist primarily of granite plutons (large igneous rocks that form when magma cools and crystallizes underneath the Earth surface), granodiorite (igneous rock composed of feldspar, quartz, and mafic minerals), and rhyolite (high content of silica, some sodium, aluminum, and potassium).2 The presence of granite and rhyolite indicate signs of volcanic activity roughly 1.5 billion years ago.In the Paleozoic Era, 570 to 245 million years ago, most of Illinois’ landscape started to sink and fill with a shallow tropical ocean. Two key elements were formed from this event. The Illinois Basin located in the state's southeastern quarter is an accumulation of Paleozoic sediment. Over a period of 300 million years, layers and layers of marine sediment settled on the gradually sinking ocean floor creating a perfect environment for raw petroleum.3 The second element was the formation of St. Peter sandstone which was deposited around the coastline during the Ordovician period. These deposits are found in the northwestern and north-central counties of the state, with the largest mining operation near Ottawa. St. Peter sandstone is prized for its purity of silica oxide (99.85%), grain size that allows for easy separation, and thickness of over one hundred feet. Due to these attributes, the Ottawa mine has historically served as a major silica source in the United States.4The Cenozoic Era, 66 million to the present, was dominated by glaciations. Three major glacial stages mark the Pleistocene epoch: the Pre-Illinoian stage, the Illinoian and the Wisconsinan. Very little physical remains exist of the Pre-Illinoian because later glaciers and erosion from wind and water have destroyed most of the evidence. The existence of the Illinoian stage, on the other hand, is much more evident. During this phase, the ice sheet advanced to its southernmost point in America near Carbondale. These Pleistocene glaciers radically changed the lands they covered. As the glaciers advanced, they filled and leveled the landscape. As the ice melted, rock debris was dropped at the points of melting. When the glaciers were stagnant, these rocks accumulated into a pile called a ground or an end moraine. The relatively flat and rolling terrain that ripples out from Lake Michigan are indication of these moraines.5Only approximately 15 percent of Illinois was not covered by glacial ice: the northwestern most part of the state (most of Jo Daviess County and the northwest quadrant of Carroll County), named the Wisconsin Driftless Section, and the southernmost part of the state, just a few miles north of the Shawnee Hills. Geologists were able to identify these areas due to the lack of glacial erratic rocks. (An erratic is a rock that differs from the type of native rocks present in the area.) Because glaciation missed these areas, they are comprised of very diverse physical landscapes from bluffs, canyons, palisades, ravines, to rolling hills. The Wisconsin Driftless Section has the highest elevation point in Illinois named Charles Mound, which sits at 1,257 feet above sea level.6In the southernmost part of the state, the physical landscape is radically different from the flat rolling farmland to the north. Glaciers missed this area, but geological upheavals did not. Between 1 billion and 570 million years ago, the southern part of Illinois experienced a continental rift, which created long narrow, continental troughs but failed in ripping apart the continent. The southern part of the Illinois Basin contains grabens (a graben is a trough generated from the displacement of a block of land downward in relation to its sides): the Dixon Springs Graben in Pope County and the Rock Creek Graben in Hardin County.7 The geological upheavals that created the rocky topography in Southern Illinois provide visitors views of ravines, cliffs, and waterfalls.As one can see, Illinois is not just corn and soybean fields. Crossing this great state one may encounter a very diverse physical landscape. For Americans seeking a diverse and unique natural environment, the Prairie State offers a montage of canyons and palisades in the northwest corner, the rolling moraines in central Illinois, and finally the rocky cliffs and outcroppings in the south.
Elizabeth I. Kershisnik (Thu,) studied this question.